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.
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