Explosive speciation at the base of the adaptive radiation of Miocene grazing horses

Abstract

Adaptive radiations are generally considered to be periods of rapid evolutionary change¹. Simpson² viewed rates of evolution as either taxonomic or morphological and various methods for their quan-tification are widely used^1,3–5. In contrast to classic studies of extant organisms^6–7, adaptive radiations of fossil groups are usually analysed on the superspecific (for example, generic or familial) level. But relatively few studies have dealt with the patterns and processes of adaptive radiations for fossil species. The principal adaptive radiation of grazing horses occurred in North America 15 to 18 million years ago (Myr BP), when at least 19 species originated by cladogenesis. We report here that the pattern of horse taxonomic evolution is logistic, with very high origination rates observed early during the radiation. By 15 Myr BP diversity reached a plateau of about 16 contemporaneous grazing species, and this taxonomic carrying capacity continued until about 6 Myr BP. In contrast, rates of morphological (dental) evolution during the period were 'normal' (or holotelic) relative to other known fossil groups. Thus, adaptive radiations are times of rapid cladogenesis, but not necessarily rapid morphological change.