AP Biology

## Learning Outcomes

- Describe how Darwin’s observations during the voyage of the HMS Beagle contributed to his theory of natural selection.
- Explain how Wallace independently developed a similar explanation for evolutionary change.
- Distinguish between evolution in populations and natural selection acting on individuals.
- Use the natural selection logic chain: variation, heritability, competition, differential reproductive success, and change over generations.
- Apply Darwin and Wallace’s reasoning to examples such as Galápagos finches, giraffes, moths, and antibiotic-resistant bacteria.
- Avoid common AP Biology errors, including Lamarckian explanations and unclear use of the word fitness.

## AP Biology Syllabus

In AP Biology Unit 7, Darwin and Wallace are mainly tested through the mechanism they helped establish: evolution by natural selection. You are rarely asked to memorize dates or biography. Instead, you must explain how heritable variation, environmental selection pressures, and differential reproductive success cause populations to change over generations.

- Unit 7, Natural Selection, is a major AP Biology unit and is commonly assessed in both multiple-choice and free-response questions.
- Natural selection is a major mechanism of evolution.
- Evolution is a change in the genetic makeup of a population over time.
- Natural selection acts on phenotypic variation among individuals.
- Only heritable variation can cause evolutionary change across generations.
- Competition for limited resources, predation, disease, climate, and other environmental factors can create selective pressures.
- Evolutionary fitness is measured by reproductive success, not physical strength or survival alone.
- AP responses should connect evidence to claims using clear cause-and-effect reasoning.

## 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. Why did Darwin’s observations of finches and tortoises on the Galápagos Islands support the idea that populations change over time?
2. What did Alfred Russel Wallace contribute to the development of the theory of natural selection?
3. Why is it incorrect to say that an individual giraffe evolved a longer neck because it needed to reach leaves?
4. In natural selection, why must a favorable trait be heritable in order to affect evolution?
5. What does fitness mean in AP Biology, and why should you define it when using the term?

## Introduction

Charles Darwin and Alfred Russel Wallace are associated with one of the most important explanations in biology: evolution by natural selection. Both naturalists studied organisms in different parts of the world and recognized the same basic pattern. Individuals in a population vary. Some of that variation is inherited. In a particular environment, certain traits help individuals survive and reproduce more successfully than others. Over many generations, the traits that increase reproductive success become more common in the population.

For AP Biology, Darwin and Wallace matter less as historical figures and more as a way to understand the logic of natural selection. Their ideas explain why populations change, why adaptations match particular environments, and why individuals do not evolve during their lifetimes.

> **Key Term: evolution**
> A change in the genetic makeup of a population over time, often described as a change in allele frequencies across generations.
>
> **Key Term: natural selection**
> A process in which individuals with heritable phenotypes that improve survival or reproduction in a particular environment leave more offspring, causing those traits to become more common over generations.

### Darwin’s observations and the origin of natural selection

Charles Darwin was a British naturalist who traveled on the HMS Beagle during the nineteenth century. During this voyage, he studied organisms in many locations, including the Galápagos Islands. He noticed that organisms on different islands were similar to one another, but not identical. For example, finches on different islands had beaks of different shapes and sizes, and tortoises showed differences in neck length and shell form.

These observations raised a key question: why would related organisms living in different places have different traits? Darwin reasoned that organisms did not simply change their bodies because they needed to. A finch could not choose to grow a longer beak during its life and then pass that acquired beak length to its offspring. Instead, Darwin argued that traits such as beak shape, neck length, and body form were inherited. If individuals with certain inherited traits survived and reproduced more successfully, those traits would become more common in later generations.

Darwin’s reasoning can be applied to finch beaks. Suppose a population of finches contains inherited variation in beak length. During a period when large, hard seeds are the main food source, birds with larger or stronger beaks may be able to feed more efficiently. These birds are more likely to survive long enough to reproduce and may produce more offspring than birds with less effective beaks. If beak size is heritable, offspring will tend to resemble their parents. After many generations, the population may contain a higher proportion of birds with larger beaks.

> **Key Term: heritable variation**
> Genetically based differences among individuals in a population that can be passed from parents to offspring.

Darwin’s explanation also applies to the classic giraffe example. A Darwinian explanation does not say that giraffes stretched their necks and passed the stretched trait to offspring. Instead, it says that ancestral giraffe populations contained variation in neck length. Individuals with longer necks may have reached more food, survived at higher rates, and produced more offspring. Over many generations, alleles associated with longer necks became more common.

