Sexual Selection

Sexual selection is selection arising from competition for mates and mate choice, and it explains elaborate ornaments, weapons, and mating behaviors that ordinary survival selection cannot.

Definition

Sexual selection is the component of natural selection that results from differences in mating success. It operates through intrasexual competition, typically among males for access to mates, and intersexual choice, typically females choosing among males, favoring traits that improve mating success even at a survival cost.

Scope

This topic covers the two main mechanisms of sexual selection, competition within a sex and choice by the other sex, the theories explaining female preference for costly male traits, the resulting sexual dimorphism and mating systems, and sexual conflict between the evolutionary interests of the sexes.

Core questions

• How do intrasexual competition and mate choice drive the evolution of mating traits?
• Why do females prefer costly male ornaments, and what do these signals indicate?
• How does sexual selection produce sexual dimorphism and diverse mating systems?
• How does sexual conflict shape the coevolution of male and female traits?

Key theories

Mate choice and honest signaling

Preferences for exaggerated traits can evolve because the traits are costly and therefore honest indicators of quality (the handicap principle), or through Fisherian runaway where preference and trait reinforce each other.

Sexual conflict

Because the sexes often have different optimal reproductive strategies, selection can drive antagonistic coevolution between male and female traits rather than mutual benefit.

Mechanisms

Sexual selection arises whenever individuals vary in mating success. Intrasexual selection favors traits, such as large size or weapons, that win contests for mates. Intersexual selection favors traits preferred by the choosing sex. Female preferences for costly male traits can evolve through good-genes or handicap mechanisms, in which only high-quality males can bear the cost, or through Fisherian runaway, in which genetic correlation between preference and trait causes both to escalate. The resulting traits frequently reduce survival, so sexual selection can oppose natural selection. Where male and female interests diverge, sexual conflict drives antagonistic coevolution of reproductive traits.

Clinical relevance

Sexual-selection theory informs understanding of mating-system effects on effective population size and genetic diversity in conservation, and the evolution of reproductive traits and behaviors relevant to managing captive and wild populations.

History

Darwin introduced sexual selection in 1871 to explain traits unfavorable to survival. Fisher proposed the runaway model in 1930, and Zahavi's handicap principle in the 1970s, formalized in the 1990s, supplied a basis for honest signaling. Andersson's 1994 synthesis consolidated the empirical and theoretical foundations of the field.

Debates

Good genes versus runaway

Whether female preferences for ornaments persist mainly because the traits signal genetic quality or because of self-reinforcing Fisherian dynamics remains debated, with both processes likely contributing.

Key figures

• Charles Darwin
• Ronald A. Fisher
• Amotz Zahavi
• Malte Andersson

Seminal works

• andersson1994
• daviesKrebsWest2012
• futuyma2017

Frequently asked questions

Why do peacocks have such costly tails if they hinder survival?

Because the tail improves mating success through female choice; sexual selection can favor traits that reduce survival as long as the mating advantage they confer outweighs their survival cost.

What is sexual conflict?

Sexual conflict occurs when the reproductive interests of males and females differ, leading to the evolution of traits in one sex that benefit it at the expense of the other and driving antagonistic coevolution between the sexes.