Sexual Selection
Sexual selection is the evolutionary mechanism by which traits evolve because they increase mating success — not survival. Proposed by Charles Darwin in The Descent of Man (1871) as a complement to natural selection.
Primary source for this wiki: miller-2001-mating-mind
Darwin’s Two Mechanisms
Darwin distinguished two processes within sexual selection:
- Intrasexual competition (male-male competition) — individuals of one sex compete directly with each other for mating access (antlers, body size, fighting ability). Winner gets mates.
- Intersexual choice (mate choice) — individuals of one sex are choosy and select among potential mates based on displayed traits. Choosy sex (usually females in most species) drives evolution of display traits in the chosen sex.
Most of Miller’s The Mating Mind focuses on intersexual choice in human evolution — specifically, how female choice of male mental qualities shaped the evolution of human cognitive capacities.
Why Sexual Selection Produces Extravagant, Costly Traits
Natural selection typically eliminates costly traits that reduce survival. Sexual selection can preserve or amplify them because mating success outweighs survival cost — if a male peacock dies young but produces many offspring because females prefer his tail, his genes (including the tail genes) propagate.
Two theoretical frameworks explain the dynamics:
1. Runaway Selection (Fisher Process)
Geneticist R.A. Fisher showed that a preference gene and a trait gene can co-evolve in a self-reinforcing loop. If females with a preference for males with long tails produce sons with long tails and daughters with the same preference, the preference-trait combination spreads even if the tail is arbitrary. The runaway can carry traits to extreme lengths far beyond initial functional utility. (See runaway-selection.)
2. Fitness Indicators / Handicap Principle (Zahavi)
Amotz Zahavi proposed that only genuinely costly traits can be honest signals of genetic quality — cheap displays can be faked, so choosy mates only trust signals that impose real costs. A peacock bearing a large tail despite the survival handicap thereby advertises: “I am so genetically fit that I thrive even with this burden.” (See fitness-indicators.)
The handicap principle resolves the evolutionary paradox: costly, seemingly useless traits persist because they convey reliable information about the organism’s quality.
Miller’s Application: The Human Mind
geoffrey-miller argues in The Mating Mind that the human mind’s most elaborate capacities — general intelligence, language, art, music, humour, morality — evolved primarily as courtship displays (fitness indicators) through intersexual mate choice, not purely through natural selection for survival:
- Problem: hominid brains tripled in size in ~2 million years — too fast for survival advantages alone
- Solution: runaway selection for mental ornaments once mate choice started operating on cognitive display traits
This means human intelligence is not just a tool for building shelters and hunting; it is also a fitness indicator — a costly signal of genetic quality selected by choosy mates over evolutionary time.
Mate Choice in Humans
Human mate choice operates on multiple domains simultaneously:
- Physical ornaments: symmetry, secondary sexual characteristics (evolved through natural + sexual selection)
- Mental ornaments: verbal creativity, humour, musical ability, moral character, artistic skill — all energetically costly, hard to fake, and advertise neurological quality
The symmetry between physical and mental ornaments is central to Miller’s thesis: the same evolutionary logic explains both the peacock’s tail and the poet’s verses.
Comparison with Survival Selection
| Feature | Natural Selection | Sexual Selection |
|---|---|---|
| Selector | Environment (predators, disease, food) | Mate choice (other individuals) |
| Currency | Survival and reproduction indirectly | Direct mating success |
| Typical outcome | Efficient, functional, conservative traits | Extravagant, costly, display traits |
| Speed | Slow (environmental changes) | Can be fast (runaway dynamics) |
| Bias direction | Reduces costly ornaments | Amplifies costly ornaments |
Related Concepts
- evolutionary-psychology — broader field applying evolutionary logic to human behaviour
- fitness-indicators — the costly honest-signal mechanism in mate choice
- runaway-selection — the Fisher process producing rapid trait amplification
- social-skills — social display behaviour possibly partly rooted in sexual selection pressures