> **Exam Warning:**
> Do not write that organisms “try,” “need,” “want,” or “learn” to evolve a trait. AP Biology expects natural selection to be explained through preexisting heritable variation and differential reproductive success.

Darwin later published his ideas in _On the Origin of Species_. The central claim was that natural selection can explain how populations become adapted to their environments and how organisms share common ancestry.

> **Key Term: adaptation**
> A heritable trait that increases an organism’s fitness in a specific environment.

### Wallace’s independent insight and shared theory

Alfred Russel Wallace was another nineteenth-century naturalist. He studied organisms in South America and, later, the Malay Archipelago. Like Darwin, Wallace was struck by the way species were distributed geographically and by the close match between organisms and their environments.

Wallace independently developed an explanation similar to Darwin’s natural selection. He recognized that populations contain variation and that environmental conditions affect which individuals survive and reproduce. Wallace sent Darwin an essay describing this idea, prompting a joint presentation of Darwin’s and Wallace’s work in 1858. Darwin’s book followed the next year.

Wallace’s role matters because it shows that natural selection was not based on a single observation or one special organism. Similar reasoning came from studying many species across different regions. Both Darwin and Wallace saw that the environment can favor some inherited traits over others.

A simple Wallace-style example is camouflage. Imagine a population of insects living on tree bark. Some individuals are lighter and some are darker because of inherited differences. If the bark is dark, darker insects may be harder for birds to see. Those darker insects are less likely to be eaten, more likely to reproduce, and more likely to pass alleles for darker coloration to offspring. Over generations, darker insects may increase in frequency.

This does not mean the environment directly creates the needed trait. The environment does not make individual insects become darker because darkness would help them. Instead, the environment affects which existing phenotypes survive and reproduce.

> **Key Term: selective pressure**
> An environmental factor that affects survival or reproduction, such as predation, competition, disease, food availability, antibiotics, or climate.

Darwin and Wallace’s shared theory can be summarized in one sentence: in a population with heritable variation, individuals with phenotypes that increase reproductive success in a given environment contribute more offspring to future generations, causing the population to evolve.

### The natural selection logic chain

AP Biology often tests whether you can explain natural selection as a sequence rather than as a vague statement. A strong response includes the following steps.

First, variation exists within a population. Individuals are not genetically identical. Variation may affect visible traits such as color, beak shape, fur thickness, or body size, or less visible traits such as enzyme structure, toxin resistance, or immune response.

Second, some of this variation is heritable. This is essential. If a trait helps an individual survive but is not genetically passed to offspring, it will not become more common in future generations by natural selection.

Third, more offspring are often produced than can survive. Food, space, mates, nesting sites, and other resources may be limited. Predators, parasites, disease, and abiotic conditions can also reduce survival.

Fourth, individuals with certain phenotypes survive and reproduce more successfully. This is called differential reproductive success. In AP Biology, this is the most useful way to define fitness.

> **Key Term: fitness**
> Relative reproductive success; the extent to which an organism passes its genes to the next generation compared with other individuals in the population.

Fifth, the alleles associated with favorable phenotypes increase in frequency over generations. This is the population-level change that counts as evolution.

> **Key Term: differential reproductive success**
> The unequal ability of individuals with different phenotypes to survive, reproduce, and pass genes to the next generation.

A full explanation might look like this:

> In the original population, some bacteria already had alleles that made them resistant to an antibiotic. When the antibiotic was added, nonresistant bacteria were more likely to die, while resistant bacteria were more likely to survive and reproduce. Because resistance was heritable, resistant bacteria passed the resistance alleles to offspring. Over generations, the frequency of antibiotic-resistant bacteria increased in the population.

This example is especially useful because it shows what natural selection is not. Bacteria do not become resistant because they “need” to survive the antibiotic. Resistant individuals are already present, usually at low frequency, before selection occurs. The antibiotic changes which bacteria survive and reproduce.

> **Test Tip:**
> When explaining natural selection, include the words or ideas “variation,” “heritable,” “survive and reproduce,” and “over generations.” These phrases help you earn points in free-response answers.

