Evolution of the nervous system: role of ontogenetic mechanisms in the evolution of matching populations.

Nervous systems are composed of populations of cells that are synaptically connected in a highly predictable manner, and we have called two interconnected populations a pair of matching populations. Heritable genetic changes that affect a pair of matching populations can be evolutionary only when this matching quality is not disrupted. We distinguish two types of heritable change. Concordant heritable changes autonomously preserve the match and are thus automatically candidates for what we call type I evolutionary change. Nonconcordant heritable changes, on the other hand, are those that do not autonomously preserve the match. Those nonconcordant heritable changes that can use other normally present ontogenetic mechanisms to preserve the match are candidates for what we call type II evolutionary change. One example of such an ontogenetic mechanism consists of the production of excess neuroblasts and the subsequent weeding out (via cell death) of those that do not successfully match. Because normal ontogeny is an integral part of type II evolutionary change, ontogenetic manipulations can give evolutionary insights. Embryonic graft experiments, in particular, can elucidate the nature of ontogenetic mechanisms that participate in type II changes. Thus, some developmental experiments can be considered to be evolutionary experiments.