When Sexual Selection Runs Away
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The concept of runaway sexual selection illustrates one of the ways that sexual selection is hypothesized to work.

The quandary of female choice:
This leads to an interesting question: how did female choice for traits like a long, colorful tail evolve? After all, if a female chooses a male with a long, awkward tail, her sons will probably have a similar tail—and that tail might hurt their chances of survival by attracting predators. How could natural selection act to produce a preference for a disadvantageous trait?

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?

• Peacock photo courtesy of Rock Maple Farm.

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Spanish translation of Understanding Evolution For Teachers from the Spanish Society of Evolutionary Biology.