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Species with reproduction barriers that can both reproduce with a third species


To start with, I do not have a sound knowledge in biology or any formal education in the area.

I was told that one of the definition of a species is a reproductive barrier, which means that if two animals can't reproduce, they are of different species. The barrier can be either the inability of the sperm to fertilize an egg, or a physical trait that inhibits reproduction, e.g. a cricket species that has a different mating song than another species or two species of flies, one that mates on yellow flowers, and the other on red flowers.

But what happens when, while two species can't reproduce, but there is a "chain" of "intermediate" sexual partners that can produce reproductive connection step by step. Like 6 degrees to Kavin Bacon, but with animal sex.

I'll try to explain with an example:

A Great Dane and a Miniature Pinscher dogs can't mate due to obvious size differences. But The Pincher can mate with a German Pincher (a slightly bigger breed of Pincher), which can mate with a Doberman Pincher. And the Doberman can mate with a German Shepard which can mate with a Great Dane.

I've also heard that such things happen with birds and crickets, where there is an original species, from which evolved several other, and while the original species (which still exists) can mate with all the new species. Some new species can't mate with some, or all of the other new species.

How are such species are defined, and at what point dogs stop being dogs anymore?


How are such species are defined, and at what point dogs stop being dogs anymore?

This is a bit like the is-Pluto-a-planet-discussion. A group of scientists have to come together and hold a big conference. You have a few principles that you want to adhere to and then it's big groups of people making decisions.


There are many definitions of a species, which may or may not include the concept of reproductive barrier. The Biological Species Concept (BSC) is quite popular and involves a reproductive barrier, but other concepts such as the Phylogenetic Species Concept do not include a reproductive barrier.

Disagreements and confusion also happen over just what the best criteria are for identifying new species. In 1942 the famous biologist Ernst Mayr wrote that because biologists have different ways of identifying species, they actually have different species concepts. Mayr proceeded to list five different species concepts, and since then many more have been added. The question of which species concept is best has occupied many printed pages and many hours of discussion.

Some debates are philosophical in nature. One common disagreement is over whether a species is defined by the characteristics that biologists use to identify the species, or whether a species is an evolving entity in nature. Every named species has been formally described as a type of organism with particular defining characteristics. These defining traits are used to identify which species organisms belong to. But for many species, all of the individuals that fit the defining criteria also make up a single evolving unit. These two different ways of thinking about species, as a category and as an evolving population, are quite different from each other. Wikipedia Species Problem

The Biological Species Concept does have it's problems:

It is also true that there are many cases where members of different species will hybridize and produce fertile offspring when they are under confined conditions, such as in zoos. One fairly extreme example is that lions and tigers will hybridize in captivity, and at least some of the offspring have been reported to be fertile. (see also Lions and Tigers)

Mayr's response to cases like these is that the reproductive barriers that are important for species are the ones that occur in the wild. But even so it is also the case that there are many cases of different species that are known to hybridize and produce fertile offspring in nature. Wikipedia Species Problem

The Phylogenetic Species Concept also has its problems:

it permits successive species to be defined even if they have evolved in an unbroken line of descent, with continuity of sexual fertility. However, because slight differences can be found among virtually any group of organisms, the concept tends to encourage extreme division of species into ever-smaller groups. Phylogenetic Species Concept

When does a dog stop being a dog? Technically all dogs are in the same species: See the definition of Dog, which is a subspecies of the species, Canis lupus, or Grey Wolf. The line between dog and wolf is poorly-defined as there may be hybrid wolf-dogs, since they are both from the same species. Up one level is the family Canidae, which includes wolves, foxes, jackals, and coyote species.


The Biological Species Concept is largely useful in looking at the process of evolution. Since speciation isn't typically (though this isn't always true either!) an instantaneous process, it is useful to observe it in action. The BSC is the best way of approaching this (at least for sexually reproducing organisms). Back to your original question. It might be useful for you to search "ring species" as this is what you are talking about. The most famous example is the Ensantina salamander complex. They are essentially several species/one species depending on your perspective. It is more-or-less a spatial version of the hypothetical dog analogy, but it is also a natural phenomenon that occurs in many other species…


Essay on Macroevolution | Species | Biology

Evolution is not progress. The popular notion that evolution can be represented as a series of improvements from simple cells, through more complex life forms, to humans (the pinnacle of evolution), can be traced to the concept of the scale of nature. This view is incorrect.

All species have descended from a common ancestor. As time went on, different lineages of organisms were modified with descent to adapt to their environments. Thus, evolution is best viewed as a branching tree or bush, with the tips of each branch representing currently living species.

No living organisms today are our ancestors. Every living species is as fully modern as we are with its own unique evolutionary history. No extant species are “lower life forms,” atavistic stepping stones paving the road to humanity.

A related, and common, fallacy about evolution is that humans evolved from some living species of ape. This is not the case—humans and apes share a common ancestor.

Both humans and living apes are fully modern species the ancestor we evolved from was an ape, but it is now extinct and was not the same as present day apes (or humans for that matter). If it were not for the vanity of human beings, we would be classified as an ape. Our closest relatives are, collectively, the chimpanzee and the pygmy chimp. Our next nearest relative is the gorilla.

Microevolution can be studied directly. Macroevolution cannot. Macroevolution is studied by examining patterns in biological populations and groups of related organisms and inferring process from pattern. Given the observation of microevolution and the knowledge that the earth is billions of years old — macroevolution could be postulated. But this extrapolation, in and of itself, does not provide a compelling explanation of the patterns of biological diversity we see today.

Evidence for macroevolution, or common ancestry and modification with descent, comes from following fields of study:

i. Comparative biochemical and genetic studies.

ii. Comparative developmental biology.

iii. Patterns of biogeography.

iv. Comparative morphology and anatomy and the fossil record.

Closely related species (as determined by morphologists) have similar gene sequences. Overall sequence similarity is not the whole story, however. The pattern of differences we see in closely related genomes is worth examining.

All living organisms use DNA as their genetic material, although some viruses use RNA. DNA is composed of strings of nucleotides. There are four different kinds of nucleotides adenine (A), guanine (G), cytosine (C) and thymine (T). Genes are sequences of nucleotides that code for proteins. Within a gene, each block of three nucleotides is called a codon. Each codon designates an amino acid (the subunits of proteins).

The three letter code is the same for all organisms (with a few exceptions). There are 64 codons, but only 20 amino acids to code for so, most amino acids are coded for by several codons. In many cases the first two nucleotides in the codon designate the amino acid. The third position can have any of the four nucleotides and not affect how the code is translated.

A gene, when in use, is transcribed into RNA — a nucleic acid similar to DNA. (RNA, like DNA, is made up of nucleotides although the nucleotide uracil (U) is used in place of thymine (T). The RNA transcribed from a gene is called messenger RNA.

Messenger RNA is then translated via cellular machinery called ribosomes into a string of amino acids—a protein. Some proteins function as enzymes, catalysts that speed the chemical reactions in cells. Others are structural or involved in regulating development.

Gene sequences in closely related species are very similar. Often, the same codon specifies a given amino acid in two related species, even though alternate codons could serve functionally as well. But, some differences do exist in gene sequences. Most often, differences are in third codon positions, where changes in the DNA sequence would not disrupt the sequence of the protein.

There are other sites in the genome where nucleotide differences do not effect protein sequences. The genome of eukaryotes is loaded with ‘dead genes’ called pseudogenes. Pseudogenes are copies of working genes that have been inactivated by mutation. Most pseudogenes do not produce full proteins.

They may be transcribed, but not translated. Or, they may be translated, but only a truncated protein is produced. Pseudogenes evolve much faster than their working counterparts. Mutations in them do not get incorporated into proteins, so they have no effect on the fitness of an organism.

Introns are sequences of DNA that interrupt a gene, but do not code for anything. The coding portions of a gene are called exons. Introns are spliced out of the messenger RNA prior to translation, so they do not contribute information needed to make the protein. They are sometimes, however, involved in regulation of the gene. Like pseudogenes, introns (in general) evolve faster than coding portions of a gene.

Nucleotide positions that can be changed without changing the sequence of a protein are called silent sites. Sites where changes result in an amino acid substitution are called replacement sites. Silent sites are expected to be more polymorphic within a population and show more differences between populations.

Although both silent and replacement sites receive the same amount of mutations, natural selection only infrequently allows changes at replacement sites. Silent sites, however, are not as constrained.

Kreitman was the first to demonstrate that silent sites were more variable than coding sites. Shortly after the methods of DNA sequencing were discovered, he sequenced 11 alleles of the enzyme alcohol dehydrogenase (AdH). Of the 43 polymorphic nucleotide sites he found, only one resulted in a change in the amino acid sequence of the protein.

Silent sites may not be entirely selectively neutral. Some DNA sequences are involved with regulation of genes, changes in these sites may be deleterious. Likewise, although several codons code for a single amino acid, an organism may have a preferred codon for each amino acid. This is called codon bias.

If two species shared a recent common ancestor one would expect genetic information, even information such as redundant nucleotides and the position of introns or pseudogenes, to be similar. Both species would have inherited this information from their common ancestor.

The degree of similarity in nucleotide sequence is a function of divergence time. If two populations had recently separated, few differences would have built up between them. If they separated long ago, each population would have evolved numerous differences from their common ancestor (and each other).

The degree of similarity would also be a function of silent versus replacement sites. Li and Graur, in their molecular evolution text, give the rates of evolution for silent vs. replacement rates. The rates were estimated from sequence comparisons of 30 genes from humans and rodents, which diverged about 80 million years ago.

Silent sites evolved at an average rate of 4.61 nucleotide substitution per 10 9 years. Replacement sites evolved much slower at an average rate of 0.85 nucleotide substitutions per 10 9 years.

Groups of related organisms are ‘variations on a theme’ — the same set of bones is used to construct all vertebrates. The bones of the human hand grow out of the same tissue as the bones of a bat’s wing or a whale’s flipper and, they share many identifying features such as muscle insertion points and ridges. The only difference is that they are scaled differently. Evolutionary biologists say this indicates that all mammals are modified descendants of a common ancestor which had the same set of bones.

Closely related organisms share similar developmental pathways. The differences in development are most evident at the end. As organisms evolve, their developmental pathway gets modified. An alteration near the end of a developmental pathway is less likely to be deleterious than changes in early development.

Changes early on may have a cascading effect. Thus most evolutionary changes in development are expected to take place at the periphery of development, or in early aspects of development that have no later repercussions. For a change in early development to be propagated, the benefit of the early alteration must outweigh the consequences to later development.

Because they have evolved this way, organisms pass through the early stages of development that their ancestors passed through up to the point of divergence. So, an organism’s development mimics its ancestors although it doesn’t recreate it exactly. Development of the flatfish, Pleuronectes, illustrates this point.

Early on, Pleuronectes develops a tail that comes to a point. In the next developmental stage, the top lobe of the tail is larger than the bottom lobe (as in sharks). When development is complete, the upper and lower lobes are equally sized. This developmental pattern mirrors the evolutionary transitions it has undergone.

