Pedigrees of neurobehavioral circuits: tracing the evolution of novel behaviors by comparing motor patterns, muscles, and neurons in members of related taxa.

Comparisons of homologous elements in neurobehavioral circuits that have diverged during speciation to mediate different behaviors should reveal the nature of evolutionary changes in nervous systems. When the pedigree of a particular behavior can be traced-by comparing motor patterns and their neural substrates in related taxa whose phylogeny is known from other (non-neurobehavioral) criteria-divergent and convergent evolutionary changes can be distinguished and the order of their occurrence reconstructed. An example of reconstructing a behavioral pedigree (for the novel mode of swimming in the crab Emerita [Hippidae]) is presented, and implications about the evolution and organization of neurobehavioral circuits engendered by this and some other studies of functionally defined neuronal networks are reviewed. Specific neural differences in related animals can only be attributed to natural selection when they can be related to species differences in function or behavior. Differences that cannot be so related, as well as apparently non-adaptive characters in individual nervous systems, are attributed to ontogenetic processes, which apparently, in some cases, introduced and, in other cases, resisted change through evolutionary time. More expressly-comparative investigations of discrete neurobehavioral circuits are needed for an understanding of the interdependence of evolutionary processes and ontogenetic and functional constraints on the organization of neuronal systems.