A new study analyzing the evolution of horses suggests that patterns of migration and changes in environment drive the development of new traits, countering a theory called rapid phenotypic evolution that proposes the opposite -- that is, that development of traits is what allows a species to take over new niches. Essentially, the ongoing debate in this space is a chicken-or-egg scenario: do traits evolve first, to facilitate dispersion of a species into new environments, or does dispersal into new environments drive the development of new traits? If the former, rapid phenotypic evolution, were to take place, changes of ecologically relevant traits should be faster in the early phases of the clade's expansion, as lineages move into new environments that require new traits for survival. Yet, when J. L. Cantalapiedra et al. looked at the evolution of horses over the course of millions of years, they did not see such a pattern. Here, the researchers analyzed the fossilized features of 138 species of horses representing an evolutionary tree spanning roughly 18 million years ago until present day. They focused on body size and tooth size (important to evaluate because evolution of bigger teeth may help horses eat new types of vegetation). Their results reveal that rates of body size evolution were not significantly different in lineages exhibiting high and low speciation, and rates of tooth evolution were significantly lower in lineages with fast speciation rates. The researchers also looked at how horses adapted to the New World. They note that although it has traditionally been thought that complementary traits evolved as grasslands emerged, more recent evidence suggests that the grasslands were already established before the arrival of horses. Equipped with an understanding that this environment was already established, in combination with speciation data, the authors propose that the environment and patterns of migration drove the development of new traits.