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Thread: c Speciation: The Role of Isolation in Speciation

  1. #1

    Post c Speciation: The Role of Isolation in Speciation


    What Is a Species?

    One of the best definitions is that of the evolutionary biologist Ernst Mayr:
    A species is an actually or potentially interbreeding population that does not interbreed with other such populations when there is opportunity to do so.
    Note: sometimes breeding may take place (as it can between a horse and a donkey) but if so, the offspring are not so fertile and/or well adapted as the parents (the mule produced is sterile).

    Allopatric Speciation: the Role of Isolation in Speciation

    The formation of two or more species often (some workers think always!) requires geographical isolation of subpopulations of the species. Only then can natural selection or perhaps genetic drift produce distinctive gene pools.

    It is no accident that the various races (or "subspecies") of animals almost never occupy the same territory. Their distribution is allopatric ("other country").

    The seven distinct subspecies or races of the yellowthroat Geothlypis trichas in the continental U.S. would soon merge into a single homogeneous population if they occupied the same territory and bred with one another.

    Darwin's Finches
    As a young man of 26, Charles Darwin visited the Galapagos Islands off the coast of Ecuador. Map

    Among the animals he studied were 13 species of finches found nowhere else on earth.

    Some have stout beaks for eating seeds of one size or another.
    Others have beaks adapted for eating insects.
    One (#7) has a beak like a woodpecker's. It uses it to drill holes in wood, but lacking the long tongue of a true woodpecker, it uses a cactus spine held in its beak to dig the insect out.
    One (#12) looks more like a warbler than a finch, but its eggs, nest, and courtship behavior is like that of the other finches.
    Darwin's finches. The finches numbered 1-7 are ground finches. They seek their food on the ground or in low shrubs. Those numbered 8-13 are tree finches. They live primarily on insects.

    1. Large cactus finch (Geospiza conirostris)
    2. Large ground finch (G. magnirostris)
    3. Medium ground finch (Geospiza fortis)
    4. Cactus finch (G. scandens)
    5. Sharp-beaked ground finch (G. difficilis)
    6. Small ground finch (G. fuliginosa)
    7. Woodpecker finch (Cactospiza pallida)
    8. Vegetarian tree finch (Platyspiza crassirostris)
    9. Medium tree finch (Camarhynchus pauper)
    10. Large tree finch (Camarhynchus psittacula)
    11. Small tree finch (C. parvulus)
    12. Warbler finch (Certhidia olivacea)
    13. Mangrove finch (Cactospiza heliobates)

    (From BSCS, Biological Science: Molecules to Man, Houghton Mifflin Co., 1963)
    Since Darwin's time, these birds have provided a case study of how a single species reaching the Galapagos from Central or South America could - over a few million years - give rise to the 13 species that live there today.
    Five factors have been identified that may contribute to speciation.

    1. Ecological opportunity.
    When the ancestor of Darwin's finches reached the Galapagos, it found
    no predators (There were no mammals and few reptiles on the islands.)
    few, if any, competitors. There were only a handful of other types of songbirds. In fact, if true warblers or woodpeckers had been present, their efficiency at exploiting their niches would surely have prevented the evolution of warblerlike and woodpeckerlike finches.
    2. Geographical Isolation (allopatry).
    The proximity of the various islands has permitted enough migration of Darwin's finches between them to enable distinct island populations to arise. But the distances between the islands is great enough to limit interbreeding between populations on different islands. This has made possible the formation of distinctive subspecies (= races) on the various islands.

    The importance of geographical isolation is illuminated by a single, fourteenth, species of Darwin's finches that lives on Cocos Island, some 500 miles to the northeast of the Galapagos.
    The first immigrants there must also have found relaxed selection pressures with few predators or competitors.

    How different the outcome, though. Where one immigrant species gave rise to at least 13 on the scattered Galapagos Islands, no such divergence has occurred on the single, isolated Cocos Island.

    3. Evolutionary Change
    In isolation, changes in the gene pool can occur through some combination of
    natural selection
    genetic drift
    founder effect
    These factors may produce distinct subpopulations on the different islands. So long as they remain separate (allopatric) we consider them races or subspecies. In fact, they might not be able to interbreed with other races but so long as we don't know, we assume that they could.
    How much genetic change is needed to create a new species?

    Perhaps not as much as you might think. For example, changes at one or just a few gene loci might do the trick. For example, a single mutation altering flower color or petal shape could immediately prevent cross-pollination between the new and the parental types (a form of prezygotic isolating mechanism).
    4. Reunion
    The question of their status - subspecies or true species - is resolved if they ever do come to occupy the same territory again (become sympatric). If successful interbreeding occurs, the differences will gradually disappear, and a single population will be formed again. Speciation will not have occurred.
    If, on the other hand, two subspecies reunite but fail to resume breeding, speciation has occurred and they have become separate species.

