Write An Essay To Briefly Explain How Geographic Isolation

Write An Essay To Briefly Explain How Geographic Isolation

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Speciation

Speciation Questions

Bring on the tough stuff

1) Is the following statement true or false? Explain why.

According to the BSC, “true” species cannot produce hybrid offspring that are viable and fertile in the laboratory.

2) Imagine a series of closely related but morphologically distinct species whose geographic ranges are represented in the map below, with each occupying a distinct ecological niche around the perimeter of a large lake. The patterns of hybridization among these species suggest that they are ring species, such that Species 1 can hybridize with 2, 2 with 3, 3 with 4, and 4 with 5, but species 1 and 5 cannot hybridize. All hybrids are viable and fertile. According to each of the following species concepts—Biological, Ecological, and Phenetic—are these populations 5 distinct species or one single species?

3) Provide and explain two reasons why creating a universal species definition is difficult.

4) Briefly explain the difference between hybridization and hybrid speciation.

5) Would you expect to see greater intrinsic or extrinsic postzygotic isolation between two species that recently arose through sympatric speciation?

6) Briefly describe allopatric and sympatric speciation, and provide a real-world example of each.

7) What does it mean to be sister species? Identify the sister species from the tree below.

8) Describe the dominant evolutionary pattern according to the punctuated equilibrium view of evolution.

9) Define secondary contact and describe the mechanism thought to maintain distinct species identities when secondary contact occurs.

10) Identify whether assortative mating is a type of pre- or post-zygotic reproduction isolation and describe its importance in sympatric and allopatric speciation.

Possible Answers

1) Is the following statement true or false? Explain why.

According to the BSC, “true” species cannot produce hybrid offspring that are viable and fertile in the laboratory.

False. The BSC claims that “true” species cannot produce hybrid offspring under natural conditions; it makes no claims for the weird things that can take place in a laboratory or with biotechnology. The BSC’s criterion for distinguishing between two species is that there must be some barrier that prevents gene flow between species. Under natural conditions, the reproductive barrier usually involves geographic separation (allopatry), though there are other isolating mechanisms that can be important, too. If two species that are reproductively isolated (that is, they share no gene flow) are brought together in a lab to hybridize, this does not violate the species definition of the BSC.

2) Imagine a series of closely related but morphologically distinct species whose geographic ranges are represented in the map below, with each occupying a distinct ecological niche around the perimeter of a large lake. The patterns of hybridization among these species suggest that they are ring species, such that Species 1 can hybridize with 2, 2 with 3, 3 with 4, and 4 with 5, but species 1 and 5 cannot hybridize. All hybrids are viable and fertile. According to each of the following species concepts—Biological, Ecological, and Phenetic—are these populations 5 distinct species or one single species?

The Biological Species Concept: They are one single species. Even though species 1 and 5 cannot hybridize, they can share genes via hybridization with intermediate species, therefore there is no reproductive isolation between them.

The Ecological Species Concept: They are five distinct species. The ESC makes no issue of hybridization, as species are distinguished according to their ecological niches. Since the 5 species all occupy distinct niches, they are distinct species according to the ESC.

The Phenetic Species Concept: Could go either way, depending on which morphological characters were used to distinguish between species. Assuming the morphological traits used to characterize species are distinct, then they are five distinct species according to the phenetic species concept.

3) Provide and explain two reasons why creating a universal species definition is difficult.

Asexual reproduction: scientists often rely on reproduction and gene flow to define species, but there are many organisms that do not reproduce sexually, and therefore traditional species concepts do not apply to them.

Misleading morphology: It is often unclear in nature which organisms are capable of mating with which others. It can be that morphologically distinct individuals—what we expect to be distinct species—are actually one species, or it can be that cryptic reproductive isolation prevents gene flow among morphologically similar organisms that we would assume are the same species. Ultimately, morphology can be misleading.

Hybridization: Many species have low levels of hybridization with other species. Even if hybrids have reduced fitness compared to parent species, as long as they are fertile it allows low levels of gene flow between what otherwise would be considered separate species.