Natural selection can modify any stage of a life cycle, so some differences are seen in early development. Thus, evolution does not always recapitulate ancestral forms — butterflies did not evolve from ancestral caterpillars, for example. There are differences in the appearance of early vertebrate embryos.

Amphibians rapidly form a ball of cells in early development. Birds, reptiles and mammals form a disk. The shape of the early embryo is a result of different yolk concentrations in the eggs. Birds’ and reptiles’ eggs are heavily yolked. Their eggs develop similarly to amphibians except the yolk has deformed the shape of the embryo.

The ball is stretched out and lying atop the yolk. Mammals have no yolk, but still form a disk early. This is because they have descended from reptiles. Mammals lost their yolky eggs, but retained the early pattern of development. In all these vertebrates, the pattern of cell movements is similar despite superficial differences in appearance. In addition, all types quickly converge upon a primitive, fish-like stage within a few days. From there, development diverges.

Traces of an organism’s ancestry sometimes remain even when an organism’s development is complete. These are called vestigial structures. Many snakes have rudimentary pelvic bones retained from their walking ancestors. Vestigial does not mean useless, it means the structure is clearly a vestige of a structure inherited from ancestral organisms. Vestigial structures may acquire new functions. In humans, the appendix now houses some immune system cells.

Closely related organisms are usually found in close geographic proximity this is especially true of organisms with limited dispersal opportunities. The mammalian fauna of Australia is often cited as an example of this marsupial mammals fill most of the equivalent niches that placentals fill in other ecosystems.

If all organisms descended from a common ancestor, species distribution across the planet would be a function of site of origination, potential for dispersal, distribution of suitable habitat, and time since origination. In the case of Australian mammals, their, physical separation from sources of placentals means potential niches were filled by a marsupial radiation rather than a placental radiation or invasion.

Natural selection can only mold available genetically based variation. In addition, natural selection provides no mechanism for advance planning. If selection can only tinker with the available genetic variation, we should expect to see examples of jury-rigged design in living species. This is indeed the case. In lizards of the genus Cnemodophorus, females reproduce parthenogenetically.

Fertility in these lizards is increased when a female mounts another female and simulates copulation. These lizards evolved from sexual lizards whose hormones were aroused by sexual behaviour. Now, although the sexual mode of reproduction has been lost, the means of getting aroused (and hence fertile) has been retained.

Fossils show hard structures of organisms less and less similar to modern organisms in progressively older rocks. In addition, patterns of biogeography apply to fossils as well as extant organisms. When combined with plate tectonics, fossils provide evidence of distributions and dispersals of ancient species.

For example, South America had a very distinct marsupial mammalian fauna until the land bridge formed between North and South America. After that marsupials started disappearing and placentals took their place. This is commonly interpreted as the placentals wiping out the marsupials, but this may be an over simplification.

Transitional fossils between groups have been found. One of the most impressive transitional series is the ancient reptile to modern mammal transition. Mammals and reptiles differ in skeletal details, especially in their skulls. Reptilian jaws have four bones. The foremost is called the dentary.

In mammals, the dentary bone is the only bone in the lower jaw. The other bones are part of the middle ear. Reptiles have a weak jaw and a mouthful of undifferentiated teeth. Their jaw is closed by three muscles- the external, posterior and internal adductor. Each reptile tooth is single cusped.

Mammals have powerful jaws with differentiated teeth. Many of these teeth, such as the molars, are multi- cusped. The temporalis and masseter muscles, derived from the external adductor, close the mammalian jaw. Mammals have a secondary palate, a bony structure separating their nostril passages and throat, so most can swallow and breathe simultaneously. Reptiles lack this.

The evolution of these traits can be seen in a series of fossils. Procynosuchus shows an increase in size of the dentary bone and the beginnings of a palate. Thrinaxodon has a reduced number of incisors, a precursor to tooth differentiation. Cynognathus (a doglike carnivore) shows a further increase in size of the dentary bone.

The other three bones are located inside the back portion of the jaw. Some teeth are multicusped and the teeth fit together tightly. Diademodon (a plant eater) shows a more advanced degree of occlusion (teeth fitting tightly). Probelesodon has developed a double joint in the jaw. The jaw could hinge off two points with the upper skull.

The front hinge was probably the actual hinge while the rear hinge was an alignment guide. The forward movement of a hinge point allowed for the precursor to the modern masseter muscle to anchor further forward in the jaw. This allowed for a more powerful bite.

The first true mammal was Morgonucudon, a rodent-like insectivore from the late Triassic. It had all the traits common to modern mammals. These species were not from a single, unbranched lineage. Each is an example from a group of organisms along the main line of mammalian ancestry.

The strongest evidence for macroevolution comes from the fact that suites of traits in biological entities fall into a nested pattern. For example, plants can be divided into two broad categories- non­vascular (ex. mosses) and vascular. Vascular plants can be divided into seedless (ex. ferns) and seeded.

Vascular seeded plants can be divided into gymnosperms (ex. pines) and flowering plants (angiosperms). Angiosperms can be divided into monocots and dicots. Each of these types of plants has several characters that distinguish them from other plants. Traits are not mixed and matched in groups of organisms.

For example, flowers are only seen in plants that carry several other characters that distinguish them as angiosperms. This is the expected pattern of common descent. All the species in a group will share traits they inherited from their common ancestor. But, each subgroup will have evolved unique traits of its own. Similarities bind groups together. Differences show how they are subdivided.

The real test of any scientific theory is its ability to generate testable predictions and, of course, have the predictions borne out. Evolution easily meets this criterion. In several of the above examples I stated, closely related organisms share X. If I define closely related as sharing X, this is an empty statement.

It does however, provide a prediction. If two organisms share a similar anatomy, one would then predict that their gene sequences would be more similar than a morphologically distinct organism.

This has been spectacularly borne out by the recent flood of gene sequences — the correspondence to trees drawn by morphological data is very high. The discrepancies are never too great and usually confined to cases where the pattern of relationship was debated.


Evolution of Organisms: Genetic Variation, Frequency of Genes and Other Details

Evolution refers to the process by which early organisms of the earth diversified into various new forms through slow but continuous variations.

Ever since the appearance of the first living beings on the earth some 3.5 billion years ago, new forms have continuously originated.

And, the different forms have undergone modifications and given rise to new forms.

The newer forms are sufficiently different to be recognized as new species. They breed amongst their own members and not with the ancestral forms or any other forms. The newly formed species may give rise to still newer species over a period of time.

This process is called descent with modification. This is the main theme of evolution. Evolution occurs due to the survival of advantageous variations produced in reproduction.

Sources of Genetic Variation:

You know that there are two main types of variations—somatic and germinal (genetic). Genetic variation arises due to mutation and it can account for the creation of a new species. Mutation is any change in the structure of a gene. Mutation may lead to a change in the expression of a gene. Such a change may even produce harmful effects in the organism.

Another source of variation is genetic recombination. It is a natural process due to which the arrangement of genes in the progeny is in a combination that differs from that of the parents. This is because the offspring receive genes from both the parents, and this ensures the transmission of some genetic variability from the parents to the offspring.

Mutation and genetic recombination may give rise to new characters due to change in genes. These new characters may help the individuals to adapt to their environment. Sometimes the new characters may not help individuals to adapt. Disease, competition, etc., can eliminate those less well-adapted individuals.

The survivors pass on their advantageous characters to their offspring. This enables the offspring to adapt well to their environment. Thus nature selects new characters by favouring some of them and eliminating others. In this way natural selection may lead to the evolution of a new species with new characters. Let us see how variations in a population lead to evolution.

Frequency of Genes in a Population:

The proportion of a particular allele in a population is called gene frequency. How does gene frequency in a population change? Let us consider the genes of a particular species. All the genes in a population of a species at a given time form its gene pool. The frequency of certain genes in the population of an area can change due to certain environmental factors.

Let us take an example and observe the results in different situations (see Figure 7.6). Suppose there is a population of red beetles living in some bushes in a particular area of a forest. Let us assume that they can generate heritable variations during their sexual reproduction.

In the first situation, a heritable colour variation occurs so that a green offspring is born to its red-coloured parents. The green beetle then passes its green trait on to its offspring.

These green beetles living in the green leaves of the bushes escape the notice of crows, while the red beetles, because of their bright colour, are easily spotted and eaten by the crows. As a result, the red beetles are soon eaten up by the crows, while the green beetles survive, reproduce and increase in number.

In the second situation, a blue-colour variation arises during the reproduction in red beetles. The blue beetle also gives birth to and more blue beetles. This change in colour, however, gives no survival advantage over the red variety since the crows easily find and eat both blue and red beetles.

Now, suppose a bush fire occurs suddenly and kills a large number of beetles, and all the surviving beetles are, by chance, blue. The progeny of the blue beetles are also blue.

The survival of these blue beetles is, however, not a case of natural selection, unlike the survival of the Heredity and Evolution green beetles in the first situation. This is a case of genetic drift, that is, a random change in the gene frequency.

In the third situation, many of the bushes dry up due to a prolonged dry period. The bushes that’ survive the drought have smaller leaves. This leads to a food shortage for the red beetles. Incidentally, some beetles in the population are smaller in size on account of a heritable variation.

They manage to survive, as they require less food, while most of the large beetles die of starvation. Some young beetles of the large variety survive, but these cannot grow to their full size due to undernourishment.

Hence, there are only small beetles. When the drought ends and there is enough food for all the beetles, large beetles reappear, and there are both large and small beetles in the population.

The genetically small beetles remain small even when they have more food, but the undernourished beetles, which did not undergo any genetic change, grow to their normal size.

In the above situations, we can examine where genetic drift has occurred. In the first situation, natural selection played a part in preserving a certain heritable variation. In the second situation, an accident caused genetic drift. In the third situation, an environmental factor led to the production of smaller beetles although some were also produced due to heritable variation.

Acquired and Inherited Traits:

From very early times scientists have been trying to explain the origin, evolution and diversity of life forms. In the nineteenth century, however, the idea that complex animals and plants developed by gradual change from simpler forms began to be taken seriously.

The mechanism of the origin of new species from the existing species was explained first by Jean Baptiste Pierre Antoine de Monet Lamarck (1744-1829), a French biologist, and then by Charles Robert Darwin (1809-82), a British naturalist.

Acquired traits:

Having accepted the fact that new species have arisen from pre-existing species with modifications, a number of scientists have tried to explain the mechanism by which this might have occurred. The first scientific theory concerning this came from Lamarck. His ideas, written in his book Philosophy Zoologies (meaning ‘philosophical zoology’), published in 1809, and are known as Lamarckism.

Lamarck observed the changes and adaptations in certain organs in animals. He suggested that favourable changes appear due to the use or disuse of organs over a long period of time.

For example, some organs develop in size if they are in continuous use, while their disuse has an opposite effect. He concluded that such characters acquired by an organism during its lifetime are transmitted to the next generation. This inheritance of acquired characters results in the evolution of one or more new species.

