Speciation

5 Key excerpts on "Speciation"

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  eBook - ePub

  Evolution

  A Beginner's Guide

  • Burton Guttman(Author)
  • 2005(Publication Date)
  • Oneworld Publications

    (Publisher)

  In spite of the difficulties we sometimes encounter in applying the biological species concept, it is still fundamental to our thinking. A hallmark of modern biological thought is recognizing that a species is not just an arbitrary collection of organisms but has objective boundaries defined by reproduction. This is one of our legacies from Darwin. The question of defining a species is not primarily interesting because biologists want a catalogue of the world’s organisms, but rather because the idea of a reproductively independent unit is so important. Unless a unit – a species defined biologically – becomes genetically independent of other groups, it can’t go on to produce other, more diverse groups in another evolutionary step, and it is questionable whether it can become a stable part of a community.

  the mechanism of Speciation

  All these difficulties in defining a species neatly make life tough for cataloguers, but they are a delight to the student of evolution because they show evolution in action in all its randomness and complexity. This will become clearer as we examine the general process of Speciation, the process in which new species are formed. This general model of Speciation through geographic isolation is due largely to the work of Ernst Mayr in the 1940s. The model is based primarily on studies of birds and insects. It undoubtedly applies to many sexually reproducing animals, and it applies to plants to a degree, although we will have to discuss special mechanisms in plants later.

  The Song Sparrow example shows how a single species may vary geographically, and such variation is common; as a species spreads out across a wide range, its populations often acquire many differences. They may become so different that one is tempted to call some populations distinct species, but as long as neighboring populations can interbreed, genes are still flowing from one to the other and they are still all one species. It is only meaningful, of course, to consider reproductive isolation if the populations in question are positioned so they could interbreed. Two populations are sympatric (sym- = together; patra = fatherland, thus homeland) if their ranges overlap, parapatric (para- = next to) if their ranges are adjacent, or allopatric (allo-

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  eBook - ePub

  The Princeton Guide to Evolution

  • David Baum, Douglas Futuyma, Hopi Hoekstra, Richard Lenski, Allen Moore, Cahterine Peichel, Dolph Schluter, Michael Whitlock(Authors)
  • 2013(Publication Date)
  • Princeton University Press

    (Publisher)

  On the Origin of Species by Means of Natural Selection, he took the first big steps to demystifying the process. Darwin recognized that in nature there was no sharp discontinuity between the differences one sees among populations within species and the differences observed between closely related species: “I look at the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms.” The origin of a species is not a sudden instant in the history of life but one that (usually) results from a steady accumulation of differences. Those differences, he explained, are the product of gradual evolution by natural selection of variation present in populations.

  We now recognize that Darwin’s solution was incomplete. One reason is that our concept of species has evolved. In Darwin’s day, species were designated according to the magnitude of morphological differences: “the amount of difference is one very important criterion in settling whether two forms should be ranked as species or varieties.” This means that except for the magnitudes involved, morphological criteria grouped populations into species just as species were grouped into genera, and genera into families. Under this concept, Speciation is the evolution of differences in ordinary phenotypic traits sufficiently large to warrant the taxonomic designation species. Darwin appreciated that matings between different species often produced inviable or sterile offspring, but he decided that this was not universal and was less reliable than morphological differences for classifying species.

  The focus of Speciation study changed with the development of the biological species concept (see chapter VI.1 ). In 1937, Theodosius Dobzhansky defined Speciation as the evolution of “isolating mechanisms,” traits that reduce gene flow between populations. Subsequently, Ernst Mayr defined species as “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.” Reproductive isolation doesn’t just mean hybrid sterility and inviability—it includes any

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  The Princeton Guide to Ecology

  • Simon A. Levin, Stephen R. Carpenter, H. Charles J. Godfray, Ann P. Kinzig, Michel Loreau, Jonathan B. Losos, Brian Walker, David S. Wilcove(Authors)
  • 2009(Publication Date)
  • Princeton University Press

    (Publisher)

  separation, often via disruptive selection 1. ECOLOGICAL Speciation: WHAT IT IS AND HOW TO TEST FOR IT The idea that the macroevolutionary phenomenon of Speciation is the result of the microevolutionary process of adaptation dates back at least to Charles Darwin. However, it was not until the popularization of the biological species concept in the middle of the last century, whereby Speciation was defined as the process by which barriers to genetic exchange evolve between populations, that the study of Darwin’s “mystery of mysteries,” the origin of species, became empirically tractable. The past two decades have witnessed an explosion of Speciation research, with much attention being given to understanding the role of divergent selection in Speciation. As defined by Dolph Schluter and others, ecological Speciation is the process in which barriers to genetic exchange evolve between populations as a result of ecologically based divergent natural selection. Selection is ecological when it arises from differences in the environment or from interactions between populations over resource acquisition. Ecologically based selection can thus arise, for example, from an individual’s quest to obtain food and other nutrients, attract pollinators, or avoid predators. It can also arise from an individual’s interaction with other organisms in its attempt to achieve these goals (e.g., resource competition, predation). Selection is divergent when it acts in contrasting directions in the two populations. Included here is the special case of disruptive selection on a single population, in which selection favors opposite extremes of the same trait. During ecological Speciation, populations experience divergent selection between environments or niches and thus differentiate in ecologically important traits

