Final answer:
Natural selection favoring the rarer sex in a population leads to the evolution of sex ratios through frequency-dependent selection, a type of natural selection where phenotypes that are either common or rare are favored, thereby influencing evolutionary changes in sex ratios. The correct option is b. frequency-dependent selection.
Step-by-step explanation:
The process in which natural selection favors the rarer sex in a population, leading to the evolution of sex ratios, is a result of frequency-dependent selection. This biological phenomenon affects the reproductive success of individuals by selecting phenotypes that are either common or rare within a population. For example, in frequency-dependent selection, rare phenotypes are more favored when they are less common, maintaining genetic variance within the population.
Negative frequency-dependent selection increases genetic variance by selecting for rare phenotypes, while positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes. This balance can result in dynamic shifts in population characteristics, such as the ratio of males to females, thus impacting the evolutionary trajectory of species. The correct option is b. frequency-dependent selection.