However, most scientists disagree with this, as it has not been supported by experiments. For example, the offspring of a couple of mice whose tails have been cut off are not born tailless. This was demonstrated by an experiment performed by August Friedrich Leopold Weismann (1834-1914), a, German biologist.

In sexually reproducing organisms, germ cells are produced in the reproductive organs, while the rest of the body has somatic cells. Changes in somatic cells due to environmental factors are not transmitted to the offspring. This is because a change in a somatic organ caused by a physiological response by the body does not bring about a corresponding change in reproductive organs.

For example, in the earlier illustration, if beetles starve, their size will get reduced. But if they reproduce, their progeny may not have reduced body size if they get enough food. This means that starvation of the parent beetles does not alter the DNA sequence of their germ cells so as to bring about a variation in the next generation.

Even if the reproductive cells suffer from starvation, this does not lead to any change in the DNA. The son of a wrestler is, therefore, not born muscular. Similarly, cutting off the tails of mice do not change the genes of their germ cells.

Inherited traits:

Darwinism is the first modem theory that attempted to explain the origin of new species. Charles Darwin made an extensive study of the flora and fauna of the Galapagos Islands in South America. He came to certain conclusions, which he explained in his book The Origin of Species, published in 1859.

Darwin proposed that new species arise by the slow accumulation of advantageous variations over a period of time. Though he did not say how these variations arise, he said that variations are so common in nature that no two individuals are alike.

Darwin’s second observation was that although the power of reproduction of organisms is enormous, the population size of any species always remains within a limit. He explained it by saying that overpopulation results in a competition for food and shelter, ultimately leading to a struggle for existence among the members of a species.

In such a struggle, those that survive must have some favourable qualities that enable them to overcome the difficult situation. These qualities are advantageous variations. The surviving organisms repeat the process of reproduction. Biologically, a species that can reproduce and leave a large number of offspring is considered successful. When the new generation with advantageous characters begins to reproduce, the situation of overproduction and inevitable struggle is repeated.

The survivors will have more advantageous characters that help them to compete and survive. All these new features might make them considerably different from the original forms. These differences, or variations, when accumulated over a long period of time lead to the origin of new species. Thus, selection of advantageous variations by nature leads to the origin of new species.

Charles Darwin explained the mechanism of origin of new species by natural selection. But his theory fell short of explaining the mechanism or the source of heritable variations. This was explained by Hugo de Vries (1848-1935), a Dutch botanist. According to him, heritable variations arise when there is a change in the genes of the germplasm (protoplasm of a germ cell). He called it mutation.

The manner in which heritable variations are passed on to the succeeding generations was explained by Gregor Mendel after he performed his pea-plant experiments.

If a particular trait spreads in the population, it means that it is favoured by natural selection. On the other hand, an acquired trait is not transmitted to the offspring. Those animals that do not show enough variations are likely to be wiped out, as they cannot cope with changing circumstances. Genetic variability gives an ability to adapt and adds to the chances of survival of a species.

A small population of any species would have fewer mutations, resulting in lesser variability and diminished ability to adapt. For example, the small numbers of tigers surviving in the world do not have enough variations to adapt well to changes in the environment and hence may become extinct.


Biology 171

By the end of this section, you will be able to do the following:

  • Define species and describe how scientists identify species as different
  • Describe genetic variables that lead to speciation
  • Identify prezygotic and postzygotic reproductive barriers
  • Explain allopatric and sympatric speciation
  • Describe adaptive radiation

Although all life on earth shares various genetic similarities, only certain organisms combine genetic information by sexual reproduction and have offspring that can then successfully reproduce. Scientists call such organisms members of the same biological species.

Species and the Ability to Reproduce

A species is a group of individual organisms that interbreed and produce fertile, viable offspring. According to this definition, one species is distinguished from another when, in nature, it is not possible for matings between individuals from each species to produce fertile offspring.

Members of the same species share both external and internal characteristics, which develop from their DNA. The closer relationship two organisms share, the more DNA they have in common, just like people and their families. People’s DNA is likely to be more like their father or mother’s DNA than their cousin or grandparent’s DNA. Organisms of the same species have the highest level of DNA alignment and therefore share characteristics and behaviors that lead to successful reproduction.

Species’ appearance can be misleading in suggesting an ability or inability to mate. For example, even though domestic dogs (Canis lupus familiaris) display phenotypic differences, such as size, build, and coat, most dogs can interbreed and produce viable puppies that can mature and sexually reproduce ((Figure)).


In other cases, individuals may appear similar although they are not members of the same species. For example, even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer) are both birds and eagles, each belongs to a separate species group ((Figure)). If humans were to artificially intervene and fertilize a bald eagle’s egg with an African fish eagle’s sperm and a chick did hatch, that offspring, called a hybrid (a cross between two species), would probably be infertile—unable to successfully reproduce after it reached maturity. Different species may have different genes that are active in development therefore, it may not be possible to develop a viable offspring with two different sets of directions. Thus, even though hybridization may take place, the two species still remain separate.


Populations of species share a gene pool: a collection of all the gene variants in the species. Again, the basis to any changes in a group or population of organisms must be genetic for this is the only way to share and pass on traits. When variations occur within a species, they can only pass to the next generation along two main pathways: asexual reproduction or sexual reproduction. The change will pass on asexually simply if the reproducing cell possesses the changed trait. For the changed trait to pass on by sexual reproduction, a gamete, such as a sperm or egg cell, must possess the changed trait. In other words, sexually-reproducing organisms can experience several genetic changes in their body cells, but if these changes do not occur in a sperm or egg cell, the changed trait will never reach the next generation. Only heritable traits can evolve. Therefore, reproduction plays a paramount role for genetic change to take root in a population or species. In short, organisms must be able to reproduce with each other to pass new traits to offspring.

Speciation

The biological definition of species, which works for sexually reproducing organisms, is a group of actual or potential interbreeding individuals. There are exceptions to this rule. Many species are similar enough that hybrid offspring are possible and may often occur in nature, but for the majority of species this rule generally holds. The presence in nature of hybrids between similar species suggests that they may have descended from a single interbreeding species, and the speciation process may not yet be completed.

Given the extraordinary diversity of life on the planet there must be mechanisms for speciation : the formation of two species from one original species. Darwin envisioned this process as a branching event and diagrammed the process in the only illustration in On the Origin of Species ((Figure)a). Compare this illustration to the diagram of elephant evolution ((Figure)), which shows that as one species changes over time, it branches to form more than one new species, repeatedly, as long as the population survives or until the organism becomes extinct.


For speciation to occur, two new populations must form from one original population and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists have proposed mechanisms by which this could occur that fall into two broad categories. Allopatric speciation (allo- = “other” -patric = “homeland”) involves geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation (sym- = “same” -patric = “homeland”) involves speciation occurring within a parent species remaining in one location.

Biologists think of speciation events as the splitting of one ancestral species into two descendant species. There is no reason why more than two species might not form at one time except that it is less likely and we can conceptualize multiple events as single splits occurring close in time.

Allopatric Speciation

A geographically continuous population has a gene pool that is relatively homogeneous. Gene flow, the movement of alleles across a species’ range, is relatively free because individuals can move and then mate with individuals in their new location. Thus, an allele’s frequency at one end of a distribution will be similar to the allele’s frequency at the other end. When populations become geographically discontinuous, it prevents alleles’ free-flow. When that separation lasts for a period of time, the two populations are able to evolve along different trajectories. Thus, their allele frequencies at numerous genetic loci gradually become increasingly different as new alleles independently arise by mutation in each population. Typically, environmental conditions, such as climate, resources, predators, and competitors for the two populations will differ causing natural selection to favor divergent adaptations in each group.

Isolation of populations leading to allopatric speciation can occur in a variety of ways: a river forming a new branch, erosion creating a new valley, a group of organisms traveling to a new location without the ability to return, or seeds floating over the ocean to an island. The nature of the geographic separation necessary to isolate populations depends entirely on the organism’s biology and its potential for dispersal. If two flying insect populations took up residence in separate nearby valleys, chances are, individuals from each population would fly back and forth continuing gene flow. However, if a new lake divided two rodent populations continued gene flow would be unlikely therefore, speciation would be more likely.

Biologists group allopatric processes into two categories: dispersal and vicariance. Dispersal is when a few members of a species move to a new geographical area, and vicariance is when a natural situation arises to physically divide organisms.

Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west coast of the United States, two separate spotted owl subspecies exist. The northern spotted owl has genetic and phenotypic differences from its close relative: the Mexican spotted owl, which lives in the south ((Figure)).


Additionally, scientists have found that the further the distance between two groups that once were the same species, the more likely it is that speciation will occur. This seems logical because as the distance increases, the various environmental factors would likely have less in common than locations in close proximity. Consider the two owls: in the north, the climate is cooler than in the south. The types of organisms in each ecosystem differ, as do their behaviors and habits. Also, the hunting habits and prey choices of the southern owls vary from the northern owls. These variances can lead to evolved differences in the owls, and speciation likely will occur.

Adaptive Radiation

In some cases, a population of one species disperses throughout an area, and each finds a distinct niche or isolated habitat. Over time, the varied demands of their new lifestyles lead to multiple speciation events originating from a single species. We call this adaptive radiation because many adaptations evolve from a single point of origin thus, causing the species to radiate into several new ones. Island archipelagos like the Hawaiian Islands provide an ideal context for adaptive radiation events because water surrounds each island which leads to geographical isolation for many organisms. The Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, the founder species, numerous species have evolved, including the six in (Figure).


Notice the differences in the species’ beaks in (Figure). Evolution in response to natural selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food source. The seed-eating bird has a thicker, stronger beak which is suited to break hard nuts. The nectar-eating birds have long beaks to dip into flowers to reach the nectar. The insect-eating birds have beaks like swords, appropriate for stabbing and impaling insects. Darwin’s finches are another example of adaptive radiation in an archipelago.

Click through this interactive site to see how island birds evolved in evolutionary increments from 5 million years ago to today.

Sympatric Speciation

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? The answer is yes. We call the process of speciation within the same space sympatric. The prefix “sym” means same, so “sympatric” means “same homeland” in contrast to “allopatric” meaning “other homeland.” Scientists have proposed and studied many mechanisms.

One form of sympatric speciation can begin with a serious chromosomal error during cell division. In a normal cell division event chromosomes replicate, pair up, and then separate so that each new cell has the same number of chromosomes. However, sometimes the pairs separate and the end cell product has too many or too few individual chromosomes in a condition that we call aneuploidy ((Figure)).


Which is most likely to survive, offspring with 2n+1 chromosomes or offspring with 2n-1 chromosomes?

Polyploidy is a condition in which a cell or organism has an extra set, or sets, of chromosomes. Scientists have identified two main types of polyploidy that can lead to reproductive isolation of an individual in the polyploidy state. Reproductive isolation is the inability to interbreed. In some cases, a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition that we call autopolyploidy ((Figure)). The prefix “auto-” means “self,” so the term means multiple chromosomes from one’s own species. Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of separating.


