Conservative behavioural evolution, the neural substrate

Abstract

The evolution of neural circuitry differs from that of other structures in a significant respect. Natural selection cannot physically eliminate most neuronal pathways for obsolete functions, because the individual neurons of these pathways usually are multifunctional, also being components of pathways for adaptive functions. Selection for loss of obsolete neural functions probably operates largely through mechanisms that suppress or neutralize the corresponding neuronal pathways without disabling pathways for adaptive functions. Accordingly, rather than becoming eliminated physically, obsolete neuronal pathways tend to become inhibited or have their outputs blocked; obsolete pathways not slected against merely cease to be accessed. These mechanisms of loss of neural function provide one of several intrinsically conservative bases for neural ontogeny and evolution. Since the central nervous system encodes and programmes motor outputs for behaviour, the obsolete circuits that are retained include pathways for obsolete behaviour. In vertebrates, such behaviour can sometimes be elicited directly by appropriate stimuli, but when the neuronal pathways for the obsolete behaviour are in an inhibited or blocked condition, they first must be rendered accessible (which can occur adventiously, pathologically, experimentally or in hybrids). The vertebrate central nervous system also plays a large role in programming the ontogeny and maturation of gametes. Egg formation in many species appears to be highly conservative, extensively retracing ancestral stages. This conservativeness probably derives from influences both of being under neural control and of having an evolutionary history in which the provisioning of eggs was continually on the increase.