When Fisher's runaway process first appeared in print in 1930, other scientists greeted it with suspicion. Runaway did not fit the prevailing emphasis on the good of the species, the efficiency of survival adaptations, and the modernist machine aesthetic. Yet despite its frosty initial reception, runaway has finally been invited back to the center of the evolutionary stage. Theoretical biologists in the 1980s showed that Fisher was right: runaway can work. Indeed, it works so well that it is hard to avoid when sexual selection is in play. Because runaway may have had an important role in the evolution of the human mind, it is important to understand it as fully as possible. What follows is the simplest example of runaway I can offer, although the theory is subtle, and demands some concentrated attention.
Imagine a population of birds with short tails, in which the males contribute nothing to raising the offspring. Although this makes life hard for females after mating, it allows females to choose any male they want, even a male who has been chosen by many other females already The most attractive male could mate with many females. He has no reason to turn down a sexual invitation from any female, because copulation costs so little time and energy.
Within this population, different males inevitably have different tail lengths Just as they have different wingspans, and different leg lengths. All biological traits show variation. Usually, much of that variation is heritable (that is, due to genetic differences between individuals), so longer-tailed males will tend to produce longer-tailed offspring. In other words, tail length varies and tail length is heritable, satisfying two out of Darwin's three requirements for evolution.
Now, suppose that some of the females become sexually attracted to tails that are longer than average. (It doesn't matter why they evolve this preference—perhaps there was a mutation affecting their sexual preferences, or their vision happened to respond more positively to large than to small objects.) Once this female preference for long tails arises, we have the third requirement for evolution: selection. In this case, it is sexual selection through mate choice. The choosy females who prefer long tails will tend to mate with long-tailed males, who are happy to copulate with all their admirers. The non-choosy females mate randomly, usually ending up with an average-tailed male.
After mating, the choosy females start producing offspring. Their sons have longer-than-average tails that they inherited from their fathers. (Their daughters may also inherit longer tails—a phenomenon we shall consider later.) The non-choosy females produce sons whose tails are about the same length as those of their fathers—but these mediocre tails are no longer average. They are now below average, because the average tail length has increased in this generation, due to sexual selection through mate choice. The genes for long tails have spread.
The question is, will they keep spreading? Fisher's key insight was that the offspring of choosy females will inherit not just longer tails, but also the genes for the sexual preference—the taste for long tails. Thus, the genes for the sexual preference tend to end up in the same offspring as the genes for the sexually selected trait. When genes for different traits consistently end up in the same bodies, biologists say the traits have become "genetically correlated." Fisher's runaway process is driven by this genetic correlation between sexual traits and sexual preferences in offspring, which arises through the sexual choices their parents made. This genetic correlation effect is subtle and counterintuitive, which is one reason why biologists took fifty years to prove that Fisher's idea could work.
Of course, when the sons of choosy females inherit the genes underlying their mother's sexual attraction to long tails, they may not express this preference in their own mating decisions. But they can pass their mother's sexual preferences on to their own daughters. Since their long tails make them sexually attractive, they tend to produce not only more sons than average, but more daughters as well. In this way, the sexual preference for long tails can genetically piggyback on the very trait that it prefers. This gives the runaway process its positive-feedback power, its evolutionary momentum.
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