For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2n = 6, when they should be n = 3), the gametes now have twice as many chromosomes as they should have. These new gametes will be incompatible with the normal gametes that this plant species produces. However, they could either self-pollinate or reproduce with other autopolyploid plants with gametes having the same diploid number. In this way, sympatric speciation can occur quickly by forming offspring with 4n that we call a tetraploid. These individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral species.

The other form of polyploidy occurs when individuals of two different species reproduce to form a viable offspring that we call an allopolyploid . The prefix “allo-” means “other” (recall from allopatric): therefore, an allopolyploid occurs when gametes from two different species combine. (Figure) illustrates one possible way an allopolyploid can form. Notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results.


The cultivated forms of wheat, cotton, and tobacco plants are all allopolyploids. Although polyploidy occurs occasionally in animals, it takes place most commonly in plants. (Animals with any of the types of chromosomal aberrations that we describe here are unlikely to survive and produce normal offspring.) Scientists have discovered more than half of all plant species studied relate back to a species evolved through polyploidy. With such a high rate of polyploidy in plants, some scientists hypothesize that this mechanism takes place more as an adaptation than as an error.

Reproductive Isolation

Given enough time, the genetic and phenotypic divergence between populations will affect characters that influence reproduction: if individuals of the two populations were brought together, mating would be less likely, but if mating occurred, offspring would be nonviable or infertile. Many types of diverging characters may affect the reproductive isolation , the ability to interbreed, of the two populations.

Reproductive isolation can take place in a variety of ways. Scientists organize them into two groups: prezygotic barriers and postzygotic barriers. Recall that a zygote is a fertilized egg: the first cell of an organism’s development that reproduces sexually. Therefore, a prezygotic barrier is a mechanism that blocks reproduction from taking place. This includes barriers that prevent fertilization when organisms attempt reproduction. A postzygotic barrier occurs after zygote formation. This includes organisms that don’t survive the embryonic stage and those that are born sterile.

Some types of prezygotic barriers prevent reproduction entirely. Many organisms only reproduce at certain times of the year, often just annually. Differences in breeding schedules, which we call temporal isolation , can act as a form of reproductive isolation. For example, two frog species inhabit the same area, but one reproduces from January to March whereas, the other reproduces from March to May ((Figure)).


In some cases, populations of a species move or are moved to a new habitat and take up residence in a place that no longer overlaps with the same species’ other populations. We call this situation habitat isolation . Reproduction with the parent species ceases, and a new group exists that is now reproductively and genetically independent. For example, a cricket population that was divided after a flood could no longer interact with each other. Over time, natural selection forces, mutation, and genetic drift will likely result in the two groups diverging ((Figure)).


Behavioral isolation occurs when the presence or absence of a specific behavior prevents reproduction. For example, male fireflies use specific light patterns to attract females. Various firefly species display their lights differently. If a male of one species tried to attract the female of another, she would not recognize the light pattern and would not mate with the male.

Other prezygotic barriers work when differences in their gamete cells (eggs and sperm) prevent fertilization from taking place. We call this a gametic barrier . Similarly, in some cases closely related organisms try to mate, but their reproductive structures simply do not fit together. For example, damselfly males of different species have differently shaped reproductive organs. If one species tries to mate with the female of another, their body parts simply do not fit together. ((Figure)).


In plants, certain structures aimed to attract one type of pollinator simultaneously prevent a different pollinator from accessing the pollen. The tunnel through which an animal must access nectar can vary widely in length and diameter, which prevents the plant from cross-pollinating with a different species ((Figure)).


When fertilization takes place and a zygote forms, postzygotic barriers can prevent reproduction. Hybrid individuals in many cases cannot form normally in the womb and simply do not survive past the embryonic stages. We call this hybrid inviability because the hybrid organisms simply are not viable. In another postzygotic situation, reproduction leads to hybrid birth and growth that is sterile. Therefore, the organisms are unable to reproduce offspring of their own. We call this hybrid sterility.

Habitat Influence on Speciation

Sympatric speciation may also take place in ways other than polyploidy. For example, consider a fish species that lives in a lake. As the population grows, competition for food increases. Under pressure to find food, suppose that a group of these fish had the genetic flexibility to discover and feed off another resource that other fish did not use. What if this new food source was located at a different depth of the lake? Over time, those feeding on the second food source would interact more with each other than the other fish therefore, they would breed together as well. Offspring of these fish would likely behave as their parents: feeding and living in the same area and keeping separate from the original population. If this group of fish continued to remain separate from the first population, eventually sympatric speciation might occur as more genetic differences accumulated between them.

This scenario does play out in nature, as do others that lead to reproductive isolation. One such place is Lake Victoria in Africa, famous for its sympatric speciation of cichlid fish. Researchers have found hundreds of sympatric speciation events in these fish, which have not only happened in great number, but also over a short period of time. (Figure) shows this type of speciation among a cichlid fish population in Nicaragua. In this locale, two types of cichlids live in the same geographic location but have come to have different morphologies that allow them to eat various food sources.


Section Summary

Speciation occurs along two main pathways: geographic separation (allopatric speciation) and through mechanisms that occur within a shared habitat (sympatric speciation). Both pathways isolate a population reproductively in some form. Mechanisms of reproductive isolation act as barriers between closely related species, enabling them to diverge and exist as genetically independent species. Prezygotic barriers block reproduction prior to formation of a zygote whereas, postzygotic barriers block reproduction after fertilization occurs. For a new species to develop, something must cause a breach in the reproductive barriers. Sympatric speciation can occur through errors in meiosis that form gametes with extra chromosomes (polyploidy). Autopolyploidy occurs within a single species whereas, allopolyploidy occurs between closely related species.

Art Connections

(Figure) Which is most likely to survive, offspring with 2n+1 chromosomes or offspring with 2n-1 chromosomes?

(Figure) Loss of genetic material is almost always lethal, so offspring with 2n+1 chromosomes are more likely to survive.

Free Response

Why do island chains provide ideal conditions for adaptive radiation to occur?

Organisms of one species can arrive to an island together and then disperse throughout the chain, each settling into different niches and exploiting different food resources to reduce competition.

Two species of fish had recently undergone sympatric speciation. The males of each species had a different coloring through which the females could identify and choose a partner from her own species. After some time, pollution made the lake so cloudy that it was hard for females to distinguish colors. What might take place in this situation?

It is likely the two species would start to reproduce with each other. Depending on the viability of their offspring, they may fuse back into one species.

Why can polyploidy individuals lead to speciation fairly quickly?

The formation of gametes with new n numbers can occur in one generation. After a couple of generations, enough of these new hybrids can form to reproduce together as a new species.

Glossary


Future Challenges

In this last section, we would like to highlight future challenges that inhibit the study of reproductive biology in wildlife and ideas to overcome them. Emerging issues related to climate change, spread of poorly known diseases, or overexploitation of natural habitats clearly remind us that we need to act quickly to prevent the loss of biodiversity [ 1]. Reproductive studies will always remain a high priority in the giant conservation puzzle. However, we have to be realistic about what our challenges are and make sure that we address them early on. While we can expect tremendous progress in the study of reproduction of wild species using new technologies (Table 1, Figure 3), we need to keep in mind that conservation means supporting whole organisms and reproductive fitness of entire populations. Genomic and epigenomic information will only have value when solid phenotypical data are integrated to them. Similar to the discipline of “precision medicine” currently developed in humans, it is time to create and develop a concept of “precision conservation” or “precision conservation breeding”—a more customized/tailored approach to optimize the potential of each individual in a population.


IV. Genetic changes and evolutionary drivers behind reproductive isolation

Our understanding of the evolutionary forces that underlie the origins of reproductive barriers – mutation, natural selection, and genetic drift – has been greatly enhanced through investigations of the genetic basis of barriers, particularly for postpollination barriers (Fig. 1). Therefore, it is appropriate to consider both the evolutionary drivers of reproductive barriers and their genetic basis when evaluating the causes of reproductive isolation. As will be evident below, in some cases (e.g. chromosomal rearrangements) we have a clearer understanding of the genetic changes involved but more limited understanding of the evolutionary drivers, whereas in other cases (e.g. aspects of local adaptation) the reverse holds true (Table 3). Although our knowledge is preliminary, the evidence so far suggests that mutation (e.g. polyploidy) and natural selection might be the main drivers of plant diversity. Before reviewing this evidence, we will briefly outline the roles of mutation, natural selection and stochastic processes in speciation.

Example
Natural selection clearly understood
Adaptation to edaphic conditions Mimulus on high-metal soil (Copperopolis Wright et al., 2013 ) adaptation to serpentine soils in Collinsia (Moyle et al., 2012 ) Senecio in sand dunes and rocky headlands (Melo et al., 2014 )
Adaptation to wind Gilia inland vs coastal (Nagy, 1997 Nagy & Rice, 1997 )
Adaptation to disease Necrosis (Bomblies, 2010 )
Reinforcement Phlox (Hopkins & Rausher, 2012 )
Role of natural selection not well understood
Genome duplication (polyploidy) Chamerion (Husband & Sabara, 2004 )
Chromosomal translocations Mimulus (Stathos & Fishman, 2014 )
Natural selection potentially absent
Gene duplication and differential silencing Oryza (Ouyang & Zhang, 2013 )

The mutations responsible for reproductive isolation occur at a variety of scales, from single nucleotide changes that affect protein structure or expression, to gene duplications, chromosomal rearrangements and genome duplication (polyploidy). The contribution of a single mutation to total reproductive isolation depends on both the time of origin and its absolute effect on reproductive isolation (Schluter & Nosil, 2011 ). As such, a mutation with a very strong effect on reproductive isolation (e.g. 90% infertility) might contribute only a small relative increase in reproductive isolation if it evolved late during the speciation process (e.g. increasing total RI from 0.95 to 0.98). Although new species rarely arise as a consequence of a single mutation, a major exception is whole genome duplication, where a single macromutational event can create strong isolation with parental populations almost instantaneously. Other common mutations with a strong absolute effect on reproductive isolation are chromosomal rearrangements that through their effects on proper meiotic segregation can often reduce fertility in hybrids.

Biologists often consider the role of genetic drift affecting allelic frequencies in a population, but other stochastic processes can also affect the identity and order in which mutations arise in a population (Werth & Windham, 1991 Lynch & Force, 2000 Schluter, 2009 ). Even when reproductive isolation is driven by natural selection, historical contingencies such as differences in the identity and order in which advantageous mutations arise in geographically isolated populations can lead to reproductive isolation – a process known as ‘mutation-order speciation’ (Schluter, 2009 ). Mutation limitation could thus be a significant contributor to variation between allopatric populations, making it a potentially important force during the early stages of the speciation process.

