Blackwell Publishing

Adaptations in sexual reproduction - Why do the sexes differ?


Fisher's runaway sexual selection

Fisher discussed how the preference for a costly character could evolve to begin with. After the long male tail has evolved, it is costly; but at an earlier evolutionary stage, before the female preference arose, things might have been different. Male tails would have been shorter then. Suppose that,

• before some mutant female arose who picked long tailed males, most females picked their mates at random; and

• that there was at that time a positive correlation between male tail length and survival.

Selection would then favor a mutant female with a preference for males with longer tails; she would produce sons with longer than average tails and the associated higher survival.

Then, as the mutation spreads, the males with longer tails would start to acquire a second advantage. There are increasing numbers of females in the population who prefer to mate with longer tailed males, and the males so endowed will not only survive better but also enjoy an advantage in mating. The evolution of longer tails in males, and a mating preference for them in females thus come to reinforce each other, in what Fisher called a runaway process.

Eventually, powered by female choice, the average tail length in the population will reach the optimum; but evolution does not stop there. By now the female preference will have spread through the population and the majority of females will prefer longer tailed mates. Now the mating preference alone drives the evolution of longer tails.

There are thus three stages in the evolution of the long tail:

1. an initial stage in which long tails have only a survival advantage,

2. a second stage in which the survival advantage is supplemented by a mating advantage, and

3. a third stage at which further elongation is driven purely by female choice.

As female choice grows commoner and the tail length grows past the optimum for survival, the relative importance of the two advantages shifts over.

As the population evolves past the point of optimum tail length, the selective forces at work have become almost absurd. The males are evolving ever longer tails even though the original selective advantage has ceased, and then reversed. The female mating preference will also continue to strengthen, even though it now favors a character that decreases survival. The runaway process will only come to a stop when the death rate of males, due to their feathery excess, is so high that their success in mating no longer makes up for it. The tail length will then reach an equilibrium. That equilibrium, according to Fisher, is what we are now observing in birds like peacocks and birds of paradise.

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