Sexual selection

Lore & Background

Darwin felt natural selection alone could not account for certain non-survival adaptations, famously writing that 'The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!' He divided sexual selection into male–male competition and female choice. Alfred Russel Wallace later opposed these views, arguing that male–male competitions were natural selection and that female mate choice required cognitive abilities animals like beetles lacked. The Fisherian runaway describes how female preference for a male ornament and the ornament itself increase together in a positive feedback loop, leading to extreme sexual dimorphism. Fisher noted that 'plumage development in the male, and sexual preference for such developments in the female, must thus advance together... with ever-increasing speed.'

Darwin's ideas were met with scepticism by contemporaries and considered unimportant until the 1930s, when biologists included sexual selection as a mode of natural selection. During the modern synthesis, it was still seen as secondary, with Ronald Fisher being the main exception. Only in the 1960s and 1970s, thanks to Robert Trivers and others, did biology shift to the present understanding of sexual selection as a key major driver of evolution. In the 21st century, the theory is seen as generally applicable and analogous to natural selection.

Reader's Guide

Sexual selection is significant as a fundamental evolutionary mechanism distinct from natural selection, explaining the evolution of elaborate secondary sexual characteristics such as the peacock's tail and deer antlers. Darwin's original formulation faced ideological opposition in Victorian times due to implications for human sexuality and the assumption that insects lacked aesthetic sense. The theory gained mathematical rigor through Ronald Fisher's work, which introduced the Fisherian runaway process—a positive feedback loop where female preference and male ornamentation co-evolve exponentially until physical constraints halt exaggeration. This process explains extreme sexual dimorphism in species like the long-tailed widowbird. Despite initial neglect during the modern synthesis, sexual selection was revived by Robert Trivers and others in the 1960s–1970s, leading to its current status as a key driver of evolution. A ten-year study on flour beetles showed that sexual selection can protect even inbred populations from extinction, underscoring its adaptive importance. The theory remains broadly applicable across the animal kingdom, plants, and fungi, with ongoing research into its mechanisms and consequences.

Did You Know?

• The Fisherian runaway describes a positive feedback loop where female preference and male ornamentation increase together exponentially.
• A ten-year study on flour beetles showed that sexual selection can protect an inbred population against extinction.