Question

1. Under what conditions will sexual selection lead to the development of sexual dimorphism? Why is one sex often “choosy” while the other is “showy”?

2. What are four reasons that females may choose males with particular traits and reject other males? Give one example of each. Does she always benefit from her choice?

Answer 1

1.

Thefundamental cause of sexual dimorphism is an asymmetry in the amount of parental investment in a given mating and the care of any resulting offspring. The sex that invests less time and energy in this process tends to be limited merely by access to mates, and hence is under strong selective pressure to attract as many mates as possible. The result is evolution of showy traits that attract the opposite sex, and competitive traits for competition with the same sex. This sex is often (but by no means always) the male, since ejaculates tend to be relatively cheap and males of many species do not provide parental care.
The sex that invests more time and energy in this process is usually not limited by access to mates, and hence is not under strong selective pressure to find as many mates as possible. Rather, this "parental" sex is under pressure to select just a few good mates-sometimes just one. This sex is often, but by no means always, the female, since eggs are relatively large and females often provide parental care.

In marine iguanas, many males are larger than the "optimum" body size (the size that can be maintained long-term), and experimental data confirms that survival rates are lower for the largest iguanas than for medium-sized iguanas. Similarly, in long-tailed widowbirds, males with intact long tail feathers lost weight at a greater rate than males with experimentally shortened feathers. These results indicate that sexual selection is working contrary to natural selection, and that "attractive" males may ultimately pay a price for attractiveness, in the form of lowered survival, shorter life spans, or reduced health.

2. Females may gain "good genes" for their offspring by choosing traits that indicate that the male is healthy and fit. An example is Welch's experiment on gray tree frogs, which demonstrated that offspring of long-calling males outperform those of short-calling males in several measures of health and rapid growth. Second, females may select males who provide them with a valuable resource, such as a food gift. An example is male hangingflies, who provide a food gift to the female. Third, females may have pre-existing sensory biases that can be exploited by males. An example is male water mites, which employ a mating display that appears to take advantage of females' tendency to turn toward vibrations-a trait that originally evolved for hunting, not for mating. Fourth, once a trait is preferred by a majority of females, the trait may become self-perpetuating because females that prefer that trait will tend to have "sexy sons". An example is spotted cucumber beetles, in which most females prefer fast-stroking males-a trait that appears to provide no advantage other than the fact that sons of those males will themselves be fast-strokers