Transitional forms

From: Keith B Miller (
Date: Sat Apr 21 2001 - 10:42:34 EDT

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    >One of the questions I had while reading the book was a statement by
    >Jonathan Wells (pp 137-138): "First, Darwin maintained that major
    >differences evolve from minor ones. Yet the fossil record shows that all
    >of the major animal groups appeared at approximately the same time,
    >without any fossil evidence that they diverged from a common ancestor.
    >These original groups have since diversified into many subgroups, so the
    >major differences among animals appeared before the minor ones.
    >Paleontologists James Valentine and Douglas Erwin call this a 'seeming
    >paradox,' since in this respect Darwin's theory 'does not accord with the
    >primary evidence.' "

    First the statement that "all of the major animal groups appeared at
    approximately the same time" is demonstrably false, at least as a literal
    reading of the fossil record. I have written on this before, and some of
    this data is included in Glenn Morton's recent Perspectives article.

    Much has been made of the "top-down" versus the "bottom-up" pattern of
    appearance of higher taxa by evolution critics. In the "top-down" pattern,
    phylum-level diversity reaches it peak in the fossil record before
    class-level diversity, and the class-level diversity before that of orders,
    etc. Critiques of macoevolution interpret this apparent "top-down"
    pattern as contrary to expectations from evolutionary theory. However,
    this pattern is generated by the way in which species are assigned to
    higher taxa. The classification system is hierarchical with species being
    grouped into ever larger and more inclusive categories. When this
    classification hierarchy is applied to a diversifying evolutionary tree, a
    "top down" pattern will of necessity result. Consider species belonging to
    a single evolving lineage given genus-level status. This genus is then
    grouped with other closely related lineages into a family. The common
    ancestors of these genera are by definition included within that family.
    Those ancestors must logically be older than any of the other species
    within the family. Thus the family level taxon would appear in the fossil
    record before most of the genera included within it. The "top down"
    pattern of taxa appearance is therefore entirely consistent with a
    branching tree of life.

    >A second question along this same line is whether forms which appear to
    >be transitional are truly so. The book _Of Pandas and People_ shows
    >silhouettes of three skulls: a Tasmanian wolf, a North American wolf, and
    >a dog. The cranial-cavity size increases as you go from the Tasmanian
    >wolf to the dog, suggesting a transitional relationship. However, we
    >know the Tasmanian wolf was a marsupial, while the other two are
    >placental mammals. Convergent evolution can produce forms which look
    >transitional but which we know are not, based upon soft-part anatomy,
    >which of course is rarely fossilized. Is this factor just generally
    >ignored by those inferring evolutionary relationships?

    Convergent evolution is ubiquitous. One of the primary objectives of
    comparative anatomy and phylogenic analysis is to distinguish between
    convergence and similarity due to close relatedness. Such analysis does
    not require knowledge of the soft anatomy. Most convergence involves
    aspects of the skeletal anatomy that are relatively plastic and respond
    readily to environmental selection pressures. It also involves aspects of
    the skeleton that are constrained by mechanical laws such as limb
    proportion. These features are not the ones used to determine evolutionary
    relationships. Rather features such as the shape of ear bones figure
    prominently in these analyses.

    BTW: Marsupials and placentals are readily distinguished by their skeletal


    Keith B. Miller
    Department of Geology
    Kansas State University
    Manhattan, KS 66506

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