Re: common ancestry

From: David Campbell (
Date: Tue Aug 01 2000 - 12:21:44 EDT

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    Wow! My research is actually directly relevant to the thread!

    Paul's comments have confused a couple of factors. First, the argument
    that various groups may be polyphyletic does not refute more distant common
    ancestry; rather, it means that the traditional group includes taxa with
    separate origins within a larger clade. For example, arguing that the
    tetrapods are polyphyletic is likely to mean that someone thinks that
    different lungfish independently gave rise to tetrapods. They have a
    common ancestor, but the tetrapod condition arose more than once among
    descendants of that ancestor. An example from my own research concerns the
    Myoida, a group of clams including the geoduck, steamer clams, shipworms,
    etc. They all have reduced hinge teeth, long siphons, and generally gaping
    shells with the same grade of shell structure. However, reduced hinge
    teeth and long siphons can evolve convergently as adaptations to deep
    burrowing, and the grade of shell structure has also evolved independently
    in the bivalves several times, so a lot of people have been suspicious that
    Myoida is polyphyletic. My DNA data (18S) confirm this. They have a
    common ancestor within a larger group of bivalves, but different "myoids"
    evolved from this ancestor at different times.

    > Philippe et al. also draw attention to 'artifacts' that lead to
    > such 'topological errors' as the portrayal of Drosophila and
    > Aedes [both insects] as a sister group of nematodes, which
    > they attribute to the rapid evolution of both groups...(1996:46)
    >But today, in the year 2000, the grouping of flies with worms isn't
    >an artefact (i.e., error) at all. Drosophila and C. elegans are now
    >thought to be cousins within the Ecdysozoa. Both animals molt,
    >although by classical morphological criteria they have precious
    >little else in common.

    This misunderstands the problem of the first paper. In that analysis,
    nematodes and flies are sister taxa, not nematodes and arthropods. In
    other words, the first analysis came out putting flies and nematodes not
    just at the same family reunion but as siblings, something that even most
    molecular biologists realize is highly improbable. The quoted discussion
    [in Paul's post] explains the problem, though in jargon. The basic problem
    is that DNA has five options-A, G, T, C, or a deletion. Thus, two random
    strands will have some similarity by chance. Rapidly evolving sequences
    (which can be detected by various statistical tests) are somewhat
    randomized. If the number of random similarities outweigh the number of
    inherited similarities, then the analysis can be misled. This situation is
    known as long-branch attraction or the Felsenstein zone. Another example
    from my research concerns oysters. Oysters, scallops, mussels, etc. are
    generally grouped as the Pteriomorphia, having a wide variety of
    morphological similarities. However, oyster 18S genes evolve more rapidly
    than those of others in that group. Early molecular studies on the group
    tended to associate the one oyster sequence with more rapidly evolving
    bivalves rather than with pteriomorphs. However, recent studies with more
    data for other oysters, other pteriomorphs, and others from the more
    rapidly-evolving groups place oysters where they belong morphologically.
    Adding additional taxa broke up the long branch and provided a correct

    An additional factor is that molecular biologists are notorious for not
    knowing anything about organisms or paleontology and sometimes present
    their latest results as the final word, without considering problems such
    as long-branch attraction.

    A man driving along a country road found his way blocked by a flock of
    sheep. As the shepherd came by, he asked "If I can guess exactly how many
    sheep you have, can I take one?" Confident that there were too many to
    count, the shepherd agreed, but was startled when the man confidently
    declared there were 3468. The man selected his sheep, whereupon the
    shepherd challenged him "If I can guess your profession, can I have it
    back?" He agreed, and the shepherd identified him as a molecular biologist
    and demanded the return of his sheepdog.

    David C.

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