    An example: The medium tree finch Camarhynchus pauper is found only on Floreana Island. Its close relative, the large tree finch, Camarhynchus psittacula, is found on all the central islands including Floreana.
    Were it not for its presence on Floreana, both forms would be considered subspecies of the same species. Because they do coexist and maintain their separate identity on Floreana, we know that speciation has occurred.

    Isolating Mechanisms

    What might keep two subpopulations from interbreeding when reunited geographically? There are several mechanisms.
    Prezygotic Isolating Mechanisms
    These act before fertilization occurs.
    Failure to elicit mating behavior. On Floreana, C. psittacula has a longer beak than C. pauper, and the Grants have demonstrated that beak size is an important criterion by which Darwin's finches choose their mates.
    Two subpopulations may occupy different habitats in the same area and thus fail to meet at breeding time.
    In plants, a shift in the time of flowering can prevent pollination between the two subpopulations.
    Structural differences in the sex organs may become an isolating mechanism.
    The sperm may fail to reach or fuse with the egg.
    Postzygotic Isolating Mechanisms
    These act even if fertilization does occur.
    Even if a zygote is formed, genetic differences may have become so great that the resulting hybrids are less viable or less fertile than the parental types.

    Although hybridization in angiosperms may also cause reduced fertility, it is sometimes followed by a doubling of the chromosome number. The resulting polyploids are now fully fertile with each other although unable to breed with either parental type. A new species has been created. Link to a discussion of polyploidy and example of its role in speciation.

    5. Intense Competition.
    The process of speciation is often hastened when two formerly isolated groups are reunited. Even if they no longer interbreed, they are probably still similar in many ways, including their requirements for the necessities of life.
    Thus the reuniting of the two groups may create an intense selection pressure leading to one of two possible outcomes.

    The competition may be so intense that one species becomes eliminated entirely; that is, it is driven to extinction on that island. This may have occurred to C. pauper on some of the central islands.
    Alternatively, the increased selection pressure may lead to character displacement and so lessen the competition between them.
    The range of beak sizes of C. pauper on Floreana and C. psittacula on Isabela is roughly the same. This reflects the fact that the two species feed on the same type and size of insect.

    On Floreana, however, character displacement has occurred: C. psittacula has a substantially larger beak than C. pauper does, and thus differs enough in its food requirements for the two species to coexist there.
    The histogram shows the data. The larger beak of C. psittacula on Floreana reduces the competition with C. pauper (which is not found on Isabela). (Redrawn with permission from David Lack, Darwin's Finches, Cambridge Univ. Press, 1947. The number of collected specimens (16) of C. psittacula on Floreana was too small to compute a reliable percentage but all fell within the range indicated.)

    Adaptive Radiation

    The process described above can occur over and over. In the case of Darwin's finches, it must have been repeated a number of times forming new species that gradually divided the available habitats between them. From the first arrival have come a variety of ground-feeding and tree-feeding finches as well as the warblerlike finch and the tool-using woodpeckerlike finch.
    The formation of a number of diverse species from a single ancestral one is called an adaptive radiation.

    House mice on the island of Madeira
    A recent (13 January 2000) report in Nature describes a study of house mouse populations on the island of Madeira off the Northwest coast of Africa. These workers (Janice Britton-Davidian et al) examined the karyotypes of 143 house mice (Mus musculus domesticus) from various locations along the coast of this mountainous island. Their findings:
    There are 6 distinct populations (shown by different colors)
    Each of these has a distinct karyotype, with a diploid number less than the "normal" (2N=40).
    The reduction in chromosome number has occurred through Robertsonian fusions. Mouse chromosomes tend to be acrocentric; that is, the centromere connects one long and one very short arm. Acrocentric chromosomes are at risk of translocations that fuse the long arms of two different chromosomes with the loss of the short arms.
    The different populations are allopatric; isolated in different valleys leading down to the sea.
    The distinct and uniform karyotype found in each population probably arose from genetic drift rather than natural selection.
    The 6 different populations are technically described as races because there is no opportunity for them to attempt interbreeding.
    However, they surely meet the definition of true species. While hybrids would form easily (no prezygotic isolating mechanisms), these would probably be infertile as proper synapsis and segregation of such different chromosomes would be difficult when the hybrids attempted to form gametes by meiosis.
    Sympatric Speciation
    Sympatric speciation refers to the formation of two or more descendant species from a single ancestral species all occupying the same geographic location.
    Some evolutionary biologists don't believe that it ever occurs. They feel that interbreeding would soon eliminate any genetic differences that might appear.

