Chapter 15: Sex & Society

Sex & Society from Sociobiology: The New Synthesis presents a comprehensive analysis of the complex evolutionary relationship between sexual reproduction and social behavior, arguing that sex is fundamentally an antisocial force that creates genetic conflicts of interest between mates and between parents and offspring. The text explores the evolutionary origins of sex, contrasting the high energetic costs of meiosis and gamete production against the adaptive benefits of rapid genetic diversification, which protects populations against environmental fluctuations. A significant portion of the discussion focuses on the mechanics of sex ratios, detailing Fisher’s principle which drives populations toward a 50/50 sex ratio, and the Trivers-Willard hypothesis, which suggests parents adjust the sex of their offspring based on maternal physical condition. The summary delves into the theory of sexual selection, distinguishing between intrasexual selection—manifested through male combat, sperm competition, and dominance hierarchies—and epigamic selection, where females choose mates based on displays or territory quality. Central to this analysis is the concept of parental investment and anisogamy; because females produce larger, energetically expensive gametes (eggs) compared to male sperm, they become a limiting resource, leading to Bateman’s principle where male reproductive success varies more widely than that of females. The chapter further examines the ecological determinants of mating systems, utilizing the Orians-Verner model to explain the "polygyny threshold" and the conditions under which polygamy evolves, such as resource abundance or precocial young. Conversely, it outlines the rare ecological pressures that favor monogamy, such as the necessity for cooperative defense of scarce resources or survival in harsh physical environments. Finally, the text addresses the evolution of communal mating displays (leks), the phenomenon of sexual dimorphism driven by niche division and signaling, and the strategic conflicts involving desertion and cuckoldry.