The recognition and evaluation of homoplasy in primate and human evolution

Homoplasy has been a prominent issue in primate systematics and phylogeny f or as long as people have been studying human evolution. In the past, homop lasy, in the form of parallel evolution, was often considered the dominant theme in primate evolution. Today, it receives blame for difficulties in ph ylogenetic analysis, but is essential in the study of adaptation. This pape r reviews the history of study of homoplasy, methods of defining homoplasy, and methodological and biological factors that generate homoplasy. A post hoc definition of homology and homoplasy, based on patterns of character di stributions and their congruence or incongruence on a cladogram, is the mos t consistent method of recognizing these phenomena. Defined this way, homol ogy and homoplasy are mutually exclusive. However, when different levels of analysis are examined, it is seen that homoplasy at one level, such as adu lt phenotype, often exists simultaneously with homology at a different leve l, such as developmental process. Thus, in some cases, patterns of homoplas y may point to underlying similarities that reflect the shared heritage of a particular clade. This is an old concept that is being renewed on the str ength of recent trends in developmental biology. Factors that influence hom oplasy include character definition and a host of biological factors, such as developmental constraints, allometry, and adaptation. These interact wit h one another to provide explanations of homoplastic patterns. Because of t he repetition of events, explanations of homoplastic features are often mor e reliable than those for homologous features, and serve as effective tests for hypotheses of evolutionary process. In some cases, particular explanat ions of homoplasy lead to generalizations about the likelihood of homoplasy in a type of structure. The structure may be adaptive or highly epigenetic , or it may belong to an anatomical system considered to be more prone to h omoplasy than others. However, our review shows that these generalizations are usually based on theory, and contradictory expectations can be develope d under different theoretical models. More rigorous empirical studies are n ecessary to discover what, if any, generalizations can be made about the li kelihood of homoplasy in different types of characters. (C) 1999 Wiley-Liss , Inc.