Decoupled ecomorphological evolution and diversification in Neogene-Quaternary horses

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Science  10 Feb 2017:
Vol. 355, Issue 6325, pp. 627-630
DOI: 10.1126/science.aag1772

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What drives divergence?

Horse evolution has long been held as a classic example of adaptive radiation. It has been thought that an increase in the height of cheek teeth opened up new grass resources, leading to divergence. Cantalapiedra et al., however, found that although the Equinae have experienced high levels of divergence, these splits do not appear to have been related initially to specific phenotypic changes. Instead, it seems that external environmental drivers and patterns of migration and isolation initiated population divergence, with phenotypic changes emerging once lineages had begun to divide.

Science, this issue p. 627


Evolutionary theory has long proposed a connection between trait evolution and diversification rates. In this work, we used phylogenetic methods to evaluate the relationship of lineage-specific speciation rates and the mode of evolution of body size and tooth morphology in the Neogene and Quaternary radiation of horses (7 living and 131 extinct species). We show that diversification pulses are a recurrent feature of equid evolution but that these pulses are not correlated with rapid bursts in phenotypic evolution. Instead, rapid cladogenesis seems repeatedly associated with extrinsic factors that relaxed diversity bounds, such as increasing productivity and geographic dispersals into the Old World. This evidence suggests that diversity dynamics in Equinae were controlled mainly by ecological limits under diversity dependence rather than rapid ecomorphological differentiation.

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