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