Another common example is the peppered moth in England. Peppered moth populations include lighter and darker wing color variants, and wing color is inherited. Before heavy industrial pollution, trees were often covered with light-colored lichen. Lighter moths blended into this background and were less visible to bird predators, so lighter moths were more common. During the Industrial Revolution, pollution darkened tree bark and killed much of the lichen. Darker moths became better camouflaged, survived predation more often, and reproduced more successfully. Over time, darker moths increased in frequency. Later, pollution controls allowed lighter lichens to return, and lighter moths again gained an advantage.

This example shows that natural selection depends on the environment. A trait is not automatically “good” in all situations. Dark wings were favorable on dark tree bark, while light wings were favorable when light lichen covered the trees.

> **Key Term: phenotype**
> The observable characteristics of an organism, produced by the interaction of genotype and environment.
>
> **Key Term: genotype**
> The genetic makeup of an organism for a trait or set of traits.

Natural selection acts directly on phenotypes because predators, disease, climate, and food availability interact with observable or functional traits. However, evolution is measured by changes in allele frequencies in the population. In other words, selection acts on individuals, but populations evolve.

### Darwin-Wallace reasoning in AP Biology questions

AP Biology questions about Darwin and Wallace usually ask you to apply their reasoning to new data. You might see a graph showing trait frequencies over time, a description of a changing environment, or an experiment involving artificial selection. Your job is to identify the selective pressure, explain which phenotype has higher reproductive success, and predict how the population changes over generations.

For example, a free-response question might describe a drought that changes vegetation from green to brown. A moth population contains green and beige individuals. Birds prey on moths. During the drought, beige moths become more common. A strong explanation would say that beige moths are better camouflaged against brown vegetation, so birds are less likely to see and eat them. Beige moths survive and reproduce at higher rates than green moths. If color is heritable, alleles associated with beige coloration increase in frequency over generations.

A weak answer would say, “The moths turned beige to survive the drought.” That answer loses the core Darwin-Wallace logic because it suggests that individuals changed in response to need.

> **Exam Warning:**
> Avoid saying “the species adapted” without explaining the mechanism. State what variation existed, what selected against or favored each phenotype, and how reproduction changed allele frequencies over time.

Artificial selection is also useful for understanding Darwin’s ideas. In artificial selection, humans choose which organisms reproduce. For example, people have selectively bred plants in the cabbage group for different traits, producing forms such as kale, broccoli, cauliflower, and Brussels sprouts. The mechanism still depends on heritable variation and differential reproductive success, but the selecting agent is human choice rather than the natural environment.

> **Key Term: artificial selection**
> Selection in which humans choose organisms with desired traits to reproduce, causing those traits to become more common over generations.

Darwin used artificial selection as evidence that selection can change populations. If humans can produce large changes in domesticated plants and animals by choosing breeding individuals, then natural environmental pressures can also change populations over many generations.

In AP Biology, fitness deserves special care. In everyday language, “fit” may mean strong, fast, or healthy. In evolution, fitness means reproductive success. A small organism that leaves many surviving offspring has higher fitness than a large, strong organism that leaves none. Survival matters only because it can increase the chance of reproduction.

> **Test Tip:**
> If you use the word fitness, define it as reproductive success. A phrase such as “higher fitness, meaning more surviving offspring produced” is safer than using the word alone.

Darwin and Wallace also connect to common ancestry. If populations split into different environments and experience different selective pressures, they may accumulate different adaptations. Over long periods, this can contribute to speciation. Although later topics in Unit 7 cover phylogeny and speciation in more detail, the starting point is the same: heritable variation plus differential reproductive success can change populations over time.

A final AP point is that natural selection is not random in the same way mutation is random. Mutations can create new genetic variation without regard to whether the change will help the organism. Natural selection is nonrandom because individuals with certain phenotypes consistently have higher reproductive success in a given environment. However, selection can only act on variation that exists. It cannot plan ahead or produce perfect organisms.

## Key Point Checklist

This article has covered the following key knowledge points:

- Darwin developed his theory after observing variation among organisms, including Galápagos finches and tortoises.
- Wallace independently developed a similar explanation for evolutionary change through natural selection.
- Darwin and Wallace recognized that inherited variation affects survival and reproduction.
- Evolution is a change in the genetic makeup of a population over generations.
- Natural selection acts on individuals’ phenotypes, but populations evolve.
- Favorable traits must be heritable to become more common in future generations.
- Differential reproductive success means some individuals leave more offspring than others.
- Fitness in AP Biology means relative reproductive success, not strