Genetic correlation between female preference and male trait drives escalation
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Generation
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Male trait (t̄)
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Female pref (p̄)
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Genetic covariance
Fisher (1930) Runaway process: If a female preference gene and a male trait gene become genetically correlated
(sons inherit both genes), selection on preference becomes selection on trait and vice versa — a positive feedback loop.
The phase plane (left) shows mean male trait vs. mean female preference over time; the trajectory spirals or runs away.
The sexy son hypothesis: females gain indirect fitness benefits because their sons will be preferred.
Natural selection on the male trait (survival cost) can arrest or reverse the runaway — creating a stable equilibrium.