Sexual Selection, Social Competition, and Speciation

Abstract

Rapid divergence and speciation can occur between populations with or without ecological differences under selection for success in intraspecific social competition-competition in which an individual must win in contests or comparisons with conspecific rivals in order to gain access to some resource, including (under sexual selection) mates. Sexual selection theory is extended to encompass social competition for resources other than mates. Characters used in social competition can undergo particularly rapid and divergent evolution owing to (1) their great importance in determining access to critical resources, (2) the absence of a limit to change (except by selection in other contexts), (3) the generation-to-generation relentlessness of selection on these traits. (4) the potential for mutually accelerating evolution of preference and attractiveness in contests involving "choice," and (5) the very large number of factors that can initiate trends, including mutation and drift leading to use of different physiological or behavioral characteristics as signals, the role of novelty per se in the evolution of combat and display, ecological or habitat differences influencing the form of combat and of signals, and (in species capable of learning) imitation of idiosyncratic characteristics of successful individuals. Many species-specific signals heretofore attributed to selection for species recognition ("isolating mechanisms") are probably instead products of social selection. This may help explain the rarity of reproductive character displacement and other phenomena predicted by the species recognition hypothesis. Examples from a wide variety of organisms illustrate patterns predicted by social selection theory, including (1) exaggeration and rapid divergence of traits (e.g., weapons, pheromones, plumage, flowers, and song) used in social competition, (2) a correlation between type of social system (intensity of social selection) and distinctiveness and exaggeration of social traits, (3) sexually monomorphic extreme development of socially selected traits when both sexes compete socially, (4) occurrence of distinctive signals in allopatric populations lacking sympatric congeners, and (5) more rapid divergence (less phylogenetic conservatism) of socially competitive compared to non-competitive signals. Rapid divergence under social selection may accelerate speciation due to effects on pre-mating interactions, as well as on critical social determinants of survival and reproductive success which would put hybrids at a disadvantage. Maintenance of parapatric boundaries (extensive contact with little or no geographic overlap) between socially selected species may sometimes be due to competitive exclusion in sympatry between populations whose primary divergence has been social rather than ecological. Patterns of variation in socially selected characters demonstrate the wisdom of Darwin's distinction between natural and sexual selection, and the applicability of sexual selection theory to social competition in general.