    But there is some compelling (albeit indirect) evidence that sympatric speciation can occur.
    The three-spined sticklebacks, freshwater fishes, that have been studied by Dolph Schluter (who received his Ph.D. for his work on Darwin's finches with Peter Grant) and his current colleagues in British Columbia, provide an intriguing example that is best explained by sympatric speciation.

    They have found:
    Two different species of three-spined sticklebacks in each of five different lakes.
    a large benthic species with a large mouth that feeds on large prey in the littoral zone
    a smaller limnetic species - with a smaller mouth - that feeds on the small plankton in open water.
    DNA analysis indicates that each lake was colonized independently, presumably by a marine ancestor, after the last ice age.
    DNA analysis also shows that the two species in each lake are more closely related to each other than they are to any of the species in the other lakes.
    Nevertheless, the two species in each lake are reproductively isolated; neither mates with the other.
    However, aquarium tests showed that
    the benthic species from one lake will spawn with the benthic species from the other lakes and
    likewise the limnetic species from the different lakes will spawn with each other.
    These benthic and limnetic species even display their mating preferences when presented with sticklebacks from Japanese lakes; that is, a Canadian benthic prefers a Japanese benthic over its close limnetic cousin from its own lake.
    Their conclusion: in each lake, what began as a single population faced such competition for limited resources that
    disruptive selection - competition favoring fishes at either extreme of body size and mouth size over those nearer the mean - coupled with
    assortative mating - each size preferred mates like it
    favored a divergence into two subpopulations exploiting different food in different parts of the lake.
    The fact that this pattern of speciation occurred the same way on three separate occasions suggests strongly that ecological factors in a sympatric population can cause speciation.
    Sympatric speciation driven by ecological factors may also account for the extraordinary diversity of crustaceans living in the depths of Siberia's Lake Baikal.

    Parapatric Speciation

    There is another possible way for new species to arise in the absence of geographical barriers.

    If a population ranges of a vast area (e.g., the Amazon basin in south America) and
    if the individuals in that population can disperse over only a small portion of this range,
    then gene flow across these great distances would be reduced.
    Genetic isolation arising simply from the great distance separating subpopulations could thus lead to "parapatric" speciation. Some workers feel that the enormous species diversity of the rain forests of South America (greater than that of Africa and Asia combined!) arose from parapatric speciation.

  2. #2

    Post Species and Speciation


    While Darwin entitled his book "On the Origin of Species", the book dealt primarily with a mechanism of evolution (natural selection) in which variation was critical. But what will selection do with this variation? Change the frequency of dark morphs of moths, or morphs of snow geese, or change the mean and the variance of the distribution of heights in human populations? What is the result of disruptive selection? How do we decide that natural selection has actually lead to the origin of new species? The answer to these questions depends on one's species concept. The concept of species is an important but difficult one in biology, and is sometimes referred to the "species problem". Some major species concepts are:

    Typological (or Essentialist, Morphological, Phenetic) species concept. Typology is based on morphology/phenotype. Stems from the Platonic "forms". Still applied in museum research (type method) where a single specimen (type specimen) is the basis for defining the species. In paleontology all you have is morphology: typology is practiced and species are defined as morphospecies (e.g., snail shells in fossil beds). Problems: what about sexual dimorphism: males and females might be assigned to different species. Geographic variants: different forms viewed as different species? What about life stages: caterpillars and butterflies? If typology is let run it can lead to oversplitting taxa: each variant is called a new species (Thomomys) pocket gophers with > 200 subspecies.

    Evolutionary species concept. "A species is a series of ancestor descendent populations passing through time and space independent of other populations, each of which possesses its own evolutionary tendencies and historical fate" (George Gaylord Simpson). Simpson was a paleontologist and emphasis on stability over time is best appreciated in the fossil record. Inherently morphological, but his claim is that morphologies have genetic bases, so it is indirectly a genetical definition. Problem: gaps in the fossil record impose arbitrary boundaries between species, especially those undergoing gradual size/shape evolution. Compare with Cladistic species concept (pg. 418). How speciation affects existing taxa can alter one's view of species.

    Biological species concept. from population-level thinking of the modern synthesis.

    "Species are groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups" (Ernst Mayr; Museum of Comparative Zoology, Harvard).

    "Species are systems of populations; the gene exchange between these systems is limited or prevented in nature by a reproductive isolating mechanism or several such mechanisms." (Theodosius Dobzhansky; Rockefeller and Columbia Universities).

    Not the first to claim the importance of reproductive continuity: "a set of individuals who give rise through reproduction to new individuals similar to themselves" (John Ray, 1682). "A species is a constant succession of similar individuals that can reproduce together." (George Louis Buffon, 1707-1788). Note the characteristic Mayr: "biological" species concept implies that all other species concepts are non-biological.

