RE: Definition of "Species"

From: bivalve (
Date: Tue Feb 19 2002 - 18:55:03 EST

  • Next message: John W Burgeson: "Darwinism"

    though they cannot provide a general definition of "species"
    or "speciation," evolutionists know it when they see it.<

    Recognition of species is not confined to evolutionists, and
    even many antievolutionists allow some speciation as
    "microevolution". There is general agreement that a
    barrier to reproduction is a component of the development
    of new species; the problem is deciding what is legitimate
    evidence of the barrier and how solid a barrier must be.
    Thus, I doubt that any evolutionary biologist would deny that
    the apple and hawthorn flies are in the process of
    speciation. However, there would be a lot of debate as to
    how close they are to becoming two species and what
    would be conclusive proof that they had speciated.

    For asexually reproducing forms, some simply assert that
    species do not really apply; others try to use some criterion
    of differentiation. It is especially tricky to apply to the many
    organisms in which asexual reproduction prevails, in some
    cases so much so that sexual reproduction was only
    recently discovered, yet occasionally sexual reproduction
    does occur.

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

    I do not object to "kinds" simply because they do not get a
    fixed definition. I do have a problem with creationist
    attempts to define kinds as a firm, fixed category.
    Uncertainty in evolutionary definitions does not make the
    Biblical category any less (or more) nebulous. I am also
    dubious of attempts to take the passing Biblical references
    to kinds and make them into an absolute barrier that divine
    inspiration has shown to be impassible by evolution.
    Finally, in identifying fixed kinds, antievolutionists typically
    pick forms or levels for which good transitional forms are
    known. Claiming that there is no evidence for continuity
    between X and Y when there is is irresponsible.

    As the debate over species definition is a current
    significant issue in biology, knowledgeable evolutionists
    will not claim that the definition of species is fixed in stone.
    The unambiguous examples of the production of new
    species are the hybrids; others are certainly undergoing a
    process that would eventually result in the production of
    new species, but drawing the line is not so easy.

    It may be helpful to note that evolution provides a strong
    incentive for reproductive barriers to develop between
    populations with different specializations. If mixed
    offspring typically do poorly in competition with either
    pureblooded population, then parents that mate with
    individuals from the other population are wasting their effort
    relative to those that mate with more similar individuals.
    For example, a hybrid between the apple fly populations
    and the hawthorn fly population might head for a hawthorn
    bush when apples are ripe, thus being in the wrong place
    for an apple mate and the wrong time for a hawthorn mate.
    The reproductive effort of the parents is thus wasted,
    whereas mating with an individual preferring the same fruit
    would have led to a fine crop of grandmaggots. Thus, any
    fly that gets better at rejecting potential mates that go for the
    other fruit will be more succesful. Thus, they may be
    expected to develop total barriers to reproduction, though
    they do not have them yet.

    >>* I have not seen the new varieties of bacteria called
    "the theory of evolution"? If new species are not being
    created, then how is the theory being confirmed?<

    Novel genetic features and new information are being
    created without intelligent design. Variation is developing
    in a population and change occurs as a result of selective
    pressures. Darwin spent an inordinate amount of time in
    the Origin of Species talking about pigeon breeding, not
    because he thought new species were being created but
    because distinctive varieties were appearing and being
    selected for.

    >>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? <

    If two forms must never ever ever ever interbreed to be
    separate species, than they are disqualified. They are
    well-separated, but interbreeding is not a total

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

    Some versions of the not out of Africa idea could be used to
    recognize modern humans as multiple species. This idea
    has very low support, in light of the molecular evidence for
    extensive genetic input out of Africa into the modern
    populations. (Glenn's view, that this out of Africa
    component did not entirely replace the existing
    populations, is compatible with the molecular evidence but
    also promotes recognizing more past humans as
    conspecific rather than splitting modern humans. The
    extreme out of Africa view is that the latest migrants out of
    Africa totally replaced the prior populations of Europe, Asia,
    and Australia; some of these were then the first to reach
    the Americas.). A recent popular science book on the
    history of Australia seems to claim that the Tasmanians
    might have been surviving Homo erectus, but I have not
    seen a more credible source to support that. However, the
    mobility of humans has generally maintained a high level
    of interbreeding between adjacent groups. I also saw a
    creationist book in the late 1980's that was making such an
    argument, claiming that Lapps and Ainu might be
    Neanderthals and not conspecific.

    then again, I am not sure I agree with several of your
    suggested definitions of "species."<

    As there is not agreement among biologists on defining
    species, you certainly do not have to agree either.
    However, be aware that someone using a different
    definition will recognize speciation differently.

        Dr. David Campbell
        Old Seashells
        46860 Hilton Dr #1113
        Lexington Park MD 20653 USA

    Exalted Periwinkle of the Mystic Order of Whelks-P.G.
    Wodehouse, Romance at Droigate Spa

    This archive was generated by hypermail 2b29 : Tue Feb 19 2002 - 18:39:07 EST