Geographic Variation in Sexual Behavior: Sex, Signals, and Speciation

Abstract

The chapters in this volume share the theme that our understanding of pattern, process, and consequence in the study of behavioral evolution can be advanced by examining differences among conspecific populations. Traditionally, biologists have sought such understanding by comparing different species. Although differences among species are usually greater than differences among conspecific populations, so many factors can vary interspecifically that determining selection pressures driving behavioral divergence may be difficult. As Arnold (1992) has argued, confounding variables often are less prevalent in intraspecific studies; in addition, relatively small evolutionary changes may be perceptible. From a historical perspective, there appear to be good reasons for believing that sexual behavior should show little variation among conspecific populations. First, early species concepts largely were typological, and accorded intraspecific variation with no reality, let alone importance (Mayr 1976). Partially due to such thinking, early ethologists argued that certain behavior patterns should be largely invariant. For Lorenz (1970), courtship behavior was a prime example of such a “fixed action pattern” (or FAP), a predictable and stereotyped sequence of actions that was elicited by a specific releasing stimulus. Later work revealed that such sequences are more variable than once thought, leading to the suggestion that the FAP be replaced by the MAP, or “modal action pattern.” This stresses the average or modal nature of much behavior (Barlow 1977). The second reason for expecting little intraspecific variation in sexual behavior derives in part from the modern synthesis. The “founding fathers” of modern evolutionary biology placed great emphasis on the role of species differences in preventing interspecific mating and wastage of reproductive effort in the production of unfit hybrid offspring (e.g., Dobzhansky 1937). Indeed, Tinbergen (1953) stated that one of the functions of mating behavior is to ensure such reproductive isolation among species. Presumably, selection would strongly favor the production of unambiguous signals, leading to invariance. The earliest evidence demonstrating the existence of intraspecific variation in sexual behavior patterns came from studies that were firmly rooted in concepts that characterized the early stages of the modern synthesis.