    Recognition concept. species are groups of individuals that share a common fertilization system (a "specific mate recognition system", SMRS of Hugh Paterson, South Africa). Emphasis is on those characteristics of species that tend to hold them together; something that members of a species share. Biological species concept stresses that which makes a species different from other species; cant define species without reference to other species. Contrast isolation vs. recognition. See figure 15.2, pg. 409.

    There are other species concepts (now you know why it this has been called the 'species problem'): Ecological, Pluralistic, etc. One philosophical approach is to ask whether species are "individuals" or "classes".

    There are some conceptual and practical problems with the Biological Species Concept: Are species real or are they arbitrary categories imposed by biologists? Populations: where do they begin and end; often arbitrary and grade into other populations; Genus, Family, Order, etc. are these human constructs? Is a genus of bees = a genus of birds in terms of levels of organization? What are the typological grounds for the boundaries. What about "species" that can freely mate such as species of orchids that can mate sometimes between genera (wide cross).

    What about asexual species? They don't reproduce with other species so every individual is a species?? Mayr would hold that species are real units. Views species boundaries as being defined by limits of gene exchange: each species is a group of populations held together by exchange of genes in a genetic system that allows free recombination among the chromosomes of this system. Holds that species are real objective units with definable limits - basic units of evolution. No mistake that the Biological Species concept was advanced by two zoologists who worked with organisms that did not present some of the more obvious problem of plants and bacteria (Nevertheless, there is clear discontinuity in the phenotypes of bacteria).

    Isolating "Mechanisms" (misleading term: is it a mechanism in that it evolved for the purpose of isolating; or did isolating "mechanisms" evolve in one context and serve to prevent mating on another?). Premating mechanisms prevent interspecific crosses. Temporal or Ecological isolation (don't meet due to different time of emergence or occur in different habitats). Ethological (behavioral) isolation (meet but don't mate) e.g. fireflies. Mechanical isolation (can't transfer sperm, morphological incompatibilities). See table 15.1, pg. 405.

    Postmating isolating mechanisms inhibit or prevent interspecific crosses gametic mortality (sperm transferred but does not fertilize eggs). zygote mortality (egg is fertilized but zygote dies). hybrid inviability (F1 hybrid has reduced viability: incomplete development). hybrid sterility (F1 hybrid viable but sterile) e.g., mule

    Premating isolation prevents wasting of gametes: highly susceptible to improvement by natural selection. Damselflies: character displacement of wing spot density. Rapid speciation events often involve behavioral isolation: Hawaiian Drosophila: hundreds of species in the past several million years. Postmating isolation does not prevent the wasting of gametes and its improvement by natural selection is indirect. Isolating mechanisms may work in concert; if one breaks down, another will prevent gene exchange (e.g., bird songs and plumage patterns). This issue of the opportunity for selection to act on pre- vs. postmating isolating mechanisms is important in the discussion of Reinforcement in the next lecture.

    Breakdown of isolating mechanisms will lead to hybridization (crickets in eastern North America hybridize in a hybrid zone along the Appalachian ridge). Are hybridizing "species" really species? If the hybrids backcross to either type, introgression can occur ("the incorporation of genes from one species into the gene pool of another species"). Many examples of hybridization in both plants and animals. Often referred to as semispecies, i.e., not complete species.

    Population structure: are populations the unit of evolution? (Ehrlich and Raven 1969, Science 165:1288-1232) Species are just "phenetic clusters". But why do populations cluster into "species". Checkerspot butterfly studied on Jasper Ridge near Stanford CA by Paul Ehrlich and colleagues (1975 Science 188:221-228). Different populations fluctuate independently: suggests little gene exchange between populations (But Slatkin's analysis of allele frequency data suggest otherwise.

    Geographic variation in reproductive isolation. If a series of populations can mate sequentially, but the end populations cannot, is one species two?? Mayr would say that since they do not meet the issue is not biologically relevant. Do you agree?

    Polymorphism. Mimicry complexes of African swallowtail. Papilio dardanus exists as one morph where no noxious species are found (Madagascar). Where noxious models are present the same species (Papilio dardanus) takes on different forms depending on the local model; the mimetic forms look like completely different species but are one.

    Sibling species. Morphologically indistinguishable, but are reproductively isolated. Not always easy to test for reproductive isolation and no morphological grounds on which to separate populations.

    Descriptions of the geography of population location/overlap helps focus on how geography might influence gene flow. If gene exchange between two populations is completely stopped, what will happen? Allopatric populations/species exist in different areas (do not overlap or abut); sympatric populations/species occupy the same geographic locality; parapatric populations/species have abutting but not overlapping ranges; a peripatric distribution refers to peripheral isolates.

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