Homology in Character Evolution

Abstract Homology forms the basis of organisation for comparative biology. Richard Owen's simple definition of homology as the ‘same organ in different animals under every variety of form and function’ takes on new meaning in light of Darwin's concept of descent with modification. The modern study of comparative biology and phylogenetics is grounded in the notion that organisms sharing a greater proportion of homologous (i.e. ‘derived’) characteristics share a more recent common ancestry. Instead of solely being the purview of anatomical study, homology of these characteristics can be applied at the level of molecular evolution, genomic organisation and modern evolutionary developmental biology (‘evo/devo’). Aspects of homology help us formulate hypotheses about the meaning of changes in homologous structures in the inference of evolution of organisms at all levels of biological organisation. Key Concepts Darwin's theory of descent with modification restructured Owen's concept of positional similarity. Ancestral ‘types’ are artificial attempts to combine major lineage features into a single progenitor. Evolution of traits can be used to assess lineage history through phylogenetics inference at multiple levels: DNA primary sequence, genomic organisation, developmental pathways or gross morphology. Pattern forming genes can give other insight into the factors leading to shifts in morphology. Deep homology may elucidate early evolutionary events, but these must be taken in context since such information can be only similar and descent with modification often retools preexisting pathways to be applied in novel ways. Appearance of commonality may be only similarity.