Darwin's forgotten idea: The social essence of sexual selection

• Social interaction is the essence of Darwin's idea of sexual selection.
• Extreme traits often result from social effects of signal-response systems.
• Social effects are easily forgotten when analyses focus on natural selection.
• Social consequences often dominate the evolution of communication systems.
• Social selection can overpower natural selection in the evolution of communication.

Darwinian sexual selection can now be seen in the broader context of social selection, or social competition for resources (under sexual selection, mates or fertilization success). The social-interaction aspects of sexually selected traits give them special evolutionary properties of interest for neurobiological studies of stimulus–response systems because they can account for highly complex systems with little information content other than stimulatory effectiveness per se. But these special properties have a long history of being forgotten when other factors dominate the analysis of male–female interactions, such as the mistaken belief that differential responsiveness to signals produced by competing rivals (“female choice”) requires an esthetic sense; that species recognition explains all species-specific sexual signals; and, more recently, that successful signals must reflect good survival genes; or that male–female conflict involves female resistance rather than stimulus evaluation. A “conflict paradox” results when male–female conflict is seen as driven by natural selection, whose costs should often move the hypothesized “sexually antagonistic co-evolution” of sensory-response systems toward the powerful domain of sexually synergistic co-evolution under sexual selection. Special properties of sexual selection apply to other forms of social competition as well, showing the wisdom of Darwin's setting it apart from natural selection as an explanation of many otherwise puzzling and extreme traits.