The Taxic Approach to Measuring Evolutionary Rates

Darwin (1859) predicted at first writing of The Origin that the rate of evolutionary change would be irregular. In Tempo and Mode in Evolution (1944), Simpson asked (p. 3): “How fast, as a matter of fact, do animals evolve in nature?” He confirmed Darwin's prediction that the rate of evolution is highly uneven and also concluded that bursts of morphological change are highly correlated with periods of cladogenesis. My purpose in what follows is to evaluate the means by which we measure rates of evolution in the fossil record, and what this means in our interpretation of variation of evolutionary rate. It is the conflation of speciation and morphological evolution that confuses us about the role of speciation in evolution.

Neontologists might think that paleontologists would routinely estimate the rate of evolution directly as the rate of change of morphological features such as size or number of spines. But surprisingly, these kinds of data have been collected, even to this day, rather sparsely and were not reviewed in great depth in Simpson's seminal monograph. Bed-by-bed collection is best applied to sections with continuous deposition and preservation (Hunter 1998). The greater the interval, the more one expects to see significant morphological change, but the chance of missing sections increases as well. This restricts fine-scale measurement of temporal morphological change to a few parts of the record. Much more commonly, paleontologists have used taxonomic longevity as an estimate of evolutionary rate.