Sexual selection is one of the major forces that shapes evolution. It occurs when differences among individuals in their ability to obtain mates or fertilizations cause some individuals to leave more offspring than others. Over generations, traits associated with greater reproductive success can become more common in a population.
Sexual selection is related to natural selection, but the two are not identical. Natural selection broadly favors inherited traits that improve survival or reproduction under particular conditions. Sexual selection specifically concerns success in competition for mates and success in attracting or securing mating partners. A trait can therefore spread even if it does little to help an animal survive—and can sometimes make survival more difficult.
This helps explain why populations can evolve elaborate courtship displays, weapons, mating behaviors, vocal signals, and striking physical differences between males and females.
Sexual selection acts through differences in reproductive success
Evolution occurs when heritable differences among individuals lead to differences in reproductive success. Sexual selection is one way those differences arise.
Suppose members of a population vary in a physical trait, and that variation has a genetic component. If individuals with one version of the trait consistently obtain more mates or achieve more fertilizations, they may contribute more genes to the next generation. If the advantage continues over many generations, the associated trait can increase in frequency.
The crucial point is that survival is not the only route to evolutionary success. An individual that survives for many years but produces few or no offspring may contribute less to future generations than an individual that survives for a shorter time but reproduces successfully.
Sexual selection can therefore favor traits that increase mating success even when those traits have costs.
Two major forms of sexual selection
Sexual selection is commonly divided into intrasexual selection and intersexual selection.
Intrasexual selection: competition within a sex
Intrasexual selection occurs when individuals of the same sex compete for access to mates. In many species, males compete with other males, although competition among females also occurs in some species.
Competition can involve direct physical conflict. Traits such as larger body size, horns, antlers, or specialized weapons can increase an individual’s ability to defeat rivals or control access to mating opportunities.
Competition does not have to involve fighting. Individuals may compete through territorial defense, dominance interactions, sperm competition, or other behaviors that affect which individuals ultimately achieve fertilization.
When such competition repeatedly favors particular inherited traits, those traits can become more common.
Intersexual selection: mate choice
Intersexual selection occurs when individuals choose among potential mates. Often, females are the choosier sex and males compete more intensely, but this pattern is not universal. The direction and strength of sexual selection depend on the reproductive biology and ecology of each species.
Mate choice can favor conspicuous coloration, songs, courtship displays, odors, body structures, or other characteristics. If individuals possessing a particular trait are more likely to be chosen as mates, the trait can increase in frequency when it is heritable.
Mate choice can also favor behavioral characteristics rather than obvious physical features. Courtship persistence, parental behavior, territory quality, or the ability to provide resources can influence mating success in different species.
Why can sexual selection favor costly traits?
At first glance, some sexually selected traits seem puzzling. A large ornament, loud song, or conspicuous display may require substantial energy or make an animal easier for predators to detect. Why would evolution favor something that appears harmful?
The answer is that evolution does not optimize survival alone. A trait can spread when its reproductive benefits outweigh its costs in terms of overall reproductive success.
A conspicuous display, for example, might increase the risk of predation but also substantially increase mating opportunities. If the reproductive advantage is large enough, selection can maintain the trait despite its survival cost.
This creates an important distinction between natural selection for survival-related traits and sexual selection for mating-related traits. In reality, the forces often operate simultaneously. The evolutionary outcome reflects their combined effects.
A trait may therefore represent a compromise: advantageous for attracting mates but disadvantageous in another aspect of an organism’s life.
Sexual selection can produce sexual dimorphism
One of the clearest effects of sexual selection is sexual dimorphism, meaning consistent physical differences between males and females of the same species.
When males and females experience different reproductive pressures, selection can favor different traits in each sex. If males compete intensely for mates, for example, selection may favor larger bodies or specialized weapons in males. If females experience different pressures, those traits may not be favored in females.
Over many generations, the sexes can become increasingly different in size, coloration, anatomy, or behavior.
Sexual dimorphism is not automatically evidence of sexual selection, because ecological differences between the sexes can also produce divergence. But strong differences in traits directly involved in competition or mate attraction can reflect sexual selection.
Sexual selection changes populations, not just individuals
The evolutionary significance of sexual selection becomes clearer when viewed at the population level.
Imagine that a population contains genetic variation for a courtship trait. Individuals with one version of the trait consistently attract more mates. Those individuals leave more offspring on average, and their offspring inherit some of the genetic variation underlying the trait.
The next generation may therefore contain a higher proportion of that trait. If the same process continues, the population’s average characteristics can shift.
