Intersexual selection is a process where individuals of one sex choose mates based on specific appealing characteristics. This preference drives the evolution of features in the opposite sex, shaping their appearance and behaviors. By favoring certain traits, intersexual selection contributes to the diversity of life.
The Dance of Choice: What is Intersexual Selection?
Intersexual selection, or mate choice, is a form of natural selection where individuals of one sex, typically females, actively select partners from the opposite sex based on desired traits. For example, a female might choose a male based on the brightness of his plumage or the complexity of his courtship display.
This differs from intrasexual selection, which involves competition within the same sex for access to mates, such as males fighting each other. Intersexual selection, in contrast, centers on the preferences of the choosy sex, driving evolutionary changes in the chosen sex.
Strategies of Attraction: How Intersexual Selection Plays Out
Mate choice is driven by various mechanisms and criteria. One category involves direct benefits, where the choosy individual gains immediate advantages from their mate. These benefits can include access to superior territory, better food resources, increased protection from predators, or greater parental care for offspring. For instance, a female bird might choose a male who possesses a high-quality nesting site.
Another mechanism centers on indirect benefits, often linked to the concept of “good genes.” Here, females select males with traits that signal genetic viability, which can lead to healthier and more robust offspring. These traits might be costly to produce, serving as an honest indicator of a male’s underlying genetic quality. For example, a vibrant coloration might indicate resistance to parasites.
Sensory bias occurs when a mate preference evolves because of a pre-existing bias in the choosy sex’s sensory system. This bias might have initially evolved for reasons unrelated to mate choice, such as detecting food or avoiding predators. If a male develops a trait that exploits this pre-existing sensory preference, females may show a greater attraction to him.
Runaway selection, proposed by Ronald Fisher, describes a process where a genetic correlation develops between a male trait and a female preference for that trait. As females consistently choose males with a particular trait, their sons inherit the “sexy trait” and their daughters inherit the preference for it, leading to an exaggeration of the trait over generations. This self-reinforcing process can cause traits to become increasingly elaborate, even beyond what might seem practical for survival.
The Evolutionary Payoff: Why Mate Choice Shapes Species
Intersexual selection drives significant evolutionary changes in species. It often leads to the evolution of elaborate traits, such as extensive plumage or complex courtship dances, which may seem disadvantageous for survival but are favored for attracting mates.
Intersexual selection also contributes to sexual dimorphism, which refers to the observable differences in appearance, size, or other attributes between males and females of the same species. As females consistently choose males with pronounced traits, those traits become more common in the male population over generations. This process can lead to males being significantly larger, more colorful, or possessing more elaborate structures than females.
There are often trade-offs involved for the chosen sex, particularly for males who develop exaggerated traits. Producing and maintaining these elaborate features can be metabolically costly, requiring significant energy and resources. For example, a male might invest heavily in developing an attractive ornament, which could reduce resources available for other functions like immune defense or overall survival. This creates a balance between maximizing attractiveness for reproduction and maintaining viability for survival.
Intersexual Selection in Action: Real-World Examples
The peacock’s tail is a classic example of intersexual selection. Male peacocks possess an extraordinarily long and colorful tail, adorned with iridescent “eyespots.” Peahens prefer males with larger, more elaborate, and symmetrical tails, signaling good health and genetic quality. While a large tail can hinder flight and increase vulnerability to predators, the reproductive advantage from female choice outweighs these survival costs.
Bowerbirds, found in Australia and New Guinea, offer another example. Male bowerbirds construct elaborate structures called bowers, which are decorated display arenas used to attract females. Different species build distinct bower types, from simple clearings with colorful objects to complex structures. Females inspect these bowers and choose mates based on their quality and artistic arrangement, reflecting the male’s creativity, resourcefulness, and genetic fitness.
Frog calls also demonstrate intersexual selection. Male frogs produce diverse vocalizations to attract females, with calls varying in frequency, duration, and complexity. Females often show preferences for certain call characteristics, such as louder calls, calls that are easier to locate, or calls of a specific frequency range. In some species, like the gray tree frog, females prefer males with longer calls, which can indicate better genetic quality, as longer calls require more energy to produce. This preference drives the evolution of more elaborate and energetically demanding vocal displays in males.