AP Biology

## Learning Outcomes

- Define sexual selection as a form of natural selection based on differences in mating success.
- Distinguish between intersexual selection and intrasexual selection using clear biological examples.
- Explain how sexually selected traits can increase reproductive success even when they reduce survival.
- Connect sexual selection to heritable variation, phenotype, allele frequency change, and evolution in populations.
- Interpret AP-style data on mating success, trait frequency, and differential reproductive success.
- Avoid common errors, such as saying that individuals evolve or that organisms develop traits because they need them.

## AP Biology Syllabus

Sexual selection fits within Unit 7 because it is one mechanism by which differential reproductive success changes populations over generations. For the AP Biology exam, you should be able to apply the same logic used for natural selection: populations contain heritable variation, some phenotypes lead to greater reproductive success, and trait frequencies change over time. Sexual selection specifically focuses on access to mates and success in attracting or competing for mates.

- Sexual selection is a type of natural selection.
- Natural selection acts on phenotypes of individuals, while evolution is measured as change in populations.
- Sexually selected traits must be heritable to increase in frequency across generations.
- Mate choice and competition for mates can both create differential reproductive success.
- A trait can be favored by sexual selection even if it carries a survival cost.
- AP questions may ask you to interpret graphs, compare mating success, or justify predictions using evidence.
- Avoid Lamarckian wording: individuals do not evolve traits because they need or want them.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. How is sexual selection similar to, and different from, natural selection caused by predation or resource competition?
2. Why might a brightly colored male bird have high reproductive success even if it is more visible to predators?
3. What is the difference between intersexual selection and intrasexual selection?
4. If females in a population prefer males with longer tails, what must be true for long tails to become more common over generations?
5. Why is it inaccurate to say that “male deer evolved antlers because they needed them to fight”?

## Introduction

Sexual selection occurs when individuals with certain characteristics are more likely to attract mates or gain access to mates than other individuals. Because those individuals reproduce more successfully, the traits linked to mating success can become more common in a population over time. This makes sexual selection a specific form of natural selection: the selective pressure is not directly food, climate, disease, or predation, but reproductive access.

This subtopic is exam-relevant because it tests your ability to connect phenotype, inheritance, differential reproductive success, and population change. AP Biology often rewards precise explanations. A strong answer does not simply say that a trait is “better.” It explains why a phenotype increases the number of surviving offspring produced and how that causes the genetic makeup of the population to change over generations.

> **Key Term: Sexual selection**
> A form of natural selection in which individuals with certain heritable traits have greater mating success than others, causing those traits to become more common over generations.

### Sexual selection as a form of natural selection

Natural selection depends on several core ideas. First, variation exists in populations. Second, at least some of that variation is heritable. Third, individuals with some phenotypes survive and reproduce more successfully than others. Fourth, over many generations, favorable phenotypes become more common.

Sexual selection follows the same logic, but it focuses on reproductive success through mating. An individual may be excellent at surviving but still have low fitness if it rarely mates or produces few offspring. By contrast, an individual with a risky or costly trait may have high fitness if that trait leads to many successful matings and offspring.

> **Key Term: Fitness**
> Differential reproductive success; in AP Biology, fitness refers to how successfully an organism passes its genes to the next generation.

For example, a male bird with very bright feathers may be easier for predators to see. From a survival-only point of view, dull coloration might seem more advantageous. However, if females strongly prefer brighter males and those males mate more often, the bright-color trait may increase in frequency. Sexual selection can therefore favor traits that appear harmful to survival if the reproductive advantage is large enough.

This is a key idea: natural selection is not simply “survival of the strongest.” It is about differential reproductive success. Survival matters because organisms usually must live long enough to reproduce, but reproduction is the direct link to evolution. A phenotype that increases mating success can be strongly favored if it leads to more offspring carrying the alleles associated with that phenotype.

> **Test Tip:**
> When using the word fitness, define it in your answer as reproductive success. AP Biology graders are looking for the idea that individuals with certain phenotypes leave more offspring, not just that they are stronger, healthier, or longer-lived.

Sexual selection also reinforces the difference between individuals and populations. Individuals may be selected because of their phenotypes, but populations evolve as allele and trait frequencies change over generations. A peacock does not evolve a larger tail during its lifetime because it wants more mates. Instead, males with inherited tail traits that improve mating success leave more offspring, so those alleles may become more common in later generations.

> **Exam Warning:**
> Do not write that individuals “adapt,” “evolve,” or “change their genes” because they need to attract mates. Variation already exists, often due to mutation and genetic recombination. Selection changes which variants become more common in the population.

### Mate choice and intersexual selection

One major form of sexual selection is mate choice. This occurs when individuals of one sex choose mates based on particular traits in the other sex. In many textbook examples, females choose males, but AP Biology questions may describe any pattern. The key point is that preference affects mating success.

