Ontophyletics of the nervous system: eyeless mutants illustrate how ontogenetic buffer mechanisms channel evolution.

Genetics and molecular biology have shown the mechanisms that allow the genome to provide both the continuity and the variation from generation to generation within a phylogeny. Embryology and developmental biology show the mechanisms that turn the genome into an organism. Mutations, the basis for evolutionary change, cannot in themselves ensure concordance between their products and the products of unchanged genes. Thus, mutations will not necessarily produce a viable organism. On the other hand, ontogenetic buffer mechanisms normally maintain concordance in the developing organism. In addition, ontogenetic buffer mechanisms can integrate discordant mutations into viable organisms that can then be perpetuated during evolution. The evolutionary role of one ontogenetic buffer mechanism, compensatory innervation, is well illustrated in the anopthalmic mutant mouse. In the anopthalmic mouse, a single gene mutation removes afferent axons of the dorsal lateral geniculate nucleus, and compensatory innervation by another population of axons ensures that the dorsal lateral geniculate remains integrated into the central nervous system. Within each organism's ontogeny is a hierarchy of sources of compensatory innervation, and this hierarchy will determine how any particular deafferentating mutation will be buffered. In this way, an ontogeny can channel the phylogeny of which it is a member.