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Evolutionary fitness is largely determined by the ability of an organism to survive and successfully produce offspring. Critical to this process is sexual selection, which plays a large role in the determination of mating pairs, and thus which genes are passed on to the next generation. Often, intense competition for mates within a population places selective pressure on traits related to courtship and copulation. Natural selection that results from these pressures is called sexual selection. Sexually selected traits include characteristics like ornamentation or coloration for the sole purpose of attracting mates. These traits can also serve to enhance the distinction between males and females within a species, termed sexual dimorphism. Dimorphism and other sexually selected traits help individuals of a species determine the fitness of potential mates and select an appropriate breeding partner. This type of selection occurs in most species, from insects to humans and countless others.
The two basic types of sexual selection are intersexual (between-sex) and intrasexual (within-sex) selection. Intersexual selection occurs as a result of interactions between males and females of a species. One sex, typically males, will develop and display traits or behavior patterns to attract the opposite sex. Examples of such traits include plumage on birds, the mating calls of frogs, and courtship displays in fish. In contrast, intrasexual selection occurs between members of the same sex. Competition between males is common, as with deer or horned beetles, which fight for dominance and the ability mate with nearby females. In such cases, sexual selection acts on traits that facilitate competition among individuals of the same sex. For example, the strength and size of the “horn” of horned beetles or antler size in deer. Both intersexual and intrasexual selection influence the development of sexually selected traits in numerous species.
The display of sexually-selected traits can be important signals of individual fitness to potential mates. Such signals are often energetically costly, and thus indicate the health, genetics, and nutrition status of an individual. Often, though not always, these traits are displayed by males to attract females. This is the result of the difference in energetic investment into offspring production and rearing. Female eggs are produced in fewer numbers and at a higher energetic cost than male sperm, prompting females to be more selective in mate choice. The level of competition for mates can be quantified by the operational sex ratio (OSR). The OSR is based on the ratio of sexually mature males to females in a population, and is often male-biased in groups that contain more males or for populations in which males mate with multiple females. However, the OSR can be skewed by several factors, including the relative investment by each sex into parental care, mate bonding processes, and the overall rate of reproduction1-2.
Because ORS is often male skewed, males of a species tend to experience stronger inter- and intra-sexual selective pressures. In this context, natural selection has favored the development of traits to attract females or compete with other males, even when these traits come at a significant energetic or fitness cost. Traits to attract females, for instance, are often colorful or flashy, making individuals more easily seen by predators. These traits may require significant energy to produce and can reduce immune function in individuals. Furthermore, males put themselves at risk when competing with other males. Battles between males, mating dances, vocalizations, and displays are often not only physically demanding, but may also be loud, flashy and distracting, reducing awareness to predators and potentially injuring individuals. Because of this, natural selection and sexual selection are often in conflict, seemingly pulling such traits in opposite directions. The influence of natural selection prevents sexually selected traits from becoming too extravagant. As a result, sexually associated traits that significantly reduce an individual’s ability to survive long enough to mate will be selected against. This balance between natural and sexual selection causes most individuals within a population to exhibit fitness and sexual traits of average or intermediate quality to maximize reproductive success and survival.
However, under unique circumstances, sexually selected traits may not be constrained by survival pressures. For instance, birds within the “bird-of-paradise” family include a variety of species that occupy remote areas lacking natural predators. As a result, males have developed extravagant colorations and mating displays that would normally be selected against due to predation. Such “runaway selection” can produce beautiful and highly ornamented species. Unfortunately, due to human activity, the introduction of new predators like cats threatens the survival of these endangered species.
Understanding sexual selection influences the way we view animal traits, behaviors, and mate choice, including humans3. There is often more to sexual selection than visual cues, with many species relying solely on smell or sound to find mates. In humans, the influence of smell was demonstrated in an experiment by Wedekind et al, which showed a female preference for the smell of males who were more different, genetically4. Enhanced genetic diversity, especially as it relates to immune function, may provide offspring with greater fitness. In this and other ways, sexual selection has influenced the way humans and organisms of all types evolve, behave, select mates, and reproduce.