Gene Expression

Paleontology & microevolution?

Rapid evolution in early trilobites fueled by high variation:

Webster compiled morphological data for nearly 1,000 of the 17,000 different species of trilobites, a class of marine arthropods that died out by 250 million years ago, from 49 previously published sources. By tracking different morphological features — the number of body segments, for example — Webster found that trilobite species exhibited more variation during the Cambrian than in later periods, he reported in Science July 27. “Once you go beyond the Cambrian, the diversity of forms within any one species drops off,” he says.

Early and Middle Cambrian trilobite species, especially, exhibited greater morphological variations than their descendants. This high within-species variation provided more raw material upon which natural selection could operate, Webster says, potentially accounting for the high rates of evolution in Cambrian trilobites. Such findings may have implications for our understanding of the nature of evolutionary processes, he says.

I don’t know about the nature of the debate within paleontology (paleobiology) with great detail, but it seems that men like S.J. Gould and Niles Eldredge promoted “higher level” evolutionary processes to explain speciation and deep time natural history. This researcher seems to be putting the onus on basal microevolutionary processes.


  1. #1 empiricus
    July 28, 2007

    Here’s the real paper (open access):

    I’m not a paleobiology type either, though I remember Gould in his popular writings (e.g. “Wonderful Life”) putting a lot of emphasis on unoccupied ecology niches as an explanation for the Cambrian explosion in higher taxa. That could potentially apply at lower taxa too – poorly unoccupied niches => greater chance that a variant trilobite with different number of segments might be able to reach a niche where the variant had better fitness.

    Also perhaps/presumably back then hox inhibition was less complex, so it was more likely that e.g. a SNP could cause a variant phenotype.

    Idiot level question which may or may not be relevant, but which I could not answer based on the paper: What gives Webster et al confidence in their assignment of individual fossils to the same species (i.e. why is he sure that he’s seeing intraspecific variation)? Presumably synapomorphies in e.g. details of tagmosis, fine points of cephalon morphology, and of course the eyes.

    It seems like that would be much harder in trilobites than e.g. crustaceans specifically because there’s so much less segment specialization in trilobites.

  2. #2 Oran Kelley
    July 30, 2007

    As a framework for tackling the puzzle of why so much anatomical variety arose so rapidly at this unique time, I suggested that two basic approaches should be explored (with a full answer undoubtedly requiring a balance of both). An “external,” or ecological, perspective would focus upon the uniquely “empty” ecological barrel of potential environments for mobile multicellular animals at the dawn of Cambrian time; almost any “experiment” might work for a while during an initial “filling”–at least until Darwinian forces sorted the workable from the suboptimal and placed a brake upon subsequent change of such magnitude. By contrast, an “internal” genetic or developmental perspective might view the Cambrian as a time of unique flexibility, before definite patterns of growth from egg to adult became so locked into the embryology of complex organisms that fundamental reconstructions became nearly impossible.

    I suggested in Wonderful Life (and still maintain) that scientists should devote more attention to the unconventional internal arguments than to the more familiar ecological claims. I proposed no bizarre or novel evolutionary mechanisms but only emphasized a potentially greater efficacy for ordinary processes at a unique time of organic flexibility, before major developmental pathways became irrevocably set.

    [from Gould’s back and forth with Conway Morris]

    Gould seemed particularly interested in the possibility that the Cambrian represented a period when developmental restraints may have been laxer or nascent.

    Don’t know how that goes to the “levels of selection” question . . .

    Webster does seem to generally support Gould’s more dramatic view of the Cambrian explosion (as opposed to Conway Morris’s).

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