The fossil record shows a consistent pattern of rapid (sometimes explosive) variation followed by stasis. This is found in the Early Cambrian with the dramatic appearance of animal phyla and, subsequently, with rapid radiations affecting classes, orders and families. Gould's advice to treat stasis as data has not been taken seriously by neodarwinists, who continue to treat the phenomenon as a quirk of history rather than a pervasive characteristic of living things.
Thinking on this topic deserves to be rekindled by a major study of trilobites by Mark Webster. Trilobites have a fossil record stretching from the Early Cambrian to the Late Permian, and 9 orders, 180 families, about 5000 genera and over 15,000 species of trilobites have been described to date. Many of these species are poorly documented and unsuitable for inclusion in a systematic study of morphological variation.
Webster's work involved coding for different character states and finding ways to document the variability. "Overall, approximately 35 percent of the 982 trilobite species exhibited some variation in some aspect of their appearance that was evolving. But more than 70 percent of early and middle Cambrian species exhibited variation, while only 13 percent of later trilobite species did so." The research documented both rapid morphological variation and subsequent stasis. "There's hardly any variation in the post-Cambrian," Webster said. "Even the presence or absence or the kind of ornamentation on the head shield varies within these Cambrian trilobites and doesn't vary in the post-Cambrian trilobites."
In a commentary on these findings, Hunt writes: "This study, in establishing the reality of increased Cambrian variability for trilobites, implies that evolutionary processes in the distant past may have acted differently, or in a different balance than in more recent periods of time." It is important to put the new research into context, for what Webster has done is to use trilobites to quantify a trend that is far from unique.
Hunt goes on to consider two possible ways of making sense of the findings: "These explanations fall into two broad categories: genetic and ecological. The former suggest that Cambrian genomes were less constrained, or otherwise less apt to generate profoundly novel morphologies, whereas the latter invoke the relative sparseness of early animal ecosystems in allowing large evolutionary jumps to become successfully established." This debate gets really interesting if it is recognised that the trilobite record is representative of the norm. "We need to tease apart what's controlling this pattern of high within-species variation. There's a lot more work to do," says Webster. With more and more and more evidences emerging of initial complexity, methodologies based on ID have much to offer. One hypothesis is that radiations occur because organisms are designed to vary, but the process results in genetic impoverishment that leads to stasis.
A Cambrian Peak in Morphological Variation Within Trilobite Species
Science, 317, 27 July 2007: 499-502.
Abstract: Morphological variation within species is a raw material subject to natural selection. However, temporal change in morphological diversity has usually been studied in terms of variation among rather than within species. The distribution of polymorphic traits in cladistic character-taxon matrices reveals that the frequency and extent of morphological variation in 982 trilobite species are greatest early in the evolution of the group: Stratigraphically old and/or phylogenetically basal taxa are significantly more variable than younger and/or more derived taxa. Through its influence on evolutionary tempo, high intraspecific variation may have played a major role in the pronounced Cambrian diversification of trilobites.
Hunt, G. Variation and Early Evolution, Science, 317, 27 July 2007: 459-460.
Fossils Older Than Dinosaurs Reveal Pattern Of Early Animal Evolution On Earth, Science Daily, July 26, 2007
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