Homology and homoplasy

Abstract

Homology and analogy both refers to similar parts (features) of organisms. Homology at the level of the phenotype (phenotypic or structural homology) is the continuous occurrence of the same feature (be it gene, gene network, cell type, tissue, organ, structure, or behavior) in two organisms whose common ancestor possessed the feature. Homologous features need not be identical but must share sufficient “similarity” to be recognizable as homologous. Homology is similarity that reflects common descent and ancestry. Another way of comparing and classifying features among organisms is homoplasy. Homoplasy is similarity (some might say superficial similarity) arrived at via independent evolution. Homology may or may not imply conserved development. Homoplasy implies divergent development. Consequently, divergence in developmental pathways is not an adequate criterion to establish features as homoplastic. Homoplastic features (independent evolution) can share an affiliation (shared developmental processes) with homologous features. The common basis for considering features as homoplastic is their independent evolution one from the other. However, homoplasy is a portmanteau term for classes of similarity otherwise subsumed under terms such as convergence, parallelisms, reversals, rudiments, vestiges, and atavisms.