> **Key Term: Intersexual selection**
> Sexual selection that occurs when individuals of one sex choose mates of the other sex based on certain traits.

Intersexual selection can favor traits such as bright coloration, courtship dances, songs, chemical signals, nest-building behavior, or body ornaments. These traits may help potential mates assess quality. For instance, a male bird that sings frequently and accurately may signal health, energy availability, or territory quality. If females prefer that song pattern and the trait has a heritable basis, males with the preferred song may produce more offspring.

A classic example is the peacock’s tail. Male peafowl can have long, colorful trains that are costly to grow and may make movement more difficult. Yet females may prefer males with larger or more elaborate trains. If males with those traits obtain more mates, the alleles linked to such tail features can increase in the population. This does not mean the tail is useful for escaping predators. It means the tail can increase reproductive success through mate attraction.

Another example involves birds of paradise, in which males often display bright plumage and elaborate courtship movements. If females repeatedly choose males with particular display traits, those traits may become more common in males over generations. The female preference itself may also have a genetic basis and may coevolve with the male display trait, although AP Biology questions usually focus on the simpler link between preference and reproductive success.

Mate choice can also involve direct benefits. A female might choose a mate that holds a high-quality territory, provides parental care, or offers food. In this case, the chosen trait may increase the survival of offspring as well as mating success. In other cases, a chosen trait may be an indirect signal of genetic quality. For AP Biology, you do not need to overstate what a trait “proves.” Instead, phrase cautiously: if individuals with the trait obtain more mates and produce more offspring, the trait can increase in frequency.

> **Key Term: Mate choice**
> A pattern of sexual selection in which individuals select reproductive partners based on traits such as appearance, behavior, territory, or courtship signals.

To answer AP questions about mate choice, look for data comparing reproductive success. Suppose a graph shows that males with longer tails average eight offspring per breeding season, while males with shorter tails average three. A strong explanation would be: males with longer tails have higher mating success, so if tail length is heritable, alleles contributing to longer tails will be passed to more offspring and may increase in frequency over generations.

Notice the condition “if tail length is heritable.” Without inheritance, selection cannot cause long-term evolutionary change. A trait caused only by temporary environmental conditions could affect mating success, but it would not necessarily become more common genetically in the next generation.

### Competition for mates and intrasexual selection

A second major form of sexual selection is competition between members of the same sex for access to mates. This often involves physical contests, displays of strength, territorial defense, or behaviors that exclude rivals.

> **Key Term: Intrasexual selection**
> Sexual selection that occurs when individuals of the same sex compete with one another for access to mates.

Intrasexual selection can favor traits such as larger body size, horns, antlers, tusks, aggression, or territorial behavior. Male deer, elk, and moose provide familiar examples. Antlers can be costly to grow and may require large amounts of energy and minerals. They can also make movement through dense vegetation harder. However, males with larger antlers may win more contests against other males, gain access to more females, and produce more offspring. If antler size has a heritable component, alleles associated with larger antlers may become more common.

Elephant seals show another clear pattern. In some species, large males fight or display to control access to groups of females. A small number of successful males may father many offspring, while many other males father none. This creates strong differential reproductive success. Even if large body size requires more food and carries risks, it may be favored when it greatly increases mating success.

Intrasexual selection does not always require direct fighting. Some animals defend territories that contain resources needed by mates. Others use displays that allow rivals to assess strength without constant injury. For AP Biology, focus less on the drama of the behavior and more on the selection mechanism: individuals with certain phenotypes gain greater access to mates, reproduce more, and pass on alleles linked to those phenotypes.

> **Key Term: Sexual dimorphism**
> Differences in size, appearance, or behavior between males and females of the same species.

Sexual dimorphism often results from sexual selection. Male peacocks have elaborate trains, while females are less showy. Male deer have large antlers in many species, while females may lack them or have much smaller ones. Differences can also involve body size, coloration, song, or behavior. However, do not assume every difference between sexes is caused only by sexual selection. Natural selection, ecological roles, hormones, and developmental patterns can also contribute. AP questions will usually provide evidence you should use.

> **Test Tip:**
> If a question asks for evidence of sexual selection, cite reproductive data. Strong evidence includes differences in mating success, number of offspring, mate preference, or success in contests. Survival data alone may not show sexual selection unless it is connected to reproduction.

In many cases, sexual selection is stronger in the sex with greater variation in reproductive success. For example, if some males mate many times and others do not mate at all, traits that affect male mating success can be under strong selection. Still, avoid the oversimplification that “males always compete and females always choose.” In some species, males provide major parental care or females compete for access to males. The AP exam may present unfamiliar organisms, so base your answer on the data rather than a memorized stereotype.

