Homoplasy: From Detecting Pattern to Determining Process and Mechanism

Understanding the diversification of phenotypes through time—“descent with modification”—has been the focus of evolutionary biology for 150 years. If, contrary to expectations, similarity evolves in unrelated taxa, researchers are guided to uncover the genetic and developmental mechanisms responsible. Similar phenotypes may be retained from common ancestry (homology), but a phylogenetic context may instead reveal that they are independently derived, due to convergence or parallel evolution, or less likely, that they experienced reversal. Such examples of homoplasy present opportunities to discover the foundations of morphological traits. A common underlying mechanism may exist, and components may have been redeployed in a way that produces the “same ” phenotype. New, robust phylogenetic hypotheses and molecular, genomic, and developmental techniques enable integrated exploration of the mechanisms by which similarity arises. Phenotypes and taxa are expected to divergeas evolution proceeds. Thus, when diver-gent lineages are found to be morpholog-ically similar, explanation is needed. Homoplasy is similarity that is the result not of simple an-cestry, but of either reversal to an ancestral trait in a lineage or of independent evolution (conver