Wednesday, October 01, 2008

Punctuation Error?   posted by DavidB @ 10/01/2008 04:59:00 AM

Readers who lived through the Punctuated Equilibrium controversy of the 70s and 80s will recall that it petered out rather inconclusively, largely for lack of decisive empirical evidence one way or the other. The fossil record is seldom good enough to distinguish unambiguously between punctuational and gradual modes of evolution, one problem (noted already by Darwin) being that the sudden appearance of a new form in a given locality may result from migration rather than rapid evolution in the same place.

Given these difficulties, a disproportionate amount of attention was focused on a handful of examples that seemed to show good evidence either of punctuational or gradual evolution. One of the best examples on the punctuationist side of the debate was a study of molluscs in the Turkana Basin of Africa by P. G. Williamson [Note 1] Williamson's study was criticised at the time on various grounds - for example that the changes observed might be due to environmental stress rather than genetic evolution - but the critics did not produce new evidence from the field.

That is changed by an article [Note 2] by a Dutch team in a recent issue of the journal Evolution....

The Abstract of the article is as follows:

A running controversy in evolutionary thought was Eldredge and Gould's punctuated equilibrium model, which proposes long periods of morphological stasis interspersed with rapid bursts of dramatic evolutionary change. One of the earliest and most iconic pieces of research in support of punctuated equilibrium is the work of Williamson on the Plio-Pleistocene molluscs of the Turkana Basin. Williamson claimed to have found firm evidence for three episodes of rapid evolutionary change separated by long periods of stasis in a high-resolution sequence. Most of the discussions following this report centered on the topics of (eco)phenotypy versus genotypy and the possible presence of preservational and temporal artifacts. The debate proved inconclusive, leaving Williamson's reports as one of the empirical foundations of the paradigm of punctuated equilibrium. Here we conclusively show Williamson's original interpretations to be highly flawed. The supposed rapid bursts of punctuated evolutionary change represent artifacts resulting from the invasion of extrabasinal faunal elements in the Turkana palaeolakes during wet phases well known from elsewhere in Africa.

I have read the full article (available here), which looks convincing on this particular case (but what do I know about old African molluscs?) [Added: a more easily readable pdf version is also available. Google 'bocxlaer turkana' and you should find it.] The strongest point is that it is not just armchair criticism but based on extensive new fossil collecting. But since I specialise in armchair criticism I can hardly throw any stones.

Obviously one such case doesn't disprove punctuated equilibrium, but Williamson's study was in some ways the 'poster child' for the theory (more so than even Eldredge and Gould's own studies), so its demolition (if accepted) would be a serious blow.

Note 1: P. G. Williamson, 'Palaeontological documentation of speciation in Cenozoic molluscs from Turkana Basin', Nature, 1981, 293, pp.437-43. Also reprinted in Evolution Now, ed. John Maynard Smith, 1982. I can't find any publications by Williamson after 1990, and I believe I have read somewhere that he died at a sadly early age. My apologies if I am mistaken.

Note 2: Bert van Bocxlaer, Dirk van Damme, and Craig S. Feibel, 'Gradual versus punctuated equilibrium evolution in the Turkana Basin molluscs: evolutionary events or biological invasions?', Evolution, 2008, 62, pp.511-20.

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