Fisher's Uncontrolled Choice

Fisherian runaway is a model of sexual selection in which preference for an inherited trait can amplify that trait over successive generations. A feature may begin as a useful indicator of fitness, but once it becomes attractive in its own right, preference for it can drive its growth even when the feature becomes costly. The process ends when those costs outweigh the reproductive advantage of being preferred.

From useful signal to preference

The account begins with females choosing males on the basis of apparently genuine indicators of good underlying genes. Traits such as strong muscles or long legs might suggest an ability to obtain food or escape predators, and could benefit offspring of either sex. The crucial further step is that a mother can also benefit her genes by producing a son who will himself be especially attractive to females and win many matings. Sexual attractiveness can therefore become desirable not only because it signals useful qualities, but because it helps secure reproductive success directly.

The runaway process

The example is the tail of male Birds of paradise. In an ancestral population, females might have preferred males with longer-than-average tails because tail length appeared to indicate fitness: a short tail might signal nutritional deficiency or poor ability to escape predators. The short tail need not itself be genetically inherited; it need only serve as an indicator of some inherited inferiority. If male tail length varied partly for genetic reasons, female preference would, over time, increase the population's average tail length.

As longer tails became attractive, females that chose long-tailed males were more likely to have sons who would be attractive to the next generation of females. A female that departed from the preference risked producing sons with fewer mating opportunities. The original association between the tail and useful qualities could thus give way to a self-reinforcing preference: longer tails produced greater attractiveness, and that attractiveness favored still longer tails. The trait's mating value could persist even after its practical usefulness had diminished.

Costs, limits, and competing explanations

The process is unstable because the same tail that attracts mates can encumber its bearer. The account compares the accumulating trend to a fashion, in which preference gathers momentum rather than stopping as soon as a trait's original practical value is exhausted. Runaway selection ends only when the tail becomes so exaggerated that its disadvantages outweigh the reproductive benefit of sexual attractiveness.

This proposal is presented as a difficult and disputed explanation of sexual selection, originally advanced by Charles Darwin and clearly formulated by R. A. Fisher. A. Zahavi's handicap principle offers a rival account: because females selecting for good genes create an opportunity for deceptive displays, false signals should be detected by counter-selection. Under that explanation, displays succeed by demonstrating genuine quality, since only a truly capable male can sustain them. The contrast clarifies the runaway model's distinctive claim: female preference can itself help propel an inherited ornament beyond its original usefulness, without requiring that the ornament remain a reliable indicator at every stage.