Diversification of the Ruminant Skull Along an Evolutionary Line of Least Resistance

Morphological integration is relevant to evolutionary biology and paleontology because the structure of variation within populations determines the ways in which a population can respond to selective pressures. However, understanding the macroevolutionary consequences of morphological integration is elusive because the adaptive landscape is dynamic and population-level constraints themselves evolve. By analyzing a previously published dataset of 2859 ruminant crania with 3D geometric morphometrics and phylogenetic comparative methods, we find that variation within and between ruminant species is biased by a highly conserved mammalian-wide allometric pattern, CREA, where larger species have proportionally longer faces. More tightly integrated species and species more biased towards CREA have diverged farther from their ancestors, and Ruminantia as a clade diversified farther than expected in the direction anticipated by CREA. Our analyses indicate that CREA acts as an evolutionary line of least resistance and facilitates morphological diversification due to its alignment with the browser-grazer continuum. These results demonstrate that biological processes constraining variation at the microevolutionary level can produce highly directional phenotypic evolution over macroevolutionary timescales.