The causes of repeated genetic evolution.

A general understanding of the evolutionary process is limited by the contingency of each evolutionary event, making it difficult, even retrospectively, to explain why things have unfolded the way they have. The repeated evolution of similar traits in organisms facing similar environmental conditions is a pervasive phenomenon, including for animal morphology, and is considered a strong evidence for adaptive evolution. Examples of repeated evolution of particular traits offer a unique opportunity to ask whether evolution has followed similar or different genetic paths. Case studies reveal that although multiple genetic paths were often possible to evolve a morphological trait, similar evolutionary trajectories have been followed repeatedly in independent lineages, suggesting that biases influence the course of genetic evolution. In the light of these examples we examine several factors influencing the genetic paths of adaptive evolution and in particular how the interplay between natural selection and genetic variations carves out predictable genetic trajectories of morphological evolution.