Sexual selection is a type of natural selection that acts on traits affecting mating success. It helps explain why some animals have bright colors, loud calls, large antlers, or complex courtship displays that may seem costly for survival. A male peacock's huge iridescent tail is a classic example because it can attract mates even though it may make movement harder and predators more likely to notice him.
Understanding sexual selection shows that evolution is shaped not only by staying alive, but also by successfully reproducing.
Understanding Biology: Sexual Selection
Evolution can change a population only when individuals differ in a trait and some of that difference is inherited. A bird with a slightly longer tail may gain more mates than other birds. If tail length is partly controlled by genes, its offspring may tend to have longer tails too.
Over many generations, the average tail can become longer. This does not mean every individual chooses consciously or that a trait appears because it is needed.
Small inherited differences already exist. Repeated differences in the number of offspring gradually alter which genes are common in the population.
Competition for mates can take many forms. Male deer may use antlers to push rivals away from breeding areas. Elephant seals fight for access to groups of females, and larger males often win.
In these cases, the important result is not simply winning a fight. It is gaining more chances to reproduce. Competition can involve less obvious traits too.
Sperm competition happens when sperm from more than one male may fertilise a female's eggs. Some insects have mating behaviours or reproductive structures shaped by this competition. The sex that competes most strongly varies among species.
In jacanas, females compete for males because males do most of the egg care. This shows that behaviour depends on the costs of producing and caring for young.
Mate preferences can produce striking feedback. If some females prefer a particular song, males with songs closer to that preference leave more offspring. Young females may inherit the preference while young males inherit the song trait.
The preference and the trait can then become more extreme together. This process is often called runaway selection. Preferences may begin because a feature suggests health, good nutrition, freedom from parasites, or the ability to survive despite a burden.
A bright coat or demanding display is not always an honest signal, though. Some individuals can bluff, and receivers can evolve ways to judge signals more carefully. Scientists test these ideas by measuring mating success, survival, health, and the number of offspring.
Students meet these ideas in nature documentaries, pet breeding, birdwatching, and discussions about human behaviour. Human attraction is influenced by culture, learning, personal experience, and social rules, so it cannot be explained by biology alone. It is important not to use evolutionary explanations to excuse harmful behaviour or claim that one group is naturally better than another.
When studying an example, separate the observation from the explanation. Bright feathers may be linked with mate choice, but researchers still need evidence that individuals with brighter feathers gain more mates or produce more young.
Pay attention to trade-offs, inherited variation, and measurements across generations. These details turn a simple story about an animal trait into a testable scientific explanation.
Key Facts
- Sexual selection favors traits that increase mating success, even if they reduce survival.
- Fitness in evolution means reproductive success, not physical strength alone.
- Total fitness depends on both survival and reproduction: fitness = survival success × reproductive success.
- Intrasexual selection occurs when members of the same sex compete for access to mates.
- Intersexual selection occurs when one sex chooses mates based on traits such as color, song, size, or behavior.
- The handicap principle states that costly traits can honestly signal high genetic quality because only strong individuals can afford them.
Vocabulary
- Sexual selection
- A form of selection in which traits become more common because they improve an individual's chances of obtaining mates.
- Intrasexual competition
- Competition among members of the same sex, often males, for access to mates or breeding territories.
- Mate choice
- The process in which individuals select reproductive partners based on traits that may signal quality, compatibility, or resources.
- Sexual dimorphism
- A difference in appearance or size between males and females of the same species.
- Handicap principle
- The idea that an expensive or risky trait can be an honest signal of an individual's health or genetic quality.
Common Mistakes to Avoid
- Assuming every attractive trait helps survival. Many sexually selected traits, such as a peacock's tail, can reduce survival while increasing mating success.
- Confusing natural selection with sexual selection. Natural selection broadly favors survival and reproduction, while sexual selection specifically favors traits that improve access to mates.
- Thinking mate choice is random. Mate preferences can be shaped by inherited tendencies, sensory biases, and signals linked to health or genetic quality.
- Saying the biggest or brightest individual always wins. Sexual selection depends on the environment, the preferences of potential mates, and the costs of maintaining the trait.
Practice Questions
- 1 In a population of 100 male birds, males with long tails average 6 offspring each and males with short tails average 3 offspring each. If there are 40 long-tailed males and 60 short-tailed males, how many total offspring does each group produce?
- 2 A bright color trait reduces survival from 80% to 60%, but increases average mating success from 2 offspring to 5 offspring. Using fitness = survival success × reproductive success, compare the fitness of dull individuals and bright individuals.
- 3 A male animal has a loud courtship call that attracts females but also attracts predators. Explain how this trait could still evolve by sexual selection.