Genetic drift cannot be ruled out as a factor in speciation and almost certainly contributes. However, the time to fix alleles via drift is relatively slow: 4Ne generations for neutral alleles. For many cases where we have estimates of timescales and population sizes, drift is simply too slow to explain the origins of reproductive isolation (Sambatti et al., 2012 ). Therefore, it is probable that most reproductive barriers evolve as a consequence of selection. Some barriers likely originate as a direct product of selection, such as ecogeographic isolation, immigrant inviability, phenological isolation, pollinator isolation and ecological selection against hybrids. Alternatively, reproductive barriers may be incidental byproducts of selection on linked loci or on loci with pleiotropic effects. For example, phenological isolation could be a byproduct of adaptation to serpentine soils, or it could arise from an independent set of genetic changes that were favoured by natural selection. A detailed genetic analysis of the alleles responsible for barriers should help distinguish these hypotheses, particularly when traits are correlated and both seem to contribute to reproductive isolation.

1. Genetic basis of reproductive isolation

Studying the genetics of reproductive barriers can shed light on several aspects of their origins, as well as on their effectiveness as reproductive barriers. For instance, at the level of the genome, barriers with a complex genetic architecture will confer greater genomic isolation in the face of ongoing gene flow than will traits with simple architectures (Barton & Hewitt, 1985 ). The genetic basis of reproductive isolation can also be useful in testing predictions of speciation theory such as the ‘snowball effect,’ in which hybrid incompatibilities are predicted to accumulate at a faster than linear rate (Orr, 1995 ). Information about the identity and function of speciation genes can inform us about the traits and evolutionary forces underlying reproductive isolation, particularly for postpollination barriers. Finally, knowledge of the overall genetic architecture of reproductive isolation allows us to ask questions about the genomic context of genes involved in reproductive isolation, such as how chromosomal differences affect the origins of reproductive isolation and how genetic differences accumulate over time. In the sections that follow, we examine the genetic basis and evolutionary drivers for different reproductive barriers, moving from prepollination barriers to postpollination barriers.

Prepollination reproductive isolation: local adaptation

One of the most exciting developments of the past decade has been progress toward understanding the genetic basis of local adaptation. The adaptation of plants to different environments has been understood as an important reproductive isolating barrier – although often partial – for over 90 yr (Turesson, 1922 Stebbins, 1950 Sobel et al., 2009 ). Local adaptation is likely a complex phenomenon, potentially involving evolution in response to different soils, temperatures, competitors, pathogens and herbivores. However, only one or a few aspects of the genetics of local adaptation have been examined in any given species thus far. In the past decade, quantitative trait loci increasing fitness in one environment have been identified in many taxa including Arabidopsis, Boechera, Helianthus, Mimulus and Trifolium (Lexer et al., 2003 Weinig et al., 2003 Gardner & Latta, 2006 Hall et al., 2010 Kooyers & Olsen, 2012 Anderson et al., 2013 Des Marais et al., 2013 Oakley et al., 2014 ). Traits examined include salt tolerance, cold tolerance, flowering time, cyanogenesis and growth. In several cases, experiments tested for trade-offs in different habitats. Intriguingly, relatively few trade-offs were identified, suggesting that many of the alleles underlying local adaptation may be favoured in one environment and neutral in another (Anderson et al., 2013 Oakley et al., 2014 ). The puzzling lack of trade-offs observed so far suggests that generalist genotypes might be possible, as the alleles leading high fitness in one environment need not lead to low fitness in other environments. Numerous genes associated with adaptation in natural populations have been cloned and characterized in Arabidopsis or in crop–wild relatives. We expect that field experiments testing the fitness effects of alternative alleles (as opposed to QTL) underlying local adaptation will be increasingly important (e.g. cbf2 locus underlying free tolerance in Arabidopsis Oakley et al., 2014 ).

Prepollination reproductive isolation: flower colour and pollen–pistil interactions

Our knowledge of the genetic basis of plant–pollinator interactions is considerably more advanced, particularly for flower colour. One of the best-studied systems is the sister-species pair, Mimulus lewisii and M. cardinalis. The two differ in floral shape, colouration and pollinator visitation. A quantitative trait locus, yup, explains a great deal of variation in the carotenoid (yellow) pigment concentrations between the two species (Bradshaw & Schemske, 2003 ). A gene responsible for a large amount of the variation in anthocyanin (pink) pigment concentrations, ROI1, has been confirmed as well (Yuan et al., 2013 ). These two genes alter flower colour and pollinator behaviour, and so contribute to the single most important reproductive isolating barrier between the species (Ramsey et al., 2003 ). The evolutionary hypothesis in this system is that differences in pollinator availability in different habitats led to divergent floral traits being favoured by natural selection. Evidence in Mimulus aurantiacus is consistent with this idea (Streisfeld et al., 2013 ).

Although pollen–pistil interactions can provide extremely strong reproductive barriers (Lowry et al., 2008a Sambatti et al., 2012 ), their genetic bases have been studied much less than flower colour. In the genus Solanum, evidence suggests that some of the proteins involved in self-incompatibility also function in reproductive isolation. By introducing functional self-incompatibility proteins into self-compatible tomato, it was possible to create a pollen–pistil barrier where one had not previously existed (Tovar-Méndez et al., 2014 ). It is possible that the same evolutionary events that led to mating system shifts may have affected pollen–pistil barriers – although in this case potentially weakening the barrier in the case of the selfing species. In some instances, pollen–pistil interactions create absolute barriers to cross-species pollination (e.g. Pellegrino et al., 2010 ), but in many others reproductive isolation is heightened through competition between intra- and interspecific pollen, a phenomenon known as conspecific pollen precedence (Fig. 2). The evolutionary forces leading to conspecific pollen precedence are often unclear, and identification of the loci involved may be necessary to elucidate the evolutionary forces.

One of the few systems in which progress has been made toward understanding the genetic basis of conspecific pollen precedence is Mimulus, where analyses of transmission ratio distortion in progeny derived from interspecific pollen mixtures implies that at least eight genomic regions contribute to precedence (Fishman et al., 2008 ). The precise loci and evolutionary forces involved remain unknown. Another study in Mimulus suggests an alternative route to address these questions. Aagaard et al. ( 2013 ) identified pollen proteins expressed in the pistil by labelling the pollen with 15 N, allowing the pollen to germinate and grow down the pistil, then identifying 15 N-labelled proteins using mass spectrometry. The protein sequences were then used to identify > 2000 genes, which were in turn screened for signatures of selection. Several genes were identified with elevated rates of molecular evolution and signatures of natural selection, although the selective pressures remain unclear.

Prepollination barriers: Reinforcement – natural selection against costly hybridization

Wallace argued that selection would favour the evolution of prezygotic barriers when postzygotic barriers already existed, as individuals wasting fewer gametes would have increased fitness (Wallace, 1912 ). This process is referred to as the ‘Wallace effect’ (Grant, 1966 ) or reinforcement (Dobzhansky, 1940 ), with the latter term becoming increasingly dominant in recent years (Servedio & Noor, 2003 ). Although reinforcement was initially viewed by Dobzhansky ( 1940 ) as a mechanism for completing speciation, this seemed unlikely because selection for reinforcement would weaken as hybridization rates decline. Also, reinforcement alleles would only be favoured in areas of sympatry, and so would be unlikely to spread across the range of species with partially allopatric distribution patterns (Servedio & Noor, 2003 Ortiz-Barrientos et al., 2009 ). Despite these concerns, theoretical and experimental evolution studies indicate that reinforcement can occur as a result of selection against the production of hybrids with low viability or fertility, or from mating costs associated with hybridization such as wasted gametes and stigma clogging (Servedio & Noor, 2003 Ortiz-Barrientos et al., 2009 ). In addition, considerable empirical support has been developed for the presence of reinforcement in animals (Servedio & Noor, 2003 ), although evidence for reinforcement in plants is less compelling: of 12 studies listed by Hopkins ( 2013 ) only one satisfies her four criteria. This fully developed case comes from an examination of Phlox drummondii and P. cuspidata (Hopkins & Rausher, 2011 , 2012 , 2014 ). Hybrids between the two species have decreased fitness due to intrinsic postzygotic incompatibilities. In areas where the two species are allopatric, both have light blue flowers, whereas in areas of sympatry, P. drummondii has dark red flowers. Two unlinked loci involved in the regulation of the anthocyanin biosynthetic pathway underlie these changes (Hopkins & Rausher, 2011 ). Experimental arrays decoupling colour from intensity demonstrated that the dark red flowers decrease the probability of a pollinator moving pollen between the two species, reducing the rate of formation of low fitness hybrids (Hopkins & Rausher, 2012 ). Although the fitness advantage of the red colour, if any, remains uncertain, that of the dark colour is clear.

Another example of reinforcement involves a diploid–tetraploid complex in fireweed (Chamerion). Reinforcement is predicted to evolve in this situation because there is postzygotic isolation between diploids and tetraploids, and frequent areas of sympatry or parapatry. Experiments using diploid, natural tetraploid and synthesized tetraploid Chamerion angustifolium reveal a pattern that is compatible with reinforcement. In mixed-pollen load experiments, established tetraploid plants are less likely to be fertilized by pollen from diploid plants than are neotetraploid plants (Baldwin & Husband, 2011 ). As with many pollen–pistil interactions, the genetic basis is uncertain. The Chamerion study represents an example in which conspecific pollen precedence appears to have evolved to limit maladaptive hybridization. Previously, most attention has focused on visible, but potentially costly, reproductive barriers such as temporal, pollinator and mating system isolation (Hopkins, 2013 ). Cryptic barriers such as conspecific pollen precedence provide a less costly means for reinforcement to evolve (Lorch & Servedio, 2007 ) and are worthy of greater attention in future studies.

Similarities and differences in the genetics and evolutionary drivers of reproductive isolation

The genetics and evolutionary drivers of prepollination isolation will likely differ from that of intrinsic postzygotic isolation because the fitness effects of alleles underlying the former depend mainly on the environment (G × E), whereas those underlying the latter depend largely on genetic background (G × G). This leads to a predicted genetic architecture in which intrinsic postzygotic barriers derive from deleterious interactions between two or more loci, whereas loci underlying prepollination barriers act mainly independently. These predictions have been largely confirmed in case studies (reviewed in Rieseberg & Blackman, 2010 ).

Three distinct hypotheses exist to explain why alleles contributing to postzygotic RI between populations become common within a population. The alleles might become common due to linkage to alleles favoured by natural selection or might be the same alleles with pleiotropic effects. Or, the alleles leading to postzygotic RI might fix within a population due to drift. Later we focus on the genetic evidence supporting these hypotheses in different cases.

Associations between reproductive isolating barriers: local adaptation and postzygotic isolation

If intrinsic postzygotic isolation arises as a byproduct of natural selection, then we might also expect an association between local adaptation and postzygotic isolation. We tested for this association by drawing on reviews of postzygotic isolation (Rieseberg et al., 2006 ), local adaptation (Hereford, 2007 ) or parallel speciation (Ostevik et al., 2012 ). In each case where either local adaptation or intrinsic postzygotic isolation had been established, we searched the ‘Web of Science’ (http://webofscience.com/) to locate information on other reproductive barriers. However, we found only 13 cases with documented local adaptation where results of crossing studies had been published (Supporting Information Table S1). Of these, five (38%) showed evidence of postzygotic incompatibilities. Thus, at this time, the evidence that local adaptation is a key driver of the development of other reproductive barriers is weak, but a few cases demonstrate how it might occur.

