The concept of runaway sexual selection illustrates one of the ways that sexual selection is hypothesized to work. Runaway sexual selection can help explain how traits that are appealing to members of the opposite sex, but pose a risk in terms of survival, can evolve – and how that preference could evolve in the first place.

The quandary of female choice

It makes sense for a female to choose a mate based on traits that help him survive. For example, a female bird would do well to choose a strong-looking, disease-free mate. That male likely carries “good” genes that allow him to resist disease and get sufficient food — and he will pass those genes on to his offspring.

However, there are many examples of females choosing mates based on less useful traits (e.g., song complexity) or even traits detrimental to survival (e.g., brightly colored plumage, as in the case of the peacock). These cases present evolutionary biologists with a bit of a puzzle. How did these preferences arise in the first place? If a female chooses a male with bright feathers, her sons will have bright feathers, which are likely to attract predators. A gene for choosing brightly colored males would seem to be disadvantageous. How do such genes spread through a population?

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.