What is Sexual Selection?

How does Sexual Selection relate to Natural Selection?

Sexual selection is a form of natural selection that acts through its effects on mating success. An individual's traits that favor its mating success are sexually selection, which can have interesting results. For example, traits that are maladaptive for survival (such as bright colors) can be adaptive for increased mating success and thus be selected for! Sexual selection has a stronger effect on males since females since sexual reproduction is more expensive for females and they typically get to choose their males.

Two types of sexual selection exist: intra-sexual selection and inter-sexual selection.

Intra-Sexual Selection

Intra-sexual selection occurs within a single sex of a species. This typically results in competition among males for access to females. This can occur through combat, infanticide, cuckoldry, etc.

Inter-sexual Selection

Inter-sexual selection occurs between the two sexes of a species. Often times this is in the form of the females selecting the males with which they want to mate. Females will generally choose males with traits that are more fit or that appear more desirable (sensory-bias).

What is Bateman's Principle?

Sexual reproduction often costs more in time and energy for the female than the males. For this reason, while the fitness of males increases with increased mating, the fitness of females actually decreases. Since females are often the limiting resource in sexual reproduction, the variance in reproductive success is much greater in males than in females and selection on males is greater as a result.

What is Fisher's Hypothesis of Runaway Sexual Selection?

Fisher's Hypothesis of Runaway Sexual Selection can be used to explain the sexual dimorphism that occurs between males and females and how males can develop extreme traits that are often times environmentally maladaptive. This is how it occurs:

  1. A subset of females prefers males that have an extreme trait.
  2. When these choosy females and their preferred males reproduce, they produce sons that have the extreme trait and daughters that prefer the extreme trait. This linkage disequilibrium continues to build up due to assortative mating.
  3. As this choosy female preference becomes more common the male trait becomes even more advantageous and then this direct selection males results in an indirect response of an increase in female preference!
  4. This process continues until natural selection finally is able to stop it (like a runaway train). However, the resulting trait value is much more extreme then it would have been if natural selection was acting alone.

Wikipedia article on Fisherian Runaways.

Antagonistic Co-Evolution and Sexual Selection

As we mentioned in the section over Bateman's Principle, males are encouraged to mate with as many females as possible even though this often has a detrimental effect on female fitness.

For this reason, females often evolve adaptations to prevent males from mating with them and the males will in turn evolve adaptations to these adaptations that make them more successful! It's like an arms race!

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