In crosses between some varieties of rice, and between some populations of Arabidopsis thaliana (reviewed in Bomblies, 2010 ), the hybrid offspring show low viability. The necrosis is similar to that seen following a disease response. This reproductive barrier (which exists in low frequency) is therefore hypothesized to be a byproduct of natural selection that favoured the increase in frequency of a disease resistance allele in one of the populations. In that genetic background, no damaging interactions occurred with other alleles. However, the disease resistance allele interacts negatively with alleles found in the other population, leading to an autoimmune response that damages hybrids (reviewed in Bomblies, 2010 ). In this case, it appears that one element of local adaptation – disease resistance – has a pleiotropic effect on postzygotic isolation.

A second case illustrates connections between local adaptation and postzygotic isolation, but where linkage rather than pleiotropy is involved. Copper mining in the foothills of California left soils with toxic copper concentrations. Although most of the surrounding Mimulus guttatus were unable to survive under these conditions, a distinct population evolved that could tolerate copper and which were reproductively isolated from a Welsh population. Initial efforts found that the two traits – copper tolerance and reproductive isolation – were inherited together (Macnair & Christie, 1983 ). However, either pleiotropic effects of copper tolerance (where the same genetic change caused copper tolerance and hybrid lethality) or tight linkage could have been at work. Recent work (Wright et al., 2013 ) separated the two traits, demonstrating that linkage (< 0.32 cM: fewer than 0.32% of offspring of heterozygotes were recombinants) rather than pleiotropy was key. In this case it appears that the copper tolerance allele was linked to an allele leading to hybrid lethality. As M. guttatus moved onto the high copper soils, natural selection led to an increase in the copper tolerance allele and to the tightly linked incompatibility allele in a process known as hitchhiking. Thus, postzygotic isolation arose as a byproduct of natural selection favouring copper tolerance due to linkage rather than pleiotropy.

A third case with apparent links between local adaptation and postzygotic isolation is suggestive, but has yet to see a detailed genetic dissection. Collinsia sparsiflora occurs in the hills of California's coast range, with populations on and off serpentine soils. In the field, ecotypes are strongly adapted to their home environment. Both ecotypes are viable in the glasshouse, but show reduced hybrid fertility (Moyle et al., 2012 ). It is not clear whether this postzygotic isolation originated from the adaptation to the two environments, or if it is due to independent genetic differences.

Postpollination barriers potentially arising from genomic conflicts

In the past decade, a number of genes leading to postzygotic reproductive isolation have been identified and found to have evolved due to natural selection. However, in several cases, the natural selection appears to have come from genomic conflicts rather than adaptation to the external environment (Presgraves, 2010 ). Genomic conflicts can occur when one gene within an individual can obtain a fitness advantage at the expense of other genes within the same organism.

One example in rice shows how reproductive isolation might arise as a byproduct of genetic conflict. In many crosses between two subspecies (Oryza sativa subsp. japonica and O. sativa subsp. indica), F1s have low female fertility. The genetic basis of this loss of hybrid fertility is complex, involving three linked loci. Two of the loci result in a ‘killer’ phenotype that leads to preferential abortion of ovules lacking a ‘protector’ allele at a third locus (Fig. 3). Neither subspecies has a functional ‘killer’ phenotype due to deletions in one of the two required loci in each subspecies. If the intact ‘killer’ phenotype evolved with a functional ‘protector’ allele, then this linked set of loci could have been favoured by natural selection in an ancestral population. In heterozygotes, ovules lacking the ‘killer’ package would have been eliminated, leading to increased frequency of the ‘killer’ in the population. In this scenario, the original machinery of reproductive isolation would have evolved before the split between the two subspecies, rising to fixation in the ancestor of the two. Subsequent evolution in the two subspecies silenced different parts of the ‘killer/protector’ package, leading to decreased fertility in hybrids in which some ovules will lack the protector allele (Ouyang & Zhang, 2013 ). This scenario implicates natural selection favouring a selfish genetic element (the linked killer–protector package) followed by potential nonadaptive differentiation in the two subspecies.

Cytoplasmic male sterility is a postzygotic reproductive barrier observed in multiple species (Scopece et al., 2010 ). In theory, an initial mutation in a mitochondrial protein leads to pollen sterility and an increase in seed production (Fig. 4). The increased seed set can lead to the mutation becoming more common in the population. If a nuclear restorer allele arises, it too can become common in the population, eliminating the male sterility. However, some crosses between populations (or species) result in hybrids that are male sterile due to a mitochondrial sterility allele from one species and a missing restorer allele from the other. As with the ‘killer’ system discussed earlier, the cytoplasmic male sterility alleles increase in frequency due to natural selection independent of the external environment.

Postpollination barriers: differential silencing

The preceding examples have examined how postzygotic isolation might arise due to natural selection (Table 3). Stochastic processes may also be involved. In one case, a portion of F2s created by crosses between two subspecies of rice were found to suffer from pollen infertility. Mapping of the genes involved found that one of the two copies at a duplicated locus had been reciprocally disrupted in each of the two subspecies (Fig. 5). Pollen grains with loss-of-function alleles at both paralogs failed to germinate (Ouyang & Zhang, 2013 ). This case is a potential candidate for stochastic processes leading to reproductive isolation: it is possible that the mutations leading to gene silencing in each subspecies were neutral, or even if beneficial, that silencing the alternate paralog would have equivalent effects. However, various scenarios involving natural selection are also consistent with the evidence here, including hitchhiking or different fitness effects of silencing in each genetic background (Muir & Hahn, 2015 ).

Postpollination barriers: chromosomal rearrangements

Because rearrangements are expected to harbour genic incompatibilities (Navarro & Barton, 2003 ), it can be challenging to distinguish between the effects of chromosomal rearrangement itself on F1 fertility and the effects of genes contained within rearrangements. Inducing polyploidy in a low-fertility F1 provides a simple test of whether decreased F1 fertility is due to chromosomal or genic factors (Dobzhansky, 1933 ). In some cases, such as in hybrids between Mimulus cardinalis and M. lewisii, inducing polyploidy restored hybrid fertility, suggesting that it was difficulties in meiotic pairing, rather than the effects of genes within the rearranged segments that were responsible for the partial sterility of diploid hybrids (Stathos & Fishman, 2014 ).

The mechanisms by which chromosomal rearrangements affect hybrid sterility are fairly clear. However, many questions about their origins and establishment remain. If chromosomal rearrangements lead to decreased fitness in heterozygotes (underdominance) due to meiotic difficulties, how would the rearrangements initially become common in a population? Early theories suggested that very small populations might allow rearrangements to become common due to genetic drift, but the effective population sizes of most plant species are too large to support such a hypothesis (Whitney et al., 2010 ). Alternatively, if a rearrangement occurs with a mutation with strong positive effects on fitness, then the net fitness of the combination could be neutral or positive despite the deleterious effects of the rearrangement on meiosis. Although the co-occurrence of a favoured mutation and a chromosomal rearrangement is likely to be rare, rearrangements could also be favoured if they reduce recombination between locally adapted alleles in a heterogeneous environment (Kirkpatrick & Barton, 2006 ). This scenario seems more plausible, at least for inversions, because selection would act on the cumulative fitness effects of the linked alleles. Models of recombination, selection and gene flow are complex and an active area of research, but beyond the scope of this review (see Ortiz-Barrientos et al., 2002 Butlin, 2005 and Seehausen et al., 2014 , for perspectives on this topic).

Rearrangements can also establish through association with a selfish genetic element, such as a pollen killer that biases the outcome of meiosis to make it more likely that the chromosome with the selfish element ends up in the next generation (White, 1978 ). However, this hypothesis seems unlikely to account for the high rate of karyotypic evolution seen in plants (Levin, 2002 ) because of the required concurrence of two uncommon events (as with the favourable mutation hypothesis). On the other hand, if the rearrangement itself is the cause of drive (i.e. female meiotic drive Fig. 7) then the hypothesis becomes more credible (Coyne, 1989 ). An advantage in meiosis could arise, for example, if there was a change in centromeric position or size (Chmátal et al., 2014 ). In support of this hypothesis, drive-like behaviour was reported for six of 10 newly arisen translocations in common sorrel (Rumex acetosa Wilby & Parker, 1988 ). A theoretical advantage to female meiotic drive as a selective explanation for the fixation of chromosomal rearrangements is that it could enable their spread across the entire range of a species rather than being limited to populations experiencing particular environmental conditions.

Postpollination reproductive isolation: polyploidy


Hybridization, Organismal

Hybridization as an Active Evolutionary Force

Some authors suggest that hybridization between species can play a more active role in species evolution than previously acknowledged. For example, many species of bacteria benefit from gene exchange with distantly related taxa, often gaining new adaptive traits such as resistance to antibiotics. Likewise, insecticide resistance is thought to be transferred via interspecific gene flow in mosquitoes and blackflies. Similar positive effects of hybridization on fitness has been recorded in Darwin’s finches, Heliconius and Papilio butterflies as well as in other organisms. Introgression can increase variation but new combinations of genes that arise can allow colonization of a new habitat ( Figure 1 (d)) and, eventually, lead to the origin of a new species (e.g., Helianthus sunflowers, Lycaeides and Heliconius butterflies). Hybrid speciation is usually associated with allopolyploidization (i.e., doubling of the chromosome set via fusion of two different genomes). Polyploidy is frequent in plants, whereas in animals it can only be found in some groups with undifferentiated sex chromosomes and/or parthenogenetic reproduction (fish, amphibians).

The origin and evolution of a new species may not be restricted to a unique hybridization event. Rather it can be dependent on a specific form of long-persisting interspecific hybridization called hybridogenesis. For example, the edible frog (Rana esculenta) is formed through crosses between the marsh frog (Rana ridibunda) and the pool frog (Rana lessonae). Whereas both genomes participate in formation of somatic tissues, lessonae chromosomes are eliminated from the germ line in Western European populations (conversely, chromosomes derived from ridibunda are not passed on to gametes in frogs from Eastern Europe). Thus, from the pool frog’s view (in Western populations), its gametes are ‘stolen’ by R. esculenta and so the latter species is sometimes called klepton (from Greek kleptein that means ‘to steal’) and the whole group of the three forms is called synklepton.


Species with reproduction barriers that can both reproduce with a third species - Biology

Every individual alive today, the highest as well as the lowest, is derived in an unbroken line from the first and lowest forms.
- August Frederick Lopold Weismann, German biologist/geneticist (1834-1914)

In this lesson, we wish to ask:

  • What is biological evolution?
  • How are theories of microevolution and macroevolution related?
  • What is a species, and what are the different ways it can be defined?
  • What are the limitations of each definition?
  • How is reproductive isolation important to speciation, and what forms can it take?
  • Why should natural selection reinforce reproductive isolation?
  • Can species be formed in ways other than geographic isolation?

