Brain-body coevolution in incipient versus established primate species—evaluating Simpson’s “most important distinction”

Are differences between species the long-term consequence of microevolution within species, or does speciation involve fundamentally different processes? We analyzed the brain and body sizes of present-day primate species using a novel phylogenetic comparative method that decomposes the phenotypic covariance of these traits into speciational and anagenetic components. We estimated that approximately half of speciation events are accompanied by accelerated phenotypic change. Equivalent in magnitude to approximately 7 million years of gradual microevolution, such speciational changes in brain and body size account for about 58% of the phenotypic variation among extant species. Interestingly, speciational changes in brain and body size appear significantly less correlated (r ≈ 0.83) than gradual, microevolutionary changes in these same traits (r ≈ 0.97). This indicates that the strong allometric constraint that dictates microevolution in brain and body sizes is relaxed at speciation events. These results suggest that phenotypic evolution is not only accelerated during speciation but also involves events that seldomly occur at microevolutionary timescales.