The Rate of Morphological Evolution in Mammals from the Standpoint of the Neutral Expectation

A comparison of the evolutionary rates of cranial morphology in mammals with the neutral expectation suggests that stabilizing selection is a predominant evolutionary force keeping the long-term diversification of lineages well below its potential. The rate of morphological divergence of almost all lineages, including the great apes, is substantially below the minimum neutral expectation. The divergence of the modern races of man is slightly above the minimum neutral rate but well below the maximum rate. Therefore, there is no need to invoke extraordinary mutational mechanisms, such as regulatory gene evolution, to explain what has been perceived as rapid morphological evolution in mammals. Nor does it appear that behavioral drive needs to be invoked to explain rapid morphological evolution in hominoids. Outside of man, the long-term rate of phenotypic evolution in the great apes is actually lower than that for other mammals. The data suggest that, immediately after reproductive isolation, most lineages diverge morphologically at approximately the neutral rate and that this rate declines over evolutionary time. Such a pattern is consistent with a broad class of phenotypic evolutionary models that invoke an interaction between the forces of random genetic drift, polygenic mutation, and stabilizing selection. This pattern also suggests that the apparently rapid rates of morphological evolution in modern man relative to other mammals and in mammals relative to other vertebrates are artifacts of temporal scaling.