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  The Origin Then and Now

  An Interpretive Guide to the Origin of Species

  • David Reznick(Author)
  • 2011(Publication Date)
  • Princeton University Press

    (Publisher)

  Peripatric (literally “around or beyond the same fatherland”) Speciation is a variation on the theme of geographic Speciation because the process is generally initiated when a small number of migrants colonize a new habitat. Such new habitats, like the Galapagos Islands, tend to be sufficiently isolated for repeated colonization to be unlikely, so the movement of individuals between the source population and the new population is a very rare event. The new habitat may have a very different environment from that experienced by the original colonists before they emigrated, so the natural selection experienced by the colonists can be particularly intense. Such circumstances can promote rapid evolution and Speciation.

  There is also evidence that species can form without geographical isolation. Such “sympatric” (same fatherland) Speciation occurs when the new species form within the “cruising range” of an individual, meaning that whatever geographical separation exists does not lie beyond the possible regular movements of individuals, so that geography alone is not a barrier to reproduction. A special property of all such modes of Speciation is that natural selection must play a role in actively selecting for reproductive isolation as part of the process. Such an active role is a necessary component of vicariant allopatric Speciation only if reinforcement comes into play when once-separated populations come back into contact with one another.

  Coyne and Orr present the best available arguments that species really are objective realities, rather than simply points on a continuum as argued by Darwin, and that species can be defined by reproductive isolating mechanisms. For example, there is a close correspondence between the distinct species of birds, frogs, and reptiles recognized by scientists and those recognized by the indigenous people in different regions of New Guinea. The fact that two different groups of people using very different criteria come up with very similar classifications suggests that the distinctions are real. These categories, species, can also be distinguished with formal statistical analyses of morphology and on the basis of DNA sequences. However, at the same time, among the millions of species that are known today, many are described as “problematic” because traces of continuity between some of them remain. For example, we sometimes find regions in which different species seem to blend with one another via a bridge of populations that have properties that lie between the two, making it impossible to pigeonhole all organisms into discrete species. These problematic species are the source of Darwin’s argument that Speciation is an ongoing process. If new species can form, then we should expect to see living examples of this ongoing process that defy objective classification.

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  eBook - ePub

  Species Concepts and Phylogenetic Theory

  A Debate

  • Quentin Wheeler, Rudolf Meier(Authors)
  • 2000(Publication Date)
  • Columbia University Press

    (Publisher)

  In swimming through the murky waters that are species concepts, we might ask what process is most critical with respect to species as entities (Cracraft 1987, 1989a). Framed another way, if we use a species concept to individuate entities, which process, or processes, do we expect those entities (species) to participate in? We ask these questions because if a definition individuates entities that actually participate, as entities, in no process (leaving aside whether this is a possibility in the first place), then what use is that definition? Relatively little, it would seem. And what does it imply about the “reality” of those “species”?

  Most authors in this volume seem to agree—even if they do not state so outright—that whole species do not speciate. Parts of species speciate in the sense that populations become isolated (at least under an allopatric model) and differentiate. At some point, and depending on the particular definition one adopts, that differentiated entity is called a species. In other words, species are speciated—they are the inferred end product of a process we call Speciation. Given this, the reader should ask what kinds of entities would be expected to result from this type of process, and which definition(s) might be expected to individuate species/entities of this sort and which definition(s) might not. Let us frame the problem another way, and more directly: if species are only the end-product of a process of differentiation and participate in no other process as discrete entities, which species concept (or concepts) fits these entities and allows us to study them? Some do, and some do not. If a concept individuates things that are not a result of a process of differentiation, then that concept must exist for some other reason. This is part of the problem “how do we know what we think we know?” Such a concept might be individuating fictitious entities with regard to their participation in processes.

  Another aspect of the problem “how do we know what we think we know?” involves ancestral species (or stem species). This is an issue that raises its ugly philosophical and empirical head in quite a few essays in this book, and some authors have sharp differences of opinion. They debate, for example, whether ancestral species can be said to maintain existence after a Speciation event or whether ancestral species can be identified in the first place. The concept of ancestral species intersects with our ontological worldview (can we really say species are ancestors?) and with the epistemological tools we bring to bear on identifying taxa and determining their relationships, especially how we see and analyze character variation. It also intersects with the previous discussion. Presumably we are talking here about a discrete real entity, an ancestral species, and the process of splitting.

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