sexual selection

sexual selection is a mode of natural selection where individuals with certain heritable traits have a reproductive advantage over others of the same sex and species. First formally articulated by Charles Darwin in his 1871 work The Descent of Man, and Selection in Relation to Sex, it explains the evolution of traits that seem to hinder survival but aid in competition for mates. This process is responsible for many of the animal kingdom's most elaborate and conspicuous features, from the iridescent tail of the Indian peafowl to the antlers of the red deer.

Overview

The theory emerged from Charles Darwin's observations that could not be explained by survival selection alone, such as the bright plumage of male birds. He distinguished it from his broader concept of natural selection, focusing specifically on competition for reproduction. Key early supporters and expanders of the idea included Alfred Russel Wallace, though he disagreed with Darwin on some points, and later, the pioneering geneticist Ronald Fisher, who developed the concept of Fisherian runaway. The modern synthesis, advanced by figures like Ernst Mayr and John Maynard Smith, integrated these ideas with genetics and population genetics.

Mechanisms

Sexual selection operates primarily through two mechanisms: intrasexual competition and intersexual choice. Intrasexual competition, typically among males, involves direct contests for access to mates, leading to weapons like the horns of the African bush elephant or the large claws of the fiddler crab. Intersexual choice, often female choice, drives the evolution of displays and ornaments to attract mates, exemplified by the song complexity of the common nightingale or the bower structures of bowerbird species like the satin bowerbird. Sperm competition, a form of post-copulatory selection, is also significant, as seen in the mating strategies of chimpanzee troops.

Evolutionary consequences

This selection can lead to pronounced sexual dimorphism, where males and females of a species differ dramatically in size or appearance, as in the elephant seal or the mandrill. It can drive the evolution of traits that are costly or even maladaptive for survival, such as the long tail feathers of the male widowbird, which increase predation risk. The process can also contribute to speciation by driving divergence in mating signals and preferences between populations, a phenomenon studied in Hawaiian Drosophila species and various cichlid fish in the African Great Lakes.

In different taxa

In birds, elaborate plumage and songs are widespread, from the greater bird-of-paradise in New Guinea to the common peafowl in South Asia. Among mammals, traits range from the manes of African lions to the tusks of walrus populations in the Arctic Ocean. Insects exhibit extreme examples, such as the enlarged mandibles of the stag beetle used in male-male combat. In fish, brilliant colors and courtship dances are common, as observed in many coral reef species like the mandarinfish. Even in plants, analogous processes occur through pollinator preference, influencing flower evolution in genera like Ophrys.

Human sexual selection

In humans, potential influences are debated but may have shaped aspects of physical appearance and behavior. Some theories suggest it played a role in the reduced body hair difference between sexes, the development of beards in males, or certain vocal characteristics. Cross-cultural studies, such as those by David Buss, have investigated universal mate preferences. These dynamics are complexly interwoven with cultural evolution, as seen in practices from the courtship rituals of the Victorian era to modern phenomena influenced by media like Hollywood.

History of the concept

Charles Darwin first proposed the theory, facing skepticism from contemporaries like St. George Jackson Mivart. The concept was later revitalized by Ronald Fisher in the early 20th century with his models of runaway selection. Major empirical support came from studies like those of Niko Tinbergen on stickleback fish and Peter and Rosemary Grant on Galápagos finches. The landmark 1972 paper by Robert Trivers on parental investment provided a powerful theoretical framework, linking asymmetry in investment to the intensity of sexual selection. Contemporary research continues in institutions like the Max Planck Institute and through work by scientists such as Malte Andersson and Marion Petrie. Category:Evolutionary biology