Sexual selection Knowledge-Based Study

Study notes

1. What is Sexual Selection?

   Sexual selection is a mode of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to secure mates. First introduced by Charles Darwin, this concept accounts for the evolution of prominent traits—such as the peacock's tail or elaborate courtship dances—that might otherwise appear maladaptive for ordinary survival.

   While standard natural selection emphasizes survival and ecological fitness, sexual selection specifically acts on reproductive success through differential mating access.

   Intrasexual and Intersexual Selection

   Sexual selection generally operates through two distinct mechanisms:

   • Intrasexual selection (male-male competition): Direct competition between members of the same sex for access to mates. This often leads to the evolution of anatomical armaments, such as horns, antlers, or enlarged body size used in combat or territorial defense.
   • Intersexual selection (mate choice): Selection whereby individuals of one sex (typically females) are choosy in selecting their mates. This mechanism drives the evolution of conspicuous ornaments, nuptial gifts, and complex behavioral displays.

   Why Are Females Usually Choosy?

   The evolutionary basis of mate choice often stems from anisogamy—the fundamental asymmetry in gamete size. Eggs are large, metabolically expensive, and limited in number, whereas sperm are small, energetically cheap, and produced in vast quantities.

   According to Robert Trivers' theory of parental investment, the sex that invests more energy and time in offspring production and care (frequently, though not exclusively, females) represents a limited reproductive resource. Consequently, the higher-investing sex tends to be discriminating, while the lower-investing sex competes more intensely for mating opportunities.

   Allele Frequencies and Selective Trade-offs

   In population genetics, sexual selection alters allele frequencies by generating non-random reproductive success. The fitness advantage conferred by preferred alleles can increase their frequency in the gene pool across generations.

   However, sexually selected traits often involve an evolutionary trade-off:

   • Viability cost: Highly elaborate plumage or intense vocalizations can increase predation risk and metabolic demands.
   • Mating benefit: The same traits yield greater reproductive output by securing more mates.

   An equilibrium is reached when the net reproductive advantage of the display trait balances the survival costs imposed by natural selection.