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. 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. Immediately after reproductive isolation, most lineages diverge morphologically at approximately the neutral rate, and this rate declines over evolutionary time, a pattern consistent with a broad class of phenotypic evolutionary models that invoke an interaction between the force of random genetic drift, polygenic mutation, and stabilizing selection. -from Author