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Runaway Selection (Fisher Process)

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Runaway Selection (Fisher Process)

Runaway selection (also the Fisher process) is geneticist R.A. Fisher’s model (1930) of how sexual-selection can generate extreme, costly traits through positive feedback between a female preference and a male trait.

Primary source: miller-2001-mating-mind (Chapter 3: The Runaway Brain)

The Mechanism

  1. A female preference for a trait (say, slightly longer tail) arises for any reason — perhaps because it genuinely signals fitness, or by arbitrary genetic drift.
  2. Females with the preference produce sons with longer tails and daughters with the same preference (both genes are linked in offspring).
  3. Sons with longer tails have more mates → reproduce more → preference gene spreads further.
  4. Self-reinforcing loop: the preference and the trait co-evolve, each driving the other higher.
  5. The process can “run away” to extremes far beyond any original functional utility — a positive feedback loop.

Why Runaway Is Explosive

Natural selection acts as a brake (costly tails reduce survival), but sexual selection accelerates the runaway. At equilibrium, the tail length stabilises when sexual selection advantage (more mates) exactly balances natural selection disadvantage (higher predation). This equilibrium can be at extreme trait lengths.

Miller’s Application to the Human Brain

In The Mating Mind, geoffrey-miller proposes that runaway selection explains the extraordinarily rapid tripling of hominid brain size in ~2 million years:

  • Once mate choice began operating on cognitive capacities (verbal skill, problem-solving display, creativity), a preference-trait loop initiated
  • The loop ran away, driving brain expansion far beyond immediate survival needs
  • Result: a brain far larger and more elaborate than any survival account requires

This is the Runaway Brain hypothesis: the human mind as the peacock’s tail of the primate world.

Relationship to fitness-indicators

The two mechanisms (runaway + fitness indicators) may both operate:

  • Fitness indicators maintain the informational content of displays (prevents degeneration to pure arbitrary ornament)
  • Runaway amplifies the intensity of selection for those displays

Graph View

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