Demystifying phenotypes: The comparative genomics of evo-devo

Developmental geneticists have spearheaded the synthesis of evolutionary and developmental biology, a.k.a 'evo-devo', leading to a wealth of recent insights about how morphological diversity has evolved. However, there exists a gap between these disciplines, and evo-devo has not benefited from an integration of the principles derived from population genetics and molecular evolution. In order to contribute to the remediation of this deficiency, we recently performed a study investigating how genes diverge among closely related species of Drosophila as a function of when they are expressed during development. We found that patterns of genetic divergence parallels morphology: interspecific divergence accumulates as development progresses. We also sought to test whether this positive gradient of divergence over ontogeny is best explained by purifying selection constraining the divergence of early-expressed genes or positive selection driving the evolution of those expressed later. Interestingly, we found evidence that both processes occur simultaneously. We argue that comparative genomics approaches, by juxtaposing gene- and phenotypelevel divergence, particularly among closely related species, have much to contribute to the ongoing evo-devo synthesis, complementing traditional genetics-based techniques with largescale screening analyses uncovering the mechanisms underlying developmental change.