Evolutionary Radiation of Equids

Horses became ubiquitous elements of Cenozoic communities and reached high diversity in Neogene times. Most accounts of their diversification history have so far focused on the Early Miocene cladogenesis of the subfamily Equinae (the so-called “grazing horses”), interpreting raw diversity counts at face value. In this chapter, we reconstruct speciation and extinction in horses, while accounting for sampling biases, and interpret these trends in the context of the evolution of body size and relative tooth crown height (hypsodonty). We found that fast species proliferation (speciation rate) is a shared feature in anchitehres and equines, likely stemming from the more changeable environments of the Neogene. The fast early-phase radiation in the subfamily Equinae was likely the result of reduced extinction rates and higher lineage survival rather than a substantial acceleration in speciation. The evolution of body size and hypsodonty was not faster in regions of the phylogeny with faster diversification, suggesting a broad-scale decoupling of ecomorphological and taxonomic diversification. Our models show that major phenotypic trends were not caused by phyletic progression, derived from sustained, ubiquitous directional selection, but more likely was the outcome of differential lineage-level survival and multiplication (species sorting).