Evo-devo mechanisms underlying the continuum between homology and homoplasy.

The different manifestations of equivalence and similarity in structure throughout evolution suggest a continuous and hierarchical process that starts out with the origin of a morphological novelty, unit, or homologue. Once a morphological unit has originated, its properties change subsequently into variants that differ, in magnitude, from the original properties found in the common ancestor. We will look into the nature of morphological units and their degrees of modification, which will provide the starting point for restructuring the concept of "homology," keeping the use of homology as the identity of an anatomical part, and homogeny, as the specific variation of that anatomical part during evolution. We will also show that parallelism has a distinct placement within an evolutionary continuum between homology and homoplasy, whereas the phenomenon of evolutionary convergence is left outside this continuum. We will then provide some epistemological and developmental criteria to justify these distinctions, showing that there is a direct relation between the nature of these concepts and the constraints that developmental mechanisms impose on evolution. Finally, we will propose a hierarchical model that places homology, homogeny, homoplasy, and parallelism, as distinct phenomena within an evolutionary continuum.