Sexual selection drives the evolution of traits that enhance an individual’s success in acquiring mates. Unlike natural selection, which focuses on survival, sexual selection centers on reproductive advantage, often leading to the development of remarkable characteristics. This evolutionary process operates through two distinct mechanisms: intrasexual selection, involving competition among individuals of the same sex, and intersexual selection, where individuals of one sex choose mates from the opposite sex.
Intrasexual Selection Explained
Intrasexual selection involves competition among members of the same sex, typically males, for direct access to mates. This form of selection favors traits that provide an advantage in direct rivalry, leading to the evolution of weaponry, larger body size, and aggressive behaviors. The primary pressure comes from within the same sex, as individuals vie for reproductive opportunities.
Direct combat is a common mechanism, where males physically fight to establish dominance or secure mating rights. Male deer, for instance, engage in intense antler locking contests during the breeding season, with the victor gaining access to females. Similarly, male elephant seals are significantly larger than females and fiercely battle each other, sometimes to the death, to control harems of females on breeding beaches.
Intrasexual selection also involves ritualized displays that minimize injury while determining dominance. Males might engage in vocalizations or posturing to signal strength and deter rivals. Another form of intrasexual competition, less visible but equally impactful, is sperm competition. This occurs when multiple males mate with a single female, and their sperm compete to fertilize the eggs. Adaptations like larger ejaculate volumes, specialized sperm morphology, or mechanisms to remove rival sperm have evolved in various species, including some insects and birds like the dunnock.
Intersexual Selection Explained
Intersexual selection, or mate choice, involves individuals of one sex selecting partners from the opposite sex based on specific traits. This process typically sees females choosing among males, leading to the evolution of elaborate ornaments, courtship rituals, and distinctive vocalizations. The selection pressure arises from the preferences of the choosing sex, driving the development of characteristics that signal quality.
Mate choice often relies on indicators of “good genes,” where traits suggest health, vigor, or genetic fitness. The peacock’s elaborate tail is a classic example; despite being costly to produce and potentially hindering survival, it signals the male’s genetic quality and attracts peahens. Male bowerbirds construct and decorate intricate “bowers” with brightly colored objects to attract females, showcasing their creativity and resourcefulness. Females inspect these bowers and observe male displays to make their mate choices.
Vocalizations also play a role in intersexual selection. Female frogs, for instance, often choose males based on the complexity and duration of their calls, which can indicate male quality or size. These preferences can lead to the evolution of complex songs or calls, even if they make males more conspicuous to predators. The choosy sex benefits by securing a mate with superior genes, greater parental care abilities, or access to valuable resources.
Comparing the Two Forms of Sexual Selection
The fundamental distinction between intrasexual and intersexual selection lies in the nature of the interaction and the source of selective pressure. Intrasexual selection involves direct competition between members of the same sex, typically males, for access to mates. This rivalry often manifests as physical confrontation or aggressive displays, leading to the evolution of traits like weapons or increased body size.
In contrast, intersexual selection involves mate choice, where one sex, usually females, selects mates from the opposite sex. This process favors the development of attractive displays, ornaments, or behaviors designed to appeal to the choosy sex. While intrasexual selection drives the evolution of armaments for competitive prowess, intersexual selection fosters the development of signals for attraction and assessment. Both mechanisms contribute to sexual dimorphism, the observable differences between males and females within a species.