Evolution and Its Many Forms

The word "evolution" does not apply exclusively to biological evolution. The universe and our solar system have developed out of the explosion of matter that began our known universe. Chemical elements have evolved from simpler matter. Life has evolved from non-life, and complex organisms from simpler forms. Languages, religions, and political systems all evolve. Hence, evolution is an appropriate theme for a course on global change.

The core aspects of evolution are "change" and the role of history, in that past events have an influence over what changes occur subsequently. In biological evolution this might mean that complex organisms arise out of simpler ancestors - though be aware that this is an over-simplification not acceptable to a more advanced discussion of evolution.

A full discussion of evolution requires a detailed explanation of genetics, because science has given us a good understanding of the genetic basis of evolution. It also requires an investigation of the differences that characterize species, genera, indeed the entire tree of life, because these are the phenomena that the theory of evolution seeks to explain.

We will begin with observed patterns of similarities and differences among species, because this is what Darwin knew about. The genetic basis for evolution only began to be integrated into evolutionary theory in the 1930's and 1940's. We will add genetics into our understanding of evolution through a discussion activity.

Definitions of Biological Evolution

  • Definition 1:
    Changes in the genetic composition of a population with the passage of each generation
  • Definition 2:
    The gradual change of living things from one form into another over the course of time, the origin of species and lineages by descent of living forms from ancestral forms, and the generation of diversity

A full explanation of evolution requires that we link these two levels. Can small, gradual change produce distinct species? How does it occur, and how do we decide when species are species? Hopefully you will see the connections by the end of these three lectures.

Today we will discuss how species are formed. But to do this, we need to define what we are talking about.

What is a Species?

  • Morphological species concept: Oak trees look like oak trees, tigers look like tigers. Morphology refers to the form and structure of an organism or any of its parts. The morphological species concept supports the widely held view that "members of a species are individuals that look similar to one another." This school of thought was the basis for Linneaus' original classification, which is still broadly accepted and applicable today.

Mimicry complexes supplied further evidence against the concept, as organisms of the same species can look very different, depending upon where they are reared or their life cycle stage (some insects produce a spring brood that looks like one host plant and a summer brood that looks like another).

  • Biological species concept: This concept states that "a species is a group of actually or potentially interbreeding individuals who are reproductively isolated from other such groups."

This concept also emphasized that a species is an evolutionary unit. Members share genes with other members of their species, and not with members of other species.

Although this definition clearly is attractive, it has problems. Can you test it on museum specimens or fossil data? Can it explain the existence of species in a line of descent, such as the well-known lineage of fossil horses? Obviously not.

In fact, one cannot apply this definition easily, or at all, with many living organisms. What if species do not live in the same place? What about the hybrids that we know occur in zoos? These problems are serious enough that some biologists recently argued for a return to the morphological species concept.

So what is the best way to define a species?

Most scientists feel that the biological species concept should be kept, but with some qualifications. It can only be used with living species, and cannot always be applied to species that do not live in the same place. The real test applies to species that have the potential to interbreed.

Most importantly, the biological species concept helps us ask how species are formed, because it focuses our attention on the question of how reproductive isolation comes about. Let us first examine types of reproductive isolation, because there are quite a few.

Types of Reproductive Isolation

This suggests a simple and useful dichotomy, between pre-mating or prezygotic (i.e., pre-zygote formation) reproductive isolating mechanisms, and post-mating or postzygotic isolating mechanisms. Remember that a zygote is the cell formed by the union of two gametes and is the basis of a developing individual.

Prezygotic isolating mechanisms

  1. Ecological isolation: Species occupy different habitats. The lion and tiger overlapped in India until 150 years ago, but the lion lived in open grassland and the tiger in forest. Consequently, the two species did not hybridize in nature (although they sometimes do in zoos).
  2. Temporal isolation: Species breed at different times. In North America, five frog species of the genus Rana differ in the time of their peak breeding activity.
  3. Behavioral isolation: Species engage in distinct courtship and mating rituals (see Figure 1).
  4. Mechanical isolation: Interbreeding is prevented by structural or molecular blockage of the formation of the zygote. Mechanisms include the inability of the sperm to bind to the egg in animals, or the female reproductive organ of a plant preventing the wrong pollinator from landing.
  1. Hybrid inviability. Development of the zygote proceeds abnormally and the hybrid is aborted. (For instance, the hybrid egg formed from the mating of a sheep and a goat will die early in development.)
  2. Hybrid sterility. The hybrid is healthy but sterile. (The mule, the hybrid offspring of a donkey and a mare, is sterile it is unable to produce viable gametes because the chromosomes inherited from its parents do not pair and cross over correctly during meiosis (cell division in which two sets of chromosomes of the parent cell are reduced to a single set in the products, termed gametes - see Figure).
  3. Hybrid is healthy and fertile, but less fit, or infertility appears in later generations (as witnessed in laboratory crosses of fruit flies, where the offspring of second-generation hybrids are weak and usually cannot produce viable offspring).

** Post-zygotic mechanisms are those in which hybrid zygotes fail, develop abnormally, or cannot self-reproduce and establish viable populations in nature. **

So species remain distinct due to reproductive isolation. But how do species form in the first place?

An abbreviated illustration of meiosis, by which reproductive cells duplicate to form gametes.

Species Formation

This question is critical, because it is what produces many species from few, and results in evolutionary trees of relatedness. The most common way for species to split, especially in animal species (we will talk more about the origin of new plant species later), is when the population becomes geographically isolated into two populations. This is referred to as allopatric (geographic) speciation (see Figure).

One model of allopatric speciation. A single population (a) is fragmented by a barrier (b) geographical isolation leads to genetic divergence (c) when the barrier is removed, the two populations come back into contact with each other, and there is selection for increased reproductive isolation (d) if reproductive isolation is effective, speciation is complete (e).
  1. Different geographic regions are likely to have different selective pressures. Temperature, rainfall, predators and competitors are likely to differ between two areas 100's or 1,000's of kilometers apart. Thus, over time, the two populations will differentiate.
  2. Even if the environments are not very different, the populations may differentiate because different mutations and genetic combinations occur by chance in each. Thus, selection will have different raw material to act upon in each population.

Differentiation also depends upon the strength of selective pressures. Strong selection can cause rapid change.

Given time and selection, the two populations become two species. They may, at some later time, spread back into contact. Then we can ask, are these two "good biological species"?

The real test of the biological species concept is when two populations, on the threshold of becoming two species, come back into contact. They may simply merge. They may be so different that they do not even recognize one another as species.

Often, though, species may come into contact when not yet fully reproductively isolated. In that event, natural selection should reinforce the reproductive barriers. Why? Because individuals that waste their reproductive effort -- their gametes -- on individuals with whom they will produce inferior offspring are less likely to pass on their genes to the next generation.

Natural selection should reinforce reproductive isolation. Probably, species that are isolated only by post-zygotic barriers will subsequently evolve pre-zygotic barriers. Why should that occur?

To review: allopatric (geographic) speciation is the differentiation of physically isolated populations to the point that reunion of the two populations does not occur if contact is re- established.

Speciation as a Gradual Process

If speciation is a gradual process, species may not yet be fully separated. A continuum must exist from species that are in the process of splitting into two, to species that are fully formed. Surely we only expect the latter to behave as "good species."

We still haven't fully explained the speciation process. In our next lesson, we will examine the theory of natural selection, which helps to explain how localized populations become adapted to local conditions. By adapting to local conditions and accumulating genetic differences, isolated geographic races start down the path to becoming separate species and creating another pair of branches on the tree of life.

But now I want to point out that there are alternative models of species formation, and finally I want to conclude by linking the concept of species formation to the hierarchical structure of life.

Alternative Models of Species Formation -- Hybridization and Polyploidy

Even if hybrids are unable to undergo sexual reproduction because their chromosomes do not sort out properly in meiosis, they may reproduce vegetatively. The total chromosome number also may double by combining the chromosome sets of a single species.

Of the 260,000 known species of plants, as many as half may have originated in this way. Many commercially important plants are examples of polyploidy (e.g. bread wheat, cotton, tobacco, sugar cane, bananas, potatoes). Polyploidy is an example of sympatric speciation defined as species arising within the same, overlapping geographic range.

Conclusion: Species Formation and the Hierarchy of Life

There are two ways to construct a phylogenetic tree (see Figure). We can use a "perfect" fossil record to trace the sequence from beginning to end, or we can use similarities and differences among living things to reconstruct history, working from the endpoint toward the beginning.

In this course, we will not consider these two methods in detail. I introduce them to make the point that, ultimately, we want to understand how evolution produces not just two species from one but the entire tree of life. This requires that we make the transition from microevolution to macroevolution. To Darwin, and to modern evolutionary biologists as well, the answer simply is time. Given enough time and successive splittings, the processes that produce two species from one will result in the entire diversity of life.

In reality, deducing the historic record of branching is very difficult. Data are incomplete, scientists debate the pace of change, and sometimes species separated by many branching steps look more similar to one another than those separated by one or a few branches. Molecular biology offers exciting new opportunities to address these issues, by looking at similarities and differences in DNA sequences.

From here we will turn away from the macroevolutionary view and look more closely at how small changes occur and accumulate, by the processes of natural selection and genetic change.

Summary

The definition of a species is debatable. Most scientists adhere either to the morphological species concept (members of a species look alike and can be distinguished from other species by their appearance), or to the biological species concept (a species is a group of actually or potentially interbreeding individuals who are reproductively isolated from other such groups). Both definitions have their weaknesses.

Reproductive isolating mechanisms are either prezygotic or postzygotic. These mechanisms ensure that species remain distinct in nature.

Species formation can occur either through allopatric (geographic) speciation or through sympatric speciation.

We can construct phylogenetic trees that show the evolutionary relatedness among living things, though the building of such trees is as yet an imperfect science.


The mating game: ligers, zorses, wholphins, and other hybrid animals raise a beastly science question: what is a species?

What has a mane like a lion, the sleek muscular body of a tiger, stripes and spots, and weighs up to 1,000 pounds? Answer: A liger. The punch line sounds like a joke, but ligers--produced by a female tiger mating with a male lion--are actual animals and one of the world's more bizarre-looking hybrids, or mixed animal species.

If these ferocious cats met in the jungle, a tiger would probably not choose to visit a pride of lions a raucous brawl--not romance--would be the more likely result. But with little choice in captivity--like an open zoo--the odd coupling may occur. In the wild, animals rarely interbreed for one potent reason: The offspring are usually infertile, or unable to reproduce--which can spell extinction for a species. "Infertile offspring don't pass on their genes [hereditary instructions in all cells] to the next generation," says University of Maine biologist Judith Rhymerat.

But even more threatening to species preservation are hybrids that can reproduce. For example, over the past decade Midwestern barred owls have pushed westward to the Pacific coast where they've settled in the forest habitat of endangered spotted owls--and bred with them to create sparred owls. "It's a nasty situation," says Susan Haig, a wildlife ecologist at Oregon State University.

Hybrids can result in loss of genetic diversity, she explains, and there's no protection for them under the 1973 Endangered Species Act. By traditional species' definition--in which organisms with common traits breed to create fertile offspring--they shouldn't be mating: Sparred owls could trigger the Northern spotted owl's extinction.

