Sexual selection represents a specific form of natural selection where individuals compete for mating opportunities to pass on their genes to future generations. This evolutionary process acts on an organism’s ability to obtain or successfully reproduce with a mate, leading to the development of traits that might not directly aid survival but enhance reproductive success. Charles Darwin first articulated this concept, noting a “second agency” beyond natural selection that shapes species traits through competition over mates. It explains how certain characteristics, behaviors, or physiological features can become more pronounced over time if they contribute to mating success.
Understanding Intersexual Selection
Intersexual selection occurs when individuals of one sex, typically females, choose mates based on specific traits exhibited by the other sex, often males. This process involves selection between sexes, where the preferences of the choosy sex drive evolutionary changes in the opposite sex. Such mate choice influences the evolution of exaggerated traits, elaborate displays, or distinct behaviors. These features, like bright plumage or complex courtship rituals, can increase a male’s chances of being selected for mating.
Mechanisms of Mate Choice
Mate choice operates through various mechanisms, with choosy individuals evaluating potential partners based on specific criteria. One mechanism involves direct benefits, where the choosy mate gains immediate advantages from the chosen partner. Examples include nuptial gifts provided by the male, access to superior territory, or direct parental care that reduces the burden on the choosy individual. This directly improves the reproductive success and survival of the choosy individual or their offspring.
Another significant mechanism is the pursuit of indirect benefits, often linked to “good genes.” Here, the choosy individual selects mates whose traits signal superior genetic quality, which can then be inherited by the offspring. Costly signals, such as elaborate and energy-intensive displays, exemplify this by indicating a male’s underlying health and vigor, as only high-quality individuals can afford such displays. The “handicap principle” expands on this, suggesting that exaggerated traits, even if they slightly hinder survival, reliably signal genetic fitness because only the most robust individuals can survive despite such a “handicap.”
Sensory bias also plays a role, where a mate preference evolves because the chosen trait exploits a pre-existing sensory sensitivity in the choosy sex. This sensitivity might have originally evolved in a non-mating context, such as foraging. For instance, a preference for a particular color might stem from an association with a food source, and then males evolve to display that color to attract mates.
Real-World Examples in Nature
The elaborate tail of the male peacock is a classic example of intersexual selection, driven by the female peahen’s preference for males with more numerous, larger, and densely distributed eyespots on their trains. Peahens selecting males with such showy features tend to produce offspring that are larger at birth and exhibit better survival rates. This costly display serves as an honest signal of the male’s genetic quality and overall health, aligning with the handicap principle.
Bowerbirds offer another compelling illustration, where males construct highly decorated structures called bowers to attract females. These architectural displays, often adorned with colorful objects like berries, flowers, or even human-made items, demonstrate the male’s ability to gather resources and his dedication. Females inspect these bowers and choose mates based on the quality and arrangement of the decorations, indicating direct benefits or a male’s skill. Similarly, the complex mating calls of certain frog species are crucial in intersexual selection. Females often prefer males with calls that indicate larger body size or greater vigor, signaling good genes that could lead to more robust offspring.
Distinguishing Intersexual from Intrasexual Selection
Sexual selection encompasses two primary forms: intersexual selection and intrasexual selection. Intrasexual selection involves competition within the same sex for access to mates, often manifesting as direct physical contests or displays of dominance. For example, male elephant seals engage in fierce fights for control over harems of females, with the strongest males monopolizing mating opportunities. Traits like larger body size, antlers in deer, or tusks in elephants evolve due to this competition, enhancing fighting ability or intimidation. While intersexual selection is about attraction and choice, intrasexual selection is characterized by direct competition and rivalry for reproductive access.