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Author Topic:   Definition of Species
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Message 1 of 2 (420825)
09-09-2007 5:26 PM

I think everyone is fairly familiar with the biological species definition, and some may be familiar with the morphological definition. Here is a site for reference on these definitions:
U of Michigan Lectures - The Process of Speciation
  • 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."
  • 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.
  • Where we can study living populations of sexual species we can use the first definition, but when we deal with the fossil record or with asexual species we would have to use the second definition.
    There is also another definition in the forum glossary:
    A basic taxonomic category for which there are various definitions. Among these are an interbreeding or potentially interbreeding group of populations reproductively isolated from other groups (the biological species concept) and a lineage evolving separately from others with its own unitary evolutionary role and tendencies (Simpson's evolutionary species concept). Employing the terms of population genetics, some definitions can be combined into the concept that a species is a population of individuals bearing distinctive genes and gene frequencies, separated from other species by biological barriers preventing gene exchange.
    Buried in Message 118 is this comment:
    The whole field of classification began with Morphology. The biological definition was first proposed by Ernst Mayer in 1942. Scientists are still attempting to find better ways to distinguish species using genetics (I’ll talk about that later). In any case, while the biological definition is nice on paper it is not as commonly used in practice as indicated by this university:
    To quote “However, in the real world, it is time-consuming and expensive to make the observations of organisms in their real habitat that would allow us to say with confidence that such-and-such a set of organisms really is a species. And in the fossil world, it is impossible. So instead, most biologists and all paleontologists make a good-faith guess about the boundaries of the set of organisms they propose to name a species. Typically, the species is defined on the morphology it has, not on the genetics and behavior that is specified in the biological species concept. ... "
    The news article listed by Murkywater above has a link to the actual Journal article:
    Page not found – Allen Press
    We define a genetic species as a group of genetically compatible interbreeding natural populations that is genetically isolated from other such groups. This focus on genetic isolation rather than reproductive isolation distinguishes the Genetic Species Concept from the Biological Species Concept. Recognition of species that are genetically isolated (but not reproductively isolated) results in an enhanced understanding of biodiversity and the nature of speciation as well as speciation-based issues and evolution of mammals. We review criteria and methods for recognizing species of mammals and explore a theoretical scenario, the Bateson-Dobzhansky- Muller (BDM) model, for understanding and predicting genetic diversity and speciation in mammals. If the BDM model is operating in mammals, then genetically defined phylogroups would be predicted to occur within species defined by morphology, and phylogroups experiencing stabilizing selection will evolve genetic isolation without concomitant morphological diversification. Such species will be undetectable using classical skin and skull morphology (Morphological Species Concept).
    -- Journal of Mammalogy, 87(4):643-662, 2006 p.643 (abstract)
    Now my first impression is that this is really just the biological definition of species using genetics to determine reproductive isolation, and one that would be useful for finding "cryptic" species, one that could be applied to asexual species, and even extended to some fossils (where DNA is recoverable).
    We define genetic species as a group of genetically compatible interbreeding natural populations that is genetically isolated from other such groups. Under our definition of the Genetic Species Concept, speciation is the accumulation of genetic changes in 2 lineages (Bateson 1909) that produce genetic isolation and protection of the integrity of the 2 respective gene pools that have independent evolutionary fates. Therefore, the process of speciation depends on divergence in genes, the genome, and chromosome structure (Check 2005), and species exist when the integrity of 2 gene pools is protected as a consequence of genetic differences in their respective genomes (e.g., as outlined in the Bateson-Dobzhansky-Muller [BDM] model but not restricted to those conditions).
    -- ibid p.645
    Reading further it seems that they establish a somewhat arbitrary delineation for "type" species concept based on <0.5% difference in the mitochondrial cytochrome-b gene:
    Sister species of mammals that have been recognized as species based on morphology often have cytochrome-b distance values .5% and this magnitude of divergence in the cytochrome-b gene has been associated with taxa recognized as species (Bradley and Baker 2001).
    -- ibid p.649
    I would think you really want to compare the total genome to ensure you are picking up where the genetic change is occurring within the population(s). I also think you would want to do a statistical mapping of all the variations by frequency and see if you have one or more peaks in the data, and let those peaks define your species (or incipient species depending on the degree of overlap and geographical separation).
    Furthermore, where you draw the line (>0.5%) could determine arbitrary species designations, with several monophylic species with wide viable hybrid zones, as opposed to one polyphylic species, being a matter of somewhat subjective interpretation.
    Genetic isolation is not simply an off-and-on switch because genetic changes in allopatric populations are accumulated slowly across the genome and may involve a substantial number (estimated at 200 for Drosophila”Presgraves 2003) of loci affecting isolation. Accumulation of adequate change in independent sister lineages that results in genetic isolation will be a chance event occurring rapidly in some cases but requiring long periods of separation in other cases. Genetic isolation resulting from the BDM model will be expected to produce intermediate and incomplete stages of reproductive isolation before the completion of reproductive isolation. We predict that genetic profiles of interactions between members of mammalian phylogroups will reveal examples of complete genetic isolation and examples of essentially no genetic isolation even with the same genetic divergence in the mitochondrial marker used to select phylogroups for more intensive study. But, more commonly in phylogroups with .5% genetic distance in the cytochrome-b gene, various combinations of genetic isolation will be apparent. As a result, when phylogroups are sympatric, there will often be hybridization, and data documenting the genetic basis for any level of isolation will be difficult to organize into well-defined stages (Table 2). Genetically defined hybrid zones will be common.
    -- ibid p.651
    Now it is not surprising to me that there would be grades of separation to be found in the data, as various populations would be at different points in developing isolating mechanisms to achieve speciation, as this is a gradual process after all.
    Nor am I surprised to think that there could be subpopulations that don't have the same proportions of alleles as other subpopulations, due to geographic factors that would make direct mating difficult, and where gene transfer would lag temporally.
    The question though is whether any member of subpopulation (A) could mate with a member of subpopulation (B) and produce viable (hybrid) offspring ... and this is not really answered when you have the subpopulations defined by average genetic similarity around mean values (there will always be a distribution in any population) and only observe mating between some of the two populations (that may or may not be statistically high in either subpopulation). You don't know who is in the "hybrid zone" when mating occurs.
    Why is the definition of species important and what is the use for the definition of species?
    Speciation is is the dividing line between what are considered microevolutionary and macroevolutionary processes and mechanisms, between the generation of homogeneous change within a population (evolution), and the generation of heterogeneous change (diversification) between diverging (especially for new) species.
    Thus I would define any population with a single peak frequency distribution as a species, any population with two peaks and a high "saddle" between them as incipient species, and any population with two peaks and a low "saddle" as different species. Analysis of this type of pattern for species like horses, zebras and donkeys would give you an idea of the saddle height necessary for speciation.

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    Message 2 of 2 (420832)
    09-09-2007 5:41 PM

    Thread copied to the Definition of Species thread in the Biological Evolution forum, this copy of the thread has been closed.

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