This is evolution by natural selection operating through reproductive differences. The individual does not evolve because it needs a particular trait. Rather, differences among individuals in reproductive success alter the genetic composition of generations that follow.
Sexual selection can change behavior as well as anatomy
Evolutionary changes driven by sexual selection are not limited to physical appearance.
Courtship rituals, mating calls, territorial displays, timing of reproduction, mate-searching strategies, and social behaviors can all be shaped by differences in reproductive success.
For example, if a particular courtship behavior consistently increases the likelihood of attracting a mate, individuals genetically predisposed toward that behavior may leave more descendants. Over time, the behavior can become more characteristic of the population.
Sexual selection can also influence how individuals allocate energy. Reproduction involves trade-offs among mating effort, parental investment, growth, maintenance, and survival. Selection acts on these trade-offs through their consequences for reproductive success.
Mate choice can influence the direction of evolution
Mate choice is especially important because preferences themselves can evolve.
Suppose individuals prefer mates with a particular characteristic, and that characteristic is heritable. Individuals possessing the characteristic may then obtain more mating opportunities and produce more offspring. If the preference is also heritable, descendants may inherit both the trait and some tendency to prefer it.
This can create a feedback process in which a trait and a mating preference reinforce each other.
However, mate choice does not necessarily produce runaway exaggeration. Preferences are influenced by many factors, including environmental conditions, sensory systems, costs and benefits of mating, and interactions with other forms of selection. The traits that become favored depend on the specific biological circumstances of the population.
Sexual selection can affect genetic diversity and population structure
Because sexual selection creates unequal reproductive success, it can influence how genetic variation is distributed within a population.
If a small number of individuals obtain most of the matings, their genes can become disproportionately represented in subsequent generations. Strong reproductive inequality can therefore have substantial effects on population genetics.
Sexual selection can also contribute to differences among populations. If populations experience different environments, social systems, mate preferences, or competitive conditions, different traits may be favored in each. Over time, those differences can contribute to evolutionary divergence.
In some circumstances, differences in mating preferences can become particularly important because they reduce mating between populations. If populations increasingly choose different partners or use different courtship signals, reproductive isolation can develop. Reproductive isolation is a major component of the process by which new species can arise.
Sexual selection can sometimes conflict with other forms of selection
Evolutionary pressures do not always point in the same direction.
A trait may improve an individual’s ability to attract mates while reducing survival. Another trait may help an individual survive but make it less successful at obtaining mates. The population’s evolutionary trajectory depends on the balance of these pressures.
This conflict can help maintain intermediate trait values rather than producing unlimited exaggeration. For instance, increasing the size or intensity of a mating signal might initially improve reproductive success, but beyond some point its energetic cost or predation risk could become too great.
Natural and sexual selection are therefore not separate evolutionary worlds. Sexual selection is part of the broader process of selection, and its effects are constrained by the other challenges organisms face.
The strength of sexual selection depends on reproductive biology
Sexual selection is particularly strong when reproductive success varies substantially among individuals.
In a population where most individuals obtain mates and reproduce at similar rates, there may be relatively little opportunity for sexual selection to produce large differences. Where a small fraction of individuals obtains a large share of mating opportunities, selection can be much stronger.
The balance of reproductive investment between males and females also matters. Differences in how much time, energy, or biological resources each sex typically invests in producing and raising offspring can influence competition and mate choice.
These patterns are tendencies rather than universal rules. Sexual selection can take many forms, and the same species may experience different pressures under different environmental or social conditions.
Sexual selection can accelerate evolutionary change in some traits
Because sexual selection can create strong differences in mating success, it can drive rapid changes in traits closely connected to reproduction.
Courtship signals are especially capable of changing over evolutionary time because they are often subject to interactions between traits and preferences. At the same time, the evolutionary rate of any particular trait depends on genetic variation, the strength of selection, inheritance patterns, environmental conditions, and constraints imposed by other aspects of biology.
Sexual selection therefore does not guarantee rapid evolution. It provides a mechanism through which reproductive differences can produce evolutionary change.
What sexual selection tells us about evolution
Sexual selection demonstrates that evolution is shaped by more than the struggle to survive. Organisms must also reproduce, find or compete for mates, attract partners, and successfully fertilize or produce offspring.
Those pressures can transform populations in striking ways. They can produce elaborate ornaments, specialized weapons, complex courtship behaviors, strong differences between the sexes, and substantial variation in reproductive success.
The central principle is simple: when inherited differences affect who reproduces and how successfully, those differences can change the population over generations. Sexual selection is the part of that process driven specifically by competition for mates and differences in mating or fertilization success.