Horizontal Gene Transfer: There is considerable evidence of shared genes between unrelated groups of organisms as a result of gene transfer in the absence of reproduction. While this is most common in prokaryotes (like bacteria), there are known cases in eukaryotes (like plants, animals, and fungi), as well.

4) Briefly explain the difference between hybridization and hybrid speciation.

Hybridization occurs when two parent species mate to form a hybrid offspring. If the hybrid offspring is inviable or infertile then the designation of parents as separate species is sound according to the BSC. However, the hybrid is an evolutionary dead end, so the end result is two species—the original parents.

In hybrid speciation, the hybrids are perfectly fit and they can mate with other hybrids to make fit offspring, but usually some mechanism isolates hybrids from backcrossing with parent species. The end result is 3 distinct species: the original parent species and the new hybrid species.

5) Would you expect to see greater intrinsic or extrinsic postzygotic isolation between two species that recently arose through sympatric speciation?

Extrinsic isolation. Because the species arose in sympatry, it is unlikely that intrinsic barriers are acting to obstruct gene flow. It is more common that niche partitioning and assortative mating prevent gene flow between sympatric species, which are both examples of extrinsic reproductive isolation.

6) Briefly describe allopatric and sympatric speciation, and provide a real-world example of each.

Allopatric speciation occurs when populations of a single species become reproductively isolated from one another by a geographic barrier. A classic example is the snapping shrimp that live around the Isthmus of Panama. When the isthmus closed several million years ago, it created a geographic barrier between a previously continuous species of shrimp, leading to the formation of distinct shrimp species on each side of the land bridge.

Sympatric speciation occurs when two populations that have overlapping geographical ranges become reproductively isolated. For example, the cichlid fishes in African rift lakes are believed to have sympatrically speciated. They took advantage of different niches and developing strong pre-zygotic isolating barriers between species.

7) What does it mean to be sister species? Identify the sister species from the tree below.

Sister species are two species that share a most recent common ancestor. They are the result of the most recent speciation event in their lineage. In the tree above, species A and C are sister species.

8) Describe the dominant evolutionary pattern according to the punctuated equilibrium view of evolution.

The dominant evolutionary pattern, as observed in the fossil record, is that there are long periods of relative evolutionary stasis, with intermittent short periods of relatively fast evolutionary change and speciation.

9) Define secondary contact and describe the mechanism thought to maintain distinct species identities when secondary contact occurs.

Secondary contact occurs when two species that arose via allopatric speciation are brought back into contact with each other.

If sufficient time has passed and evolutionary divergence has taken place during allopatry such that hybrids have reduced fitness compared to their parent species, reinforcement may occur upon secondary contact. This will maintain the distinction between the two species.

10) Identify whether assortative mating is a type of pre- or post-zygotic reproduction isolation and describe its importance in sympatric and allopatric speciation.

Assortative mating is a mechanism of prezygotic isolation, because it affects which individuals will mate with one another.

It plays a large role in sympatric speciation by maintaining reproductive isolation among species whose ranges overlap. In allopatry, however, where geography provides an adequate barrier to gene flow, assortative mating is not necessary at all for the maintenance of reproductive isolation.

People who Shmooped this also Shmooped.

Speciation and the Fossil Record

Species vs. Time
In this example, a snail species, A, is isolated by a geographic barrier to form subspecies X and Y. Eventually, this geographic separation leads to genetic isolation, and two distinct new species, B and C, form.
© AMNH

Is evolutionary change mostly dependent upon speciation, which occurs when one or more descendant species split from an ancestral one? Do most anatomical changes occur during these events? Or can species evolve as single lineages? And how long does it take the process of speciation to work in geological time?

Conventional wisdom: evolution is gradual and constant

The traditional view in evolutionary biology, which stems largely from Darwin’s On the Origin of Species. is that adaptive change, or genetically based anatomical change, will tend to accumulate more or less regularly and gradually throughout a species’ history. This assumption underlies the concept of the “molecular clock”: that if genetic change on the whole accumulates at an steady rate, genetic “distance,” or the amount of genetic difference between two species, should be directly proportional to the amount of time the two have been phylogenetically separate. According to this view, even when lineages divide, the slight differences between the newly separate species will tend to grow steadily and gradually increase over time [Figure 1] .