### Trade-offs, allele frequencies, and AP-style reasoning

Sexual selection often involves trade-offs. A trait may increase mating success while decreasing survival. Bright coloration may attract mates and predators. Large antlers may help in contests but require energy to grow. Loud calls may attract females and also alert predators or rivals. Whether the trait increases in frequency depends on the net effect on reproductive success.

> **Key Term: Trade-off**
> A situation in which a trait has both costs and benefits, such as increasing mating success while reducing survival.

For AP Biology, the correct explanation is not that the trait is “good” in every way. A trait is favored in a particular environment or social context if individuals with that trait leave more offspring than individuals without it. If predation increases, the balance may shift. For example, a very bright color might be favored when predators are rare but selected against when predators are common. Sexual selection, like other forms of selection, depends on conditions.

Sexual selection can also interact with other mechanisms of evolution. Mutation and recombination produce genetic variation. Sexual selection acts on phenotypic variation that affects mating success. If mating is nonrandom because individuals prefer certain phenotypes, Hardy-Weinberg expectations may not apply. Nonrandom mating can change genotype frequencies, and when mate choice or competition leads to differential reproductive success tied to heritable traits, allele frequencies may change over generations.

> **Key Term: Allele frequency**
> The proportion of a specific allele among all copies of that gene in a population.

Consider an AP-style scenario: in a fish population, males vary in red coloration. Females prefer redder males, and researchers find that red males produce more offspring than dull males. Over ten generations, the allele associated with red coloration increases from 0.30 to 0.62. This is evidence of evolution because the allele frequency changed over time. The likely mechanism is sexual selection because the advantage was linked to mate choice and reproductive success.

A clear free-response answer might read:

Red males had greater mating success because females preferred them. Since coloration is heritable, red males passed alleles associated with red coloration to more offspring than dull males did. Over generations, the frequency of the red-color allele increased in the population, showing evolution by sexual selection.

This answer earns credit because it includes variation, heritability, differential reproductive success, and population-level change.

Now consider a different scenario: males with longer calls attract more mates, but a predator also uses the call to locate them. If predation increases, the average call length might decrease if long-calling males are killed before reproducing. Or an intermediate call length might be favored if it attracts mates while reducing predation risk. The AP exam may ask you to predict outcomes from data. Your prediction should match the given evidence and explain both survival and reproductive effects.

> **Exam Warning:**
> Do not assume the most extreme sexual trait will always be favored. If a trait becomes too costly, individuals with intermediate versions may have higher reproductive success.

Sexual selection can sometimes produce directional selection if one extreme is favored, such as longer tails consistently leading to more matings. It can also help maintain variation if different traits are favored under different conditions. For example, in one habitat, bright males may attract many mates; in another, predators may remove bright males before they reproduce. The direction of selection can change when the environment or mating preferences change, just as in examples such as peppered moth coloration under changing pollution conditions.

When interpreting graphs, pay attention to what is on each axis. A graph of “number of mates” or “number of offspring” directly relates to reproductive success. A graph of “survival percentage” only relates to sexual selection if the question connects survival to mating and offspring. If confidence intervals or error bars are included, avoid claiming that groups differ if their error bars strongly overlap unless the question provides statistical support.

In experimental design questions, you may be asked to test whether a trait affects mate choice. A good design would control other variables while changing the trait of interest. For instance, researchers could present female insects with male courtship songs that differ only in pulse rate, then measure the proportion of females approaching each song. The independent variable would be song pulse rate, and the dependent variable would be female response or mating frequency. A valid conclusion would connect the measured preference to potential reproductive success, while recognizing that field conditions may add other factors.

Sexual selection is not limited to animals with showy traits. It can involve chemical signals, timing of reproduction, courtship behavior, territory quality, or parental care. What matters is the same AP Biology pattern: heritable traits affect reproductive success, and populations change over generations.

## Key Point Checklist

This article has covered the following key knowledge points:

- Sexual selection is a form of natural selection based on differences in mating success.
- Natural selection acts on individuals, but populations evolve over generations.
- Fitness means differential reproductive success, not simply strength or survival.
- Sexually selected traits must be heritable to cause evolutionary change.
- Intersexual selection involves mate choice between sexes.
- Intrasexual selection involves competition among members of the same sex for mates.
- Sexual dimorphism can result from sexual selection acting differently on males and females.
- Sexually selected traits may carry survival costs but still increase reproductive success.
- AP explanations should include variation, heritability, differential reproductive success, and change in trait or allele frequency.
- Individuals do not evolve because they need traits; populations change as alleles become more or less common.
- Data on offspring number, mating frequency, or contest success can support claims about sexual selection.
- The direction of selection can change if environmental conditions, predation pressure, or mate preferences change.

## Key Terms and Concepts

- Sexual selection
- Fitness
- Intersexual selection
- Mate choice
- Intrasexual selection
- Sexual dimorphism
- Trade-off
- Allele frequency


Natural selection - Sexual selection