RE: Definition of "Species"

From: Tim Ikeda (
Date: Tue Feb 19 2002 - 22:11:04 EST

  • Next message: "Re: Definition of "Species""

    Hello Norm,
    You wrote:
    >But, all in all, you have confirmed my contention: that though they
    >cannot provide a general definition of "species" or "speciation,"
    >evolutionists know it when they see it.

    In practice, whether a group of organisms is defined as a separate
    species is more a matter of whether it helps address the particular
    question at hand. It is a functional definition, not an absolute
    one. Biologists (or taxonomists) don't assign populations to
    species willy-nilly. It is done so and done in certain, careful ways
    for particular uses.

    >Yet they are quick to hang creationists out to dry when such an approach
    >is used to define "kinds."

    The key is that biologists readily recognize that the concept of species
    is a man-made construct; an attempt at segregating biological organisms
    into "bins" or groups. This sort of grouping was practiced before Darwin
    entered the scene. Biologists see the clustering of organisms into species
    as a consequence (one might say, an artifact) of evolution and particular
    mechanisms of divergence. These mechanisms will differ, depending on the
    biologies and historical circumstances of organisms. That is why, for example,
    the partitioning of clonal organisms like bacteria into species is more
    problematic than splitting sexually reproducing organisms like mammals.
    Metrics like sequence divergence, genome structure, phage sensitivity,
    metabolic divergence and recombination frequencies have been used for
    bacterial classification. The same metrics could be applied to mammals but
    there could be obvious exceptions or criteria which are easier to discern.
    Essentially, there is _no_ single criterion which will work in all cases
    and for all uses. Life is, after all, a continuum.

    This is in contrast to the traditionally Biblical concept of "kinds" which
    assumes the existence of fundamentally separate origins for groups of
    organisms. It is an absolute definition.

    >* I have not seen the new varieties of bacteria called species.
    >But have you seen the their creation cited as a proof of "the theory of
    >evolution"? If new species are not being created, then how is the theory
    >being confirmed?

    New species do arise. In plants this may occur within a single generation
    through polyploidy. One can also observe related populations of organisms
    at various stages of speciation. For example, there are different bird and
    fish species where there may be populations found that exhibit varying
    degrees of overlap with sister populations. Some of the barriers between
    populations are related to behavior, others include the timing of
    reproduction, gamete incompatibilities and physical differentiation. It's
    hardly unexpected that animal groups observed near the time of speciation
    (divergence) are difficult to differentiate. In most instances,
    speciation is not an instantaneous process.

    >A horse and donkey can be crossed to produce a mule, but the mule is
    >almost always sterile.
    >Had anyone suggested that the caballus and the asinus were not of
    >different species?

    Yes, and for precisely the reason you describe: "the mule is _almost_
    always sterile". And as long as the particular definition used is precisely
    described, that's OK.

    >I am aware of the problems of interspecies breeding. I am just amazed how
    >speciation claims seem to be made based on, it would appear, rather
    >subjective classification.

    Well, the splitting of lions and tigers into two species seems logical.
    Granted, they do hybridize in the wild to create fertile offspring, but
    "ligers" are the exception rather than the norm. And lions & tigers do
    have distinctive physical differences.

    >BTW, has any reputable biologists (lately) crossed politically correct
    >barriers, and implied that human populations of different races may be of
    >different species?

    I'm familiar of only a few: Most of these also hold to "less than mainstream"
    interpretations of the Bible.

    >Surely, when considering, at least historically, morphological differences
    >and geographic and social barriers prohibiting "intermarriage," and the
    >various definitions of "species" which you have proposed, this conclusion
    >must be the "elephant in the parlor," that everyone sees, but no one will
    >Of course, I would not subscribe to such a concept, but, then again, I am
    >not sure I agree with several of your suggested definitions of "species."
    >(I hope I have not offended anyone's sensitivities, but sometimes to
    >discredit a concept, you have to extend it its illogical conclusion.)

    What is "illogical" about the idea of human speciation? Presumably, that's
    how modern humans arose from apes. If, perhaps, the gene flow between human
    populations stopped, or if selection drove populations to differentiate
    into separate niches, then eventual speciation would not be unexpected. That
    has, however, not occurred -- Mating barriers have not last long enough
    to reduce gene flow across the population. If we compare genomes of the
    "traditionally" recognized races, we see that we really can't distinguish
    between them. In contrast, there are animal populations in the wild
    where gene flows have stopped for a long period of time and mating
    incompatibilities are found (e.g. Equus caballus vs. Equus asinus).
    The path toward speciation is a continuum. Human populations are at
    one end; horses and asses are more toward the other end. Cheetahs and house
    cats are at the far end.

    Suggested reading:
    Ernst Mayr: The Growth of Biological Thought: Diversity, Evolution
    and Inheritance (See Chapter 6 - "Microtaxonomy, the science of species"
    pp. 251-297)

    Tim Ikeda

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