The Biological Basis of Sexual Selection

Sexual selection operates through two main mechanisms:

• Intrasexual selection: Competition among individuals of the same sex (usually males) for access to mates.

• Intersexual selection: Mate choice, often by females, based on specific traits in the opposite sex.

While both mechanisms shape the evolution of reproductive traits, intersexual selection—female choice—is particularly powerful in driving the evolution of male ornamentation and display behaviors.

Why Are Females the Choosy Sex?

The primary reason females are typically choosier than males lies in the cost of breeding. In most species, females invest significantly more resources into reproduction than males. This investment begins with the production of eggs, which are larger and more energetically expensive than sperm. Beyond gamete production, females often invest further through pregnancy, lactation, or extensive parental care.

Because each offspring represents a substantial investment of time, energy, and resources for the female, the cost of making a poor mate choice is high. Selecting a mate with superior genetic or phenotypic qualities can increase the chances of producing healthy, viable offspring. As a result, females are under strong evolutionary pressure to be selective, favoring males with traits that signal genetic fitness, health, or the ability to provide resources.

The Evolutionary Consequence: Variation in Male Reproductive Success

Since females are choosy and males compete for access to them, not all males have equal reproductive success. Males with preferred traits mate more often and leave more descendants, while others may not mate at all. This differential success is the engine of sexual selection, leading to the evolution of exaggerated male traits and behaviors.

Why the Cost of Breeding Matters

Among the options provided, the correct explanation for why sexual selection results in female choice for particular phenotypes is:

• The cost of breeding is higher for females as compared to males.

This higher cost means females have more to lose from a poor mating decision, so natural selection favors choosiness in females and competitiveness in males.

Why Other Options Are Incorrect

• Males cannot compete with other males: Intrasexual competition is important, but does not explain female choice.

• Inappropriate mating results in similar reduction in fitness of females and males: In reality, the fitness cost is typically higher for females.

• Males are a limiting resource for females: In most species, females are the limiting resource due to their higher investment and lower reproductive rate.

Conclusion

Sexual selection often results in female choice for particular male phenotypes because the cost of breeding is higher for females than for males. This evolutionary pressure drives females to be selective, leading to the development of striking male traits and behaviors that define much of the diversity seen in the natural world.

Correct answer:
(2) cost of breeding is higher for females as compared to males