Fossils tell a different story

The fossil record indicates otherwise. New species tend to appear in the fossil record already morphologically distinct from their closest relatives. And once established, species tend not to change significantly or permanently, which in the case of marine invertebrates can amount to a 5-million-year or even 10-million-year history without change.

The birth of a new theory: speciation and punctuated equilibria

In the late 1960s I was conducting my doctoral dissertation research on the evolution of a Middle Devonian trilobite called Phacops rana. Along with many other species of trilobites, mollusks, brachiopods, corals, and other invertebrates, the abundant Phacops rana inhabited the shallow-water inland seas that ran from the present-day Appalachian mountains as far west as Iowa. Many, including P. rana. were derived from species that migrated into North America from Europe and Africa when those continents collided with North America about 380 million years ago. North America lay astride the Equator in those days, and its shallow tropical seas literally teemed with life that has left a rich and dense fossil record. The fauna persisted for some 6 million to 8 million years.

Phacops rana
A fully articulated trilobite and a well-preserved trilobite head from Dr. Eldredge’s collection.
© AMNH

I had been trained to believe that evolution proceeds gradually, as depicted in Figure 1. Collecting all over the Midwest and up and down the Appalachian mountain chain over several summers, I was dismayed to detect no obvious evolutionary change across 6 million years and thousands of miles. Finally, I stumbled on some changes: very slight but persistent, like the consistent differences Darwin saw in the mockingbirds on different islands in the Galapagos. Back in the laboratory at the American Museum of Natural History, where I was privileged to work as a Columbia University graduate student, I carefully removed the rocky matrix covering my specimens and began to measure and examine the trilobites.

Having learned from another paleontologist that the lenses of the eyes of these kinds of trilobites are arranged into vertical columns holding anywhere from one to more than 15 large, bulging lenses, I counted the lenses on each specimen. One day I suddenly realized that samples from different localities—different places in both time and space—differed mainly in the number of those vertical columns of lenses. Babies would add columns, then stop at the “mature” number: Some stopped at 18; many stopped at 17; and a few stopped at 15.

Then I had an another idea: I simply plotted these numbers on a sequence of maps of the United States, moving from the earliest samples to progressively younger ones. I was delighted to see that a clear pattern emerged, one that reflected NOT gradual change through time, but geographic speciation. Like their European/African ancestors, the earliest samples, from the eastern part of the range, had 18 columns of lenses in the eye.

Interestingly, an early sample from upstate New York seemed to vary; some had 18 and some 17. With one exception, all samples up and down the Appalachians had 17, while all the samples from the shallow limey seas that once covered Ohio, Michigan, and Iowa still had the ancestral 18 columns. It was beginning to look as if “Phacops rana ” included several species, closely similar yet distinct.

Evolution of the Trilobite
This sequence shows how the continental seas of North America waxed and waned over a seven million year interval during the Devonian Period (400 million years ago). The white circles indicate some of the more important localities where the trilobite, Phacops rana. were found by Dr. Eldredge.
© AMNH

Two million years later, those Midwestern shallow seas dried up. When marine conditions were once again restored, the 18-column species of Phacops was gone. (Presumably a victim of extinction, they never reappeared in the fossil record.) The 17-column species took its place, spreading westward from the Appalachian area where the seas had NOT dried up. A similar event happened some 2 million years later when a 15-column species arose, again probably in the eastern part of the range. Then it, too, spread westward, and the 17-column form became extinct.

“Aha!” I said to myself: This is a case of allopatric speciation. Lineages are splitting into descendant species, with the ancestral species persisting alongside the new, daughter species. And though it is difficult to measure small increments of time in the fossil record, my data seemed to suggest that speciation could be quite rapid—taking, perhaps, anywhere from five to 50,000 years. Later, the daughter species spread, and in the case of Phacops, at least, eventually replaced the ancestral species. But what remained surprising were the long periods of time with little or no change at all. The 17-column species, for example, seemed to have lasted unchanged for nearly 5 million years!

