Cambrian explosion: Those gaps, they just keep getting filled in, right?
|January 11, 2014||Posted by News under Cambrian explosion, News|
From Darwin’s Doubt:
Over the past 150 years or so, paleontologists have found many representatives of the phyla that were well known in Darwin’s time (by analogy, the three primary colors) and a few completely new forms altogether (by analogy, some other distinct colors such as green and orange, perhaps). And, of course, within these phyla there is a great deal of variety. Nevertheless, the analogy holds at least insofar a the differences in form between ay member of one phylum and any member of another phylum are vast, and paleontologists have utterly failed to find forms that would fill these yawning chasms in what biologists call “morphological space.” In other words, they have failed to find the paleontological equivalent of the numerous finely graded interediate colors (Pendleton blue, dusty rose, gun barrel gray, magenta, etc.) That interior designers covet. Instead, extensive sampling of the fossil record has confirmed a strikingly discontinuous pattern in which representatives ofthe major phyla stand in stark isolation from members of other phyla, without intermediate forms filling the intervening morphological space.
Foote’s statistical analysis of this pattern documented by an ever-increasing number of paleontological investigations demonstrates just how improbable it is that there ever existed a myriad of as yet undiscovered intermediate forms of animal life–forms that could close the morphological distance between the Cambrian phyla one tiny evolutionary step at a time.
In effect, Foote’s analysis suggests that since paleontologists have reached repetdly into the proverbial barrel, sampled it from one end to the other, and found only representatives of vaious radically distinct phyla but no rainbow of intermediates, we should’t hold our breath expecting such intermediates to eventually emerge. He asks, “whether we have a representative sample of morphological diversity and therefore can rely on patterns documented in the fossil record.” The answer, he says, is yes.
By this affirmation, he doesn’t mean that there are no biological forms left to discover. He means, rather, that we have good reason to conclude that such discoveries will not alter the largely discontinuous pattern that has emerged. “Although we have much to learn about the evolution of form,” he writes, the statistical pattern created by our existing fossil data demonstrates that “in many respects our view of the history of biological diversity is mature. (pp. 70–71)
Morphological analysis of four higher taxa of fossil marine invertebrates shows that, over the history of paleontology, there is no general tendency for morphologically extreme or modal species and genera to be described preferentially early or late. Reconstructing the expected evolutionary sequences of morphological disparity that would have been estimated at various times during the past century and a half reveals features that are sensitive to sampling (for example, peak trilobite disparity in the Ordovician, peak of post-Paleozoic crinoid disparity in the Triassic, and peak blastoid disparity in the Permian), as well as more robust features (for example, increase in trilobite disparity from the Cambrian to the Ordovician, continued increase in trilobite disparity despite a drop in taxonomic diversity after the Early Ordovician, decrease in blastoid disparity from the Devonian to the Carboniferous, and increase in crinoid disparity from the Jurassic to the Cretaceous followed by decline during the Cretaceous). Although we still have much to learn about the evolution of form, in many respects our view of the history of biological diversity is mature.
Sampling, taxonomic description, and our evolving knowledge of morphological diversity. Paleobiology, 23: 181–206. Foote, M. 1997c.
Any big changes since then? Nope? Okay, so the gaps aren’t getting filled in.
The most likely reason now is that the gaps don’t exist. Which just mean that the history of evolution, whatever it is, is not Darwinian.
Unlike the God of Evolution, whose thundering opinion can be viewed here.
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