How Does Sexual Selection Operate?
Sexual selection can operate both intra- and inter-sexually, either sequentially or simultaneously (Table 1). During intrasexual selection, members of the same sex attempt to outcompete rivals, often during direct encounters. Intrasexual selection is typically responsible for the evolution of male armaments such as deer antlers, beetle horns, and large body size, that provide individuals with an advantage when fighting off potential competitors. Individuals who are better able to exclude competitors, have a greater chance to acquire mates and father offspring. For example, dominant male red deer monopolize a group of females (also known as harem) by constantly fighting off competitors, and they father most of the offspring produced by the females. By contrast, intersexual selection results from interactions between the sexes, typically involving mate choice. The evolution of elaborate behavioral displays and morphological traits can often be explained as the result of intersexual selection. Usually, females tend to be more choosy, evaluating morphological and behavioral traits from potential mates to determine which will maximize their fitness. Males tend to compete with one another to gain the female's attention. An extreme example of intersexual selection can be found in species where males form leks where multiple males gather to display to females.
Table 1: Some examples of when and how sexual selection operates
Sexual selection episodes can occur before mating takes place (pre-copulatory), or during and after mating (post-copulatory), and they can occur within a sex (intrasexual) and between the sexes (intersexual).
Choosing a Mate
Why do females choose between males rather than mate at random, or with the first male they encounter? Females can directly increase their reproductive success by mating with certain, select males and acquiring direct benefits. For example, females can gain increased access to food, protection from harassing males, or help in raising offspring, and avoid being infected with parasites or other diseases by choosing healthy males. However there are instances where females do not appear to gain any direct benefit from males, yet they still discriminate among them. Under these conditions, females likely gain indirect benefits via their offspring. These indirect benefits are usually genetic rather than resource based. By choosing certain males, their offspring will likely inherit genes that tend to increase their fitness. Males often evolve traits and displays that advertise their ability to provide direct and indirect benefits, and females evolve preferences for these traits. Two major mechanisms to account for female mate choice have been proposed: good genes, and Fisherian arbitrary processes.
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