I wrote these results up in a paper in 1971 called “The allopatric model and phylogeny in Paleozoic invertebrates,” concluding that speciation has played a critically important role in evolution since the dawn of time. It was beginning to look like evolution is mostly correlated with speciation events.

But what about this unexpected stability? In the following year (1972) I published a longer paper with Stephen Jay Gould, with whom I had worked in graduate school. (Steve was two years ahead of me and was already teaching at Harvard.) We dubbed the phenomenon of non-change “stasis ” and called the entire pattern of stasis + speciation “punctuated equilibria” [Figure 2 ].

Historical connections: the importance of geology in evolution

Gould and I were not the first to point out that patterns of stasis and change in the fossil record disagree with the standard image of evolution as slow, steady, gradual change. Darwin himself, in his earliest notebooks, thought that speciation must come in sudden “jumps.” These early “saltational” views were based in part on his experiences collecting fossils, but also on his observations of recent species. Noting, for example, that the two species of ostrich-like rheas in South America meet only in one place and do not seem to interbreed, Darwin initially imagined that the “lesser rhea” probably was derived from the larger “common rhea” in one swift evolutionary jump. Only later in his life, based on his experiences with variation and his vision of how natural selection works in the natural world, did Darwin reject saltationism for the “phyletic gradualism” [Figure 1 ] that has come down to us as the standard image of evolution at work.

A number of biologists after Darwin thought that geography and isolation underlay speciation, and indeed the entire evolutionary process. The 19th-century German biologist G.J. Romanes, for example, felt that without isolation, evolution would not be possible. Another German biologist, Moritz Wagner, who corresponded at length with Charles Darwin, independently derived the notion of punctuated equilibria in the 1870s, apparently on purely theoretical grounds. Wagner saw evolution as occurring mainly in small populations isolated geographically from the large, well-established populations of the parental species. Such isolated populations, he felt, would undergo rapid evolution; if the fledgling species survived, it might grow in numbers and expand its range—in which case, Wagner surmised, further evolution of that species would grind to a halt?

Yet the importance of geographic variation, isolation, and the emergence of new species was a largely muted, secondary theme in evolutionary biology until the 1930s, when the geneticist Theodosius Dobzhansky and the systematist Ernst Mayr propounded the “biological species concept” and re-established the importance of geographic, or allopatric. speciation in the evolutionary process. But even their concept differed little from the gradual divergence model seen in Figure 1.

Explaining stasis: the biological evidence

Perceived as radically anti-Darwinian, stasis was the most controversial component of punctuated equilibria. Nevertheless, over the 35 years or so since the original publications on punctuated equilibria, stasis has been increasingly accepted as a real, unpredicted phenomenon that must be explained by evolutionary biologists. Species may exhibit lots of molecular variation, yet their morphology is consistently found to vary only within certain limits and to remain essentially stable for millions of years. How could this be?

In the late 1990s, biologist John N. Thompson (see Week 3, Essay 1) and I convened a team of geneticists, ecologists, and paleontologists to study the problem of stasis from an interdisciplinary perspective. We concluded that the most likely cause of stasis is the simple fact that most species are distributed as local populations inhabiting a variety of physical and biotic environments. Especially in species with large-scale distributions (e.g. over a half continent or more), local populations have to contend with different environments (predators, food resources, temperature, and rainfall parameters, etc.). Since species are not homogenous mega-populations, it is statistically unlikely that natural selection will systematically modify an entire species in any one direction, especially over long periods of geological time. Stasis should have arisen as a simple prediction from population genetics as long ago as the 1930s, but its apparent anti-Darwinian nature prevented that from happening.

