© 1984 by The Society for Integrative and Comparative Biology
Evolution of the Vertebrate Central Nervous System: Patterns and Processes1
Division of Biological Sciences, The University of Michigan Ann Arbor, Michigan 48109
AS brains do not fossilize, most proposed phylogenetic sequences for central nervous system characters must be based on the patterns of variation of those characters in living organisms. Similarly, hypotheses regarding how brains change through time, and the evolutionary processes that produce these changes, are ultimately based on the character patterns recognized. It is critical in these analyses to distinguish between homologous and homoplasous characters if errors in the reconstruction and interpretation of phylogenies are to be minimized. Definitions of homology and homoplasy are reviewed, as are the concepts that bear on their application. Cladistic definitions are adopted, and criteria for distinguishing homologous from homoplasous characters are discussed. Analysis of a number of CNS characters that are usually assumed to be homologous reveals that homoplasous characters appear among them. As in other organ systems, homoplasous characters are actually common. A number of previous hypotheses regarding CNS evolution are reviewed in the context of new data on neural connections and their cladistic analysis. Some of these hypotheses may be falsified by a cladistic treatment of CNS characters, whereas sufficient data do not exist to evaluate others.