While ligers are rare, some animals in captivity are deliberately interbred for greater strength or endurance, like mules (horse + donkey) and zorses (horse + zebra). They're also interbred for food, like the beefalo (cow + buffalo) and different types of catfish and trout. Russians crossbreed dogs with jackals to create a hybrid whose superior sense of smell, for example, is put to the test sniffing out bombs in Moscow's airports.

But why don't distinct wild animal species--like lizards and frogs, or cougars and elephants--mate of their own accord? The answer: Nature imposes breeding barriers, safeguards to protect individual species and help them adapt to their environment. Animals evolve, or develop unique traits over time, to ensure their survival. So specific genes that help a species adapt to a particular climate, eat what's on the local menu, and fight off neighborhood predators, are passed on to the next generation. Mixing genes through interbreeding can eliminate survival traits--or result in infertile offspring.

To produce fertile offspring, scientists think chromosomes (cell structures that house all the genes) from both a mother and father may need to pair off evenly during meiosis, a process of cell division that produces sex cells. For the hardy mule, for example, this is impossible, since its father--a donkey--has 62 chromosomes and its mother--a horse--has 64. When the two animals mate, each contributes half its chromosomes to the mule. In turn, the mule is almost always sterile because it inherits a total of 63 chromosomes, a number that can't divide into pairs (see diagram, below).

Sometimes the main breeding obstacle is a simple difference in habitat or breeding area--one species may fare better in thick jungles, another in wide-open spaces. And even if separate species do mate--and a female's egg successfully fertilizes, or fuses, with a male's sperm--the parental genes must partner perfectly to develop a healthy embryo (living organism in its earliest stages of development). "Genes need to turn on and off at the right time, in the right places--millions of times--in order to form limbs and other body parts," notes Eric Hallerman, a geneticist at Virginia Polytechnic Institute and State University. "If they don't, the embryo dies or becomes grossly malformed--and then dies." The off-and-on gene sequence isn't the same in all species, because different species possess different genes--which means they don't coordinate properly.

Besides infertility, blindness, faulty hearts, and brief life spans are routine disorders for many hybrids. Case in point: When a 400-pound Atlantic bottlenose dolphin and a 4,000-pound false killer whale mated off the coast of Hawaii, their wholphin offspring died at age 5, decades younger than the average 40- to 50-year life span of its parents.

Many of today's newly created creatures would confuse 18th-century Swedish naturalist Carolus Linnaeus, who developed the Linnaean taxonomic, or classification, system for the natural world. Within this system, taxonomists have identified and grouped about 2 million plant and animal species based on similarities and differences. But how exactly do you define a species? "That's one of the biggest questions in science," says Rhymer. "It's what everyone is arguing about."

Traditionally, a species is a group of organisms that share at least one unique characteristic, can interbreed to produce fertile offspring, and rarely reproduce with organisms of another species. But what to make of fertile hybrids like the sparred owl? "The old definition of a species doesn't really work today," Rhymer says. "We know of related species separated by millions of years that still have the ability to reproduce successfully."

One such example: the canid family--wolves, coyotes and dogs--whose common forebear is the fox-size Eucyon that roamed prehistoric Earth around 4 million years ago. From the carnivorous Eucyon arose three distinct species of various body sizes and shapes--with different hunting and feeding habits. And unlike most related but distinct species, such as the horse and donkey, the canines share enough genetic similarities to produce healthy, fertile pups.

Interbreeding doesn't always spell doom. When Florida's panther population plummeted to fewer than 30 during the 1980s, the animals began inbreeding, mating among direct relatives who share remarkably similar gene sets. Inbreeding, which greatly increases the odds of birth defects, spawned cubs with crooked tails, heart defects, and other medical problems. In other words, it made the panther population dangerously unfit for survival. To widen the gene pool--the total collection of genes in a species--the U.S. Fish and Wildlife Service (USFWS) brought in a closely related subspecies, the Texas cougar.

Today, panthers' numbers have shot up to at least 78, and females are birthing healthy, fertile hybrid cubs. Still, Rhymer calls the hybridization effort a last desperate attempt to save some fraction of the panther gene pool. "The USFWS could either have hybridized the Florida panther or let it go extinct."

Hybridization can be a natural evolutionary process, explains Nina Fascione of the Defenders of Wildlife organization. "But problems arise when it's human-caused," she says. Leveling forests forces organisms to search out new homes and breaks down natural barriers, allowing animals to encroach on each other's habitats, as in the case of the spotted owl.

For now, the USFWS is still wrestling over a federal policy on the status of hybrid species--especially those that threaten endangered species. "As habitats become more fragmented, we're going to find more and more examples of hybrids, and it's going to be a prime problem for conservationists," warns ecologist Sue Haig.

INTERSPECIES MATING: DOES IT ADD UP?

Breeders mate horses and donkeys to get mules. The hybrids are stronger than their parents but unable to reproduce. Here's one theory why.

1 To create a mule, the female horse donates half of her 64 chromosomes. The male donkey donates half of his 62 chromosomes.

2 At the start of meiosis, a cellular division process that creates sex cells, chromosomes align and pair off.

3 Normally, meiosis produces four genetically unique daughter cells. But the mule has an odd number of chromosomes that cannot properly divide.

REASONS WHY DIFFERENT SPECIES RARELY MATE

GENETIC Different species have different genes chromosomes must align during meiosis.

BEHAVIORAL Species may not understand each other's mating and "courting" language.

PHYSICAL Reproductive organs may not be compatible.

CHEMICAL Unique hormones (chemical messengers) that help sperm (male sex cells) penetrate an egg (female sex cell) vary between species.

IMMUNOLOGICAL The immune system recognize sperm from another species as foreign and kill it.

With more than 2 million known plant and animal species-and an estimated 10 million still nameless--it's no surprise scientists can disagree on how an organism should be classified. One example is the red wolf (Canis rufus). By traditional definition, a species does not interbreed with other species, yet the red wolf can breed and produce healthy offspring with both the gray wolf (Canis lupus) and the coyote (Canus latran).

Some scientists say the red wolf is a unique species because the size and structure of its head is significantly different from that of the gray wolf and coyote. Other scientists conclude that the red wolf is a hybrid species, based on genetic data that show its DNA to be remarkably similar to the gray wolf and coyote.

DEBATE IT: Is the red wolf a separate--or hybrid--species? Support your answer with scientific evidence.

* In sub-Saharan Africa, several African clawed frog species have interbred so successfully that at least seven new species with distinct genetic makeup have been created. The original species had 36 chromosomes five new species have 72 and two more species have 108.

* "Hybrid vigor" makes some hybrids stronger or better adapted to their surroundings than their parents. Black South American fire ants came to the U.S. in 1918 aboard a ship, and mated with red imported fire ants in the South. The hybrid fire ants are hardy enough that they live longer in cold climates than either parent.

* Natural selection plays a role in the origin of species: If two populations are separated by mountains or an ocean, for example, they eventually become so distinct that they can no longer interbreed to produce fertile offspring.

* The most interbreeding in the wild occurs between fish species.

Art: Combine physical traits from different animals to create your own super-species. Submit a drawing and an essay explaining your creation to enter our contest. See page 23 and TE7 for details.

In the article, biologist Judith Rhymer is quoted as saying: "The USFWS could either have hybridized the Florida panther or let it go extinct." Turn this statement into a classroom debate.

Directions: Answer the following in complete sentences.

1. Define hybrid species. Include one example.

2. Why did scientists choose the Texas cougar to breed with the Florida panther? And why is creating hybrid cubs beneficial to the Florida panther population?

3. Give at least two reasons why different species rarely mate.

4. Why are most hybrid species infertile?

1. A hybrid is a mixed animal species. For example, a mule, which results from breeding a horse with a donkey, is a hybrid.

2. Scientists bred the Texas cougar with the Florida panther because it is a closely related subspecies. The Florida panther is dangerously close to extinction, and in the 1980s the animals began inbreeding. Inbreeding increases birth defects, which made the population unfit for survival. In a last-ditch effort to save the Florida panther, scientists brought in the Texas cougars to widen the gene pool. Hybrid cubs save some fraction of the panther gene pool.

3. Some reasons why different species rarely mate: They have different genes, may not understand each other's mating behavior, or have incompatible reproductive organs. Also, hormones that help a sperm fertilize an egg may vary, and the female's immune system may read the sperm as foreign material and kill it.

4. To produce fertile offspring, ,chromosomes from both parents must pair off evenly during meiosis, a process of cell division that produces sex cells. Scientists believe that for most hybrid species, its parents may each have a different number of chromosomes. When the two animals mate, each contributes half its chromosomes to the offspring. And an odd number of chromosomes can't pair off properly to create sex cells.

To learn more about species preservation, check out: Defenders of Wildlife online at www.defenders.org

The endangered species program of the U.S. Fish and Wildlife Service online at endangered.fws.gov

"Ligers and Tigons and Zorses . oh my!" by Beth Daley, The Boston Globe, May 29, 2001


Difference Between Geographical and Reproductive Isolation

Definition

Geographical Isolation: Geographical isolation is the separation of two populations by a physical barrier.

Reproductive Isolation: Reproductive isolation is the separation of two populations in such a way they cannot interbreed and produce a fertile offspring due to environmental, mechanical, physiological or behavioral barriers.

Cause

Geographical Isolation: The geographical isolation is caused by the geographical barriers.

Reproductive Isolation: The reproductive isolation is caused by the behavioral barriers, temporal barriers, and the geographical barriers.

Significance

Geographical Isolation: The geographical isolation is a form of reproductive isolation.

Reproductive Isolation: The reproductive isolation is the major cause of the speciation.

Examples

Geographical Isolation: The Darwin’s finches are examples of geographical isolation.

Reproductive Isolation: The temporal isolation of red-legged and yellow-legged frogs, mechanical isolation of snails with right-coiling shells and snails with left-coiling shells are examples of reproductive isolation.

Conclusion

Geographical and reproductive isolation are two types of mechanisms that lead to speciation. Geographical isolation leads to allopatric speciation through adaptive radiation. The reproductive isolation occurs due to mechanical, ecological, temporal or behavioral isolation of populations. The geographical isolation is a form of reproductive isolation. The main difference between geographical and reproductive isolation is their extent of the contribution to speciation.

Reference:

1.“Allopatric speciation: the great divide.” Understanding Evolution, Available here. Accessed 29 Sept. 2017.
2.“Reproductive Isolating Mechanisms.” Reproductive Isolation, Available here. Accessed 29 Sept. 2017.

Image Courtesy:

1. “Topography-driven Isolation PEAK&VALLEY (allopatric speciation)” By Andrew Z. Colvin – Own work (CC BY-SA 4.0) via Commons Wikimedia

2. “Reproductive Isolation (Process diagram)” By Andrew Z. Colvin – Own work (CC BY-SA 4.0) via Commons Wikimedia

About the Author: Lakna

Lakna, a graduate in Molecular Biology & Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things