The benefits of communication between comparative ethology and phylogenetic systematics: a case study using gasterosteid fishes

Early comparative ethologists stressed both phylogenetic (historical) and environmental (selection) factors when searching for explanations of behavioural evolution. The last two decades have witnessed a narrowing of ethologists' evolutionary perspective to questions concerning environmentally based maintenance of behaviours. This approach, with its reliance on subjective, a priori assumptions concerning the temporal sequence of ethological modifications, dissociates character evolution from underlying phylogenetic relationships. This, in turn, is responsible for the tendency of many researchers to confuse character maintenance (stasis) with character origin and divergence (evolution). The decline of phylogenetically based explanations of behavioural evolution is mirrored by the decline in the utilization of behavioural data in systematic analyses. Yet, since behaviour is closely intertwined with development, physiology, and morphology and thus subject to the same evolutionary processes and constraints, it should change in ways that reflect underlying phylogenetic relationships. Communication between comparative ethologists and systematists is restricted. As an example of the potential benefits of interdisciplinary communication, a phylogenetic systematic analysis of the teleostean family Gasterosteidae (sticklebacks) based solely upon behavioural characters is presented. The phylogenetic tree derived from this analysis has a consistency index of 90.3%. In this particular case, the behavioural data provide a less ambiguous picture than the morphological data. The phylogenetic tree, in turn, is a hypothesis of the temporal sequence of behavioural evolution. From this hypothesis, it is possible to trace the origin, divergence, and interrelationships of agonistic, sexual, and parental behaviours in gasterosteids, and to compare this macroevolutionary patterning with predictions of character evolution based on microevolutionary studies. Examination of behavioural evolution within a phylogenetic framework provides a more robust characterization of evolutionary history. By analyzing and comparing structural (biochemical, morphological) and functional (ecological, behavioural) aspects of the phenotype, future phylogenetic studies will enable us to develop a more comprehensive picture of the patterns of biological evolution.