Johnson, Darwin, and transitionals

From: Wesley R. Elsberry (
Date: Fri Jan 21 2000 - 02:11:44 EST

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    Chris Cogan wrote:


    CC>Evolution is *definitely* not to be regarded as always
    CC>proceeding at a constant rate (by any measure that I can think
    CC>of). This "uniformitarianism" was one of Darwin's biggest


    Except that Darwin did not espouse a "constant rate" view of
    natural selection. At least, I haven't seen any prose from
    Darwin that sets aside his identification of intermittent
    action of natural selection.

    I think, Chris, that you grant Johnson too much credit in his
    assertion that the invertebrate fossil record fails to clearly
    document transitional sequences. In fact, my reading leads me
    to almost the opposite conclusion -- the invertebrate fossil
    record yields a richer source of data concerning transitions
    than does the vertebrate record. I routinely ask those who
    deny that any transitional sequences exist to explain what,
    precisely, might disqualify the transition discussed in
    the following paper:

    Pearson, P.N.; Shackleton, N.J.; and Hall, M.A., 1997.
    Stable isotopic evidence for the sympatric divergence of
    _Globigerinoides_trilobus_ and _Orbulina_universa_ (planktonic
    foraminifera). Journal of the Geological Society, London,
    v.154, p.295-302.

    Researchers at Florida State Univerity (Arnold and Parker) put
    together a large-scale collection of foram data showing
    transitions. A longer article used to be available, but
    <> seems to
    describe the department broadly.

    Roger Cuffey's article on paleontological evidence printed by
    the ASA in 1974 cited many (>100) research papers, and the
    majority of those were of invertebrate studies.


    Planktonic foraminifera provide a unique tool for assessment
    of rates and patterns of morphological evolution. It is in
    this group of fossils where the best documented examples of
    evolutionary transitions between species have been
    described. Yet, although the fossil record of planktonic
    foraminifera allows direct tracing of ancestry, quantification
    of rates of morphological change and tracking of spatial
    patterns of speciation, the insufficient knowledge of the
    function of their shells has been gravely limiting the
    interpretation of such data. Recent advances in DNA extraction
    from modern planktonic foraminifera, as well as stable
    isotopic studies of habitat changes of their fossil relatives
    have cast additional doubts on the biological meaning of the
    well documented evolutionary patterns in this group. If there
    are cryptic species of planktonic foraminifera, if changes in
    the shape of their shells during the evolution occur out of
    phase with changes in their ecological preferences, does it
    mean that shell form is in fact not subjected to the machinery
    of natural selection?

    Here, I present data on morphological evolution in a late
    Neogene planktonic foraminifer lineage. A detailed
    investigation of the changes in the relative abundance and
    size of secondary openings in the shells of this lineage
    suggests that there was a strong selection against this
    character. Thus, it seems that at least in some cases
    morphological evolution of planktonic foraminifera may indeed
    reflect the biological processes of natural selection. This
    implies that recent findings on the functional morphology of
    planktonic foraminiferal shells do not imply a threat to
    studies of morphological evolution in this group; rather they
    provide a challenge and a great prospect for future

    [End Quote -

    Five out of six examples at
    concern invertebrates.

    Discussion of speciation in the fossil record in
    <> discusses
    four groups of invertebrates.


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