Despite the fact that the history of life records a single evolutionary process, findings from the fossil record are sometimes difficult to reconcile with genetic evidence. Recently, geneticist Mark Pagel and colleagues found that, on average, 22 percent of the DNA change in animals, plants and fungi can be attributed to punctuational evolution while the rest accumulates gradually. Their results call into question the “molecular clock” assumption of constant, gradual change through time. They also found that there is little evidence for stasis at the molecular level. In contrast, paleontological data suggest that far more than 22 percent of morphological evolutionary change is punctuational, and that stasis is a very common phenomenon. Since molecular data for fossil examples is unavailable, there’s no direct way to compare anatomical and morphological data.

Do genes show punctuated equilibrium at work?

Though the molecular data support the paleontological notion of punctuated equilibria to some degree, the differences in results need to be reconciled. This is the hypothesis about the molecular data that requires further testing: Changes in genes that code for protein products—and thus produce morphology—will be found to be disproportionately concentrated at the splitting (speciation) events, while neutral, non-coding changes not subject to natural selection will simply accumulate in a clocklike fashion through time. If further studies find that to be the case, as I anticipate, not only will most coding DNA changes be focused in splitting events, but they will also be locked into stasis in the interim—evidence of punctuated equilibrium at the molecular level.

The fossil record suggests not only that most evolutionary change occurs in speciation events, but also that speciation events themselves are non-randomly clustered in space and especially in time. In the history of life, speciation events seem to follow episodes of environmental disruption, especially when sufficiently large-scale events drive many pre-existing species to extinction. Even before Darwin’s day, extinction was considered a reality, predominantly caused by physical events. The half-dozen or so truly global mass extinctions have removed entire groups (e.g. terrestrial dinosaurs and marine ammonoids at the end of the Cretaceous), followed by evolutionary bursts of other groups (e.g. mammals and nautiloids in the Tertiary), usually after a lag of several million years.

Gradualism vs. Punctuated Equilibria
Here are two models of speciation: gradualism, where a species slowly changes over time; and punctuated equilibrium, where morphological changes occur relatively rapidly.
© AMNH


Extinctions and evolutionary rebounds: “turnovers” large and small

The same thing happens more regularly on a regional basis: Entire faunas and floras are often found to be locked in stasis, where the individual species show little or no evolutionary change through time. Then an environmental perturbation such as climate change or an asteroid impact disrupts the ecosystems, and if severe enough, drives many component species to extinction at more or less the same time. They are eventually replaced, in part by surviving species from elsewhere—as in the case of the 17-column species of Phacops migrating into vacated habitat in the American Midwest after the ancestral species had disappeared. But sometimes the reconstituted ecosystems are populated by newly evolved species that arose in isolation in the disturbed environment over several hundred thousand years or so. Paleontologist Elisabeth S. Vrba calls these associations of large-scale extinction events of species with subsequent evolutionary bursts “turnover pulses.”

Darwinian theory meets the challenge

Darwin added a note to his unpublished 1844 essay on evolution: “Better begin with this: if species really, after catastrophes, created in showers over world, my theory false.” Aware that geologists were talking about such turnovers, Darwin felt that they threatened his vision of natural selection slowly modifying species through time.

We know now that most morphological evolution occurs relatively rapidly in conjunction with speciation, and that most speciation events are concentrated into turnover events. Yet Darwin’s theory of evolution through natural selection remains essentially sound. All we need do is add concepts of isolation, speciation, and extinction; and understand the conservative action of natural selection producing stasis in stable ecological regimes, to grasp the actual context in which natural selection produces evolutionary change in the history of life.

  • Berkeley: Macroevolution
    A module on macroevolution, including discussions of stasis, character change, extinction, and speciation.
    http://evolution.berkeley.edu/evosite/evo101/VIMacroevolution.shtml
  • NGM: The Big Bloom
    Read an excerpt from an article about how flowering plants changed the world 100 million years ago.
    http://magma.nationalgeographic.com/ngm/0207/feature6/
  • PBS: Mass Extinction
    Watch a video about the Permian Extinction that wiped out 95% of all ocean- and land-dwelling species.
    http://magma.nationalgeographic.com/ngm/0207/feature6/
  • The Process of Speciation

    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)

    From the remotest past which Science can fathom, up to the novelties of yesterday, that in which Progress essentially consists, is the transformation of the homogeneous to the heterogeneous.
    Herbert Spencer, English philosopher/psychologist (1820-1903)

    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

    Today we continue a three-lecture sequence on biological, or organic, evolution. Evolution is a unifying theme of this course, and the concept of evolution is relevant to many of our topics.

    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

    We begin with two working definitions of biological evolution, which capture these two facets of genetics and differences among life forms. Then we will ask what is a species, and how does a species arise?

    • 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

    Note that the first definition emphasizes genetic change. It commonly is referred to as microevolution. The second definition emphasizes the appearance of new, physically distinct life forms that can be grouped with similar appearing life forms in a taxonomic hierarchy. It commonly is referred to as macroevolution.

    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?

    Despite our increasing ability to understand the finest details of organisms, there is still debate about what constitutes a species. Definitions of species tend to fall into two main camps, the morphological and the biological species concepts.

    • 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.

    This concept became criticized by biologists because it was arbitrary. Many examples were found in which individuals of two populations were very hard to tell apart but would not mate with one another, suggesting that they were in fact different species.

    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).

    The morphological species concept was replaced by another viewpoint that puts more emphasis on the biological differences between species.

    • 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 definition was attractive to biologists and became widely adopted by the 1940’s. It suggested a critical test of species-hood: two individuals belong to the same species if their gametes can unite with each other under natural conditions to produce fertile offspring.

    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

    There are many barriers to reproduction. Each species may have its own courtship displays, or breeding season, so that members of the two species do not have the opportunity to interbreed. Or, the two species may be unable to interbreed successfully because of failure of the egg to become fertilized or to develop.

    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.

    ** All of the above prevent the formation of hybrid zygotes. **

    Postzygotic isolating mechanisms

    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

    How do we get cladogenesis — the splitting of one lineage into two?

    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).

    Geographic isolation leads to reproductive isolation. Once two populations are reproductively isolated, they are free to follow different evolutionary paths. They are likely to differentiate for two reasons:

    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.

    In short, physical isolation turns a single population into two, which, because of their lack of connectedness, may follow different evolutionary paths. What happens next? The fate of the populations depends upon time and factors related to their different environments. If the two populations are soon rejoined, they may not differ very much, and likely will become a single population again.

    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

    Our understanding of speciation arising from reproductive isolation and the gradual evolution of reproductive isolating mechanisms should help us to appreciate why the biological species concept, and the test of reproductive isolation, may sometimes fail.

    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

    In plants, new, reproductively isolated species may arise instantaneously, due to multiplication of the entire complement of chromosomes by a process known as polyploidy. This may occur as a result of hybridization, combining the chromosome sets from two parent species in a hybrid individual. If such hybrids turn out to be well adapted to environmental conditions, hybridization is a mechanism that produces new species.

    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

    Speciation results in the splitting of an ancestral species into two (or more) descendent species. This process, continued indefinitely, results in a sequence of speciation events extending over great expanses of time, resulting in a branching tree of historical relatedness. Imagine if we had complete and certain knowledge of such a tree — it would tell us the evolutionary relatedness among living things, the pathways of divergence, even the timing of separation.

    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

    Biological evolution can be defined in two ways: as a result of changes in the genetic composition of a population with the passage of each generation (microevolution), or as a result of the gradual change of living things from one form into another over the course of time, generating species diversity (macroevolution).

    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.

    Suggested Readings:

    • Futuyma, D.J. 1986. Evolutionary Biology. Sunderland, Mass: Sinauer Associates, Inc.
    • Wessells, N.K. and J.L. Hopson. 1988. Biology. New York: Random House. Chapter 43.
    • Rosenzweig, M.L. 1995. Species Diversity in Space and Time. Cambridge: Cambridge University Press.

    Copyright Regents of the University of Michigan unless noted otherwise.