What Is Sexual Selection?
Sexual selection is a specific type of natural selection that focuses on an individual’s ability to obtain a mate and successfully reproduce. It drives the evolution of traits that enhance reproductive success, even if those traits might not always contribute to an individual’s survival. This process explains why certain characteristics, behaviors, or physical attributes become more common over generations. The core concept revolves around differential reproductive opportunities among individuals based on their mating success.
Sexual Selection Versus Natural Selection
While both sexual selection and natural selection are mechanisms of evolution, they operate under distinct pressures. Natural selection favors traits that increase an organism’s survival and reproductive output. This can include adaptations for acquiring food, avoiding predators, or tolerating environmental conditions.
In contrast, sexual selection specifically targets traits that directly enhance an individual’s success in attracting and securing a mate. These traits might even impose a survival cost on the individual. For instance, the elaborate, colorful tail of a male peacock makes him more conspicuous to predators and can hinder his escape, yet it is highly advantageous for attracting peahens. Such traits persist because their reproductive benefits outweigh their survival disadvantages, leading to their propagation within the species.
How Sexual Selection Operates
Sexual selection operates through two mechanisms: intrasexual selection and intersexual selection. Intrasexual selection involves direct competition among individuals of the same sex, usually males, for access to mates. This competition often manifests as physical contests, displays of strength, or dominance hierarchies. The victor of these contests gains preferential mating opportunities, passing on the genes associated with their competitive advantage.
This form of selection can lead to the evolution of weaponry, larger body size, or aggressive behaviors in the competing sex. Traits like antlers in deer or strong jaws in certain fish species are often products of intrasexual competition.
Intersexual selection occurs when individuals of one sex, typically females, choose their mates based on desirable traits displayed by the other sex. This process is also known as mate choice. The choosy sex evaluates potential mates based on signals that may indicate good genetic quality, robust health, or the ability to provide resources or parental care. These signals can take various forms, including elaborate ornaments, complex courtship displays, or even the construction of structures.
For example, a female might select a male with vibrant plumage as an indicator of good health and parasite resistance, suggesting he possesses beneficial genes. This preference drives the evolution of exaggerated or complex traits in the displaying sex, as individuals with more attractive signals are more likely to reproduce. The interplay between these two forms of selection shapes the diverse array of reproductive strategies observed across the animal kingdom.
Examples in the Animal Kingdom
Sexual selection is evident in numerous species, shaping a wide range of physical and behavioral adaptations. One clear example of intrasexual selection is observed in male red deer, which engage in fierce battles using their antlers to establish dominance. These contests determine which males gain access to groups of females during the breeding season, with larger antlers and greater fighting prowess leading to higher reproductive success.
Peacocks provide a clear illustration of intersexual selection, where the peahen’s choice drives the evolution of extravagant male ornamentation. Male peacocks display their large, iridescent tail feathers in elaborate courtship rituals to attract females. Females assess the size, symmetry, and vibrancy of these tail feathers, which are thought to signal a male’s genetic quality and overall health. Males with more impressive tails are more likely to be chosen as mates, despite the energetic cost and increased predation risk associated with such a display.
Bighorn sheep offer an example combining elements of both types of selection. Male bighorn sheep engage in head-butting contests, clashing their massive horns together to establish dominance and secure access to females. This direct physical combat represents intrasexual selection, as the strongest males win breeding opportunities. The size and shape of a male’s horns are often a direct result of this intense competition.
Bowerbirds, native to Australia and New Guinea, offer an instance of intersexual selection through intricate mate choice. Male bowerbirds construct elaborate structures called “bowers” from twigs, leaves, and various collected objects, decorating them with colorful items like berries, flowers, and even human-made refuse. Females visit and inspect these bowers, along with the male’s courtship display, before choosing a mate. The quality of the bower and the male’s performance during the display are key factors in the female’s decision, demonstrating sophisticated mate assessment.