Runaway selection is a powerful type of sexual selection that accounts for the evolution of exaggerated, often impractical traits in many animal species. Proposed by Ronald Fisher in the 1930s, this theory explains how persistent mate choice drives the appearance of seemingly non-adaptive ornaments. The process is characterized by a positive feedback loop between a preference for a trait in one sex, typically the female, and the trait’s expression in the other, usually the male. This mechanism leads to the coevolution and rapid exaggeration of both the preference and the trait, pushing them far beyond what natural selection alone would favor.
The Core Mechanism of Runaway Selection
Runaway selection begins with two heritable components: a variable trait in one sex (e.g., the length of a tail feather) and a variable preference for that trait in the opposite sex. Initially, the trait may have indicated general fitness, but the process relies simply on the fact that some females prefer males with a more pronounced version of the characteristic.
When a female chooses a male based on this trait, she produces offspring that inherit the genes for the attractive trait (if male) and the genes for the preference (if female). This non-random mating links the genes for the preference and the trait together within the population. As the preferred trait becomes more common, the preference for it also spreads, because females with the preference successfully produce “attractive” sons.
This creates a powerful positive feedback loop where the selection pressure on the trait increases as the preference becomes more widespread. The process accelerates because the benefit to a female of having a preference is that her sons will be more successful at mating. The more exaggerated the male trait becomes, the stronger the female preference must be to keep pace, driving the trait to ever-greater extremes.
Genetic Basis and Correlation
The engine driving the accelerating coevolution is the establishment of a genetic correlation between the preference and the trait. This correlation, often referred to as linkage disequilibrium, means that the alleles for the preference and the alleles for the trait tend to be inherited together. This genetic link is established through assortative mating, where choosy females consistently mate with attractive males.
The act of non-random mating links these genes across generations, even if they are located on different chromosomes. For example, females who prefer longer tails mate with males that have longer tails, and their offspring receive the genetic instructions for both attributes. Male offspring inherit the longer tail genes, making them “sexy sons,” and female offspring inherit the preference genes, making them “choosy daughters.”
The preference gene hitchhikes along with the trait gene, which is under direct sexual selection. The preference itself does not need to offer a survival advantage; it spreads simply because it is correlated with the genes that produce successful, attractive sons. For runaway selection to operate, both the preference and the trait must be heritable, meaning they have a genetic basis that can be passed down to offspring. Modern quantitative genetic models confirm that this process can generate rapid evolutionary divergence, provided there is sufficient genetic variation for both components.
Evolutionary Consequences and Limits
The most dramatic consequence of runaway selection is the evolution of exaggerated secondary sexual characteristics. These elaborate ornaments or behaviors—such as extremely long tails, vibrant colors, or complex courtship dances—appear to defy logic from a survival standpoint. The process continues to amplify the trait until the mating advantage it confers is exactly balanced by the survival cost it imposes.
This balancing act represents the ultimate limit to the runaway process, where sexual selection confronts natural selection. An overly long tail, for instance, might make a male more attractive to females, but it increases the metabolic cost of movement and makes the male an easier target for predators. This trade-off between reproductive success and viability prevents the trait from becoming infinitely exaggerated.
The point at which the runaway process stops is a state of equilibrium, where the benefit of being highly attractive is negated by the high cost of survival. If the environment changes, such as the introduction of a new predator, the balance shifts, and the trait may become shorter or less conspicuous. The cost of choice for the female, such as increased time spent searching for a mate, can also stabilize this equilibrium.
Classic Examples in Nature
The most frequently cited example of runaway selection is the peacock’s elaborate tail, or “train.” The male peacock’s plumage is colorful and extremely long, a feature that impairs flight and makes the bird more conspicuous to predators. This impractical ornament is maintained because the female peahen exhibits a strong preference for males with the most ornate and largest tails.
Another well-documented example is the African long-tailed widowbird, a species exhibiting extreme sexual dimorphism. Males possess an extraordinarily long tail that is non-functional for flight, and experimental evidence shows that females prefer males with the longest tails. Researchers demonstrated this preference by artificially elongating the tails of some males, who subsequently acquired mates more easily than control males.
The stalk-eyed fly also provides compelling evidence for the theory in insects. In this species, females prefer males with the widest eye spans, leading to the evolution of exaggerated eye stalks in males. These examples illustrate how female mate choice acts as a persistent directional force, resulting in the evolution of extravagant features.