When Sexual Selection Runs Away
(2 of 2)

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There are several possible answers to explain how these seemingly disadvantageous genes spread through the population, among them:

• Runaway selection:
  Imagine a bird population in which females choose mates at random. Males with slightly longer tails fly a little more adeptly, avoid predation, and so, survive better than males with slightly shorter tails. In this situation, a gene for female choosiness (longer tail = sexier) will be favored, since—by choosing a long-tailed male—she will have sons with longer tails. This trait will spread through the population until most males have long tails and most females prefer long-tailed mates. So far so good.

  

  However, once this has happened, the process may run out of control, until the male trait becomes so exaggerated that it is disadvantageous. In other words, female preference, instead of survival advantage, may begin to drive the evolution of ever-longer tails, until males are encumbered by showy plumage that no longer helps them avoid predation.

• Good genes:
  Imagine another bird population in which females choose mates at random. Some males in the population have better genes for survival than others, but it is difficult to tell whether a male has good genes or not. In this scenario, long tails make it more difficult to survive—they are costly to produce and maintain. Because they are so costly, only males with good genes have the extra resources to produce them. In this situation, a long tail is an indicator of good genes. A gene for female choosiness (longer tail = sexier) will be favored, since—by choosing a long-tailed/good gene male—she will have sons with good genes. This trait will spread through the population until most females choose long-tailed mates and males that are able to produce long tails are favored.

If females choose males with “long and costly” tails, they are guaranteed to get good genes! If they choose males with “short and cheap” tails, they may get good or bad genes.