Deep homology

In evolutionary developmental biology, the concept of deep homology is used to describe cases where growth and differentiation processes are governed by genetic mechanisms that are homologous and deeply conserved across a wide range of species.

Deep homology on the molecular level may correspond to a range of relationships between the processes they enable, including analogy/convergent, homoplasy/parallel, and true homology. In the two former cases, these processes are attributed to independent exaptation (co-option) of the same underlying genetic mechanism. Determining the nature of each case of observed deep homology and "where homology ends and novelty begins" often requires extended taxonomic sampling using living or fossil species.