Diversity and Disparity

One of the central (and controversial) aspects of the Cambrian Explosion— especially with reference to the wondrous assemblage of fossils found in the Burgess Shale localities in Western Canada (where not only early animals with hard parts but also forms without skeletons are preserved as smears on the rocks)—concerns what are called diversity and disparity. Diversity (or in this case biological diversity) is a term familiar to most of us. Overtly, it is usually understood as a measure of the number of species present. Biologists use a more sophisticated meaning, that encompasses not only the number of species present in a locality, but also the relative abundance of those species. For instance, in this more technical sense, an assemblage of organisms comprising some given number of species, each with the same number of individuals in each species, is considered more diverse than a second assemblage with the same number of species but a highly unequal distribution of the numbers of individuals that compose each species group. Disparity is a measure of the number of body plans, types, or design forms, rather than the number of species. This distinction, first articulated by paleontologist Bruce Runnegar, seems at first glance rather subtle. Surely each different species has a somewhat different body plan from every other, and thus disparity and diversity should always be equal. But this is not the case. There are millions of species on Earth today. Yet the number of general body plans is far less than this.

Among animals, the major body plans are found within the major evolutionary lineages, the phyla. As we have seen, these animal groups all arose in the Cambrian Explosion. Yet the surprising finding of paleontology is that there were very few species in the Cambrian. In his 1989 book Wonderful Life, Stephen Jay Gould describes this finding as "a central paradox of early life: How could so much disparity in body plans evolve in the apparent absence of substantial diversity in number of species?"

The history of diversity and disparity during the Cambrian Explosion (or, more properly, creating the Cambrian Explosion) is another puzzling aspect of planet Earth's diversification of animals: Is this the only way to create animals, or just one way? Will every planet with animals create them as ours did, by evolving the entire spectrum of body plans in one great evolutionary rush among a low number of species? Or could this process be more gradual, with a slowly increasing number of species over long periods of time incrementally enlarging the number of body plans?

The Burgess Shale is clearly of major importance in understanding the initial diversification of animal life. It is largely responsible for showing us that most or all of the various animal phyla (or major body plans) originated relatively quickly during the Cambrian. But the Burgess Shale may also be telling us that not only were the body plans found on Earth today around in the Cambrian, but so too were other body plan types now extinct. One of the central messages of Gould's Wonderful Life is that the Cambrian was a time not only of great origination but also of great extinction, for Gould (and others as well) assert that far more phyla were present in the Cambrian than exist today. How many were there? Some paleontologists have speculated that there may have been as many as 100 different phyla in the Cambrian, compared to the 35 still living today. Gould clearly believes that there were more Cambrian than present-day phyla: "[W]e may acknowledge a central and surprising fact of life's history—marked decrease in disparity followed by an outstanding increase in diversity within the few surviving designs."

This view—so forcefully and elegantly described in Gould's Wonderful Life—is vigorously disputed in British paleontologist Simon Conway Morris's 1998 book Crucible of Creation, about the Burgess Shale and Cambrian Explosion. Conway Morris is, ironically enough, a central and sympathetic figure in Gould's book. He is one of the architects of our new understanding of the Cambrian Explosion. But Conway Morris denies that disparity has been decreasing since the Cambrian, citing several cases that suggest just the opposite. He also attacks Gould's metaphor of "replaying the tape" by showing how convergence in evolution (where distinct lineages evolve similar body types in response to similar environmental conditions) can produce the same types of body plans from quite unrelated evolutionary lineages. Conway Morris argues that even if the ancestor of vertebrates went extinct during or soon after the Cambrian, it is likely that some other lineage would have evolved the body plan with a backbone, because this design is optimal for swimming in water. This view is quite antithetical to that espoused by Gould. We thus have several models for diversification (see Figure 7.3), and how it actually occurred on Earth is still in doubt.

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