Behavioral Drive or Behavioral Inhibition in Evolution: Subspecific Diversification in Holarctic Passerines

Abstract

Abstract. Behavioral changes have long been hypothesized to be an important driver of evolutionary diversification in animals, as they expose individuals to new environmental pressures and thus favor evolutionary divergence. There have been few empirical tests of this hypothesis, however, and the mechanisms linking behavioral changes and diversification processes remain controversial. We show here that Holarctic passerines with large brain size relative to body size, a character correlated with a high propensity for behavioral changes, generally have experienced more extensive subspecific diversification. This effect appears to be largely independent of other well-known mechanisms thought to promote diversification. As suggested by path analysis, relative brain size seems to affect diversification directly rather than indirectly through its presumed effect on range expansion, which is consistent with the original formulation of the behavioral drive hypothesis. Thus, the results support the long-held, intuitive hypothesis that behavioral changes facilitate evolutionary diversification. In evolutionary biology, behavior is classically viewed as an important driver of evolutionary change (e.g., Baldwin 1896;Miller 1956;Mayr 1963;Wyles et al. 1983;Bateson 1988;Fitzpatrick 1988;Wcislo 1989;West-Eberhard 1989;Greenberg 1990;Futuyma 1998;Futuyma and Moreno 1998). The idea is that changes in behavior can allow animals to enter new adaptive zones and that this can then favor divergent selection on morphology, physiology, and/or behavior. Mayr (1963, p. 604), for example, argued that ''behavioral changes in habitat and food selection are necessary in the shift to new adaptive zones, with structural changes in morphology acquired secondarily. '' Wyles et al. (1983, p. 4396) expressed a similar view when they wrote that ''As a consequence of adopting a new habit, the species faces a new set of selection pressures favoring those mutations that improve the individual's effectiveness at living in the new way.'' The concept that adaptation to a new niche is initiated by behavioral changes is implicit in influential models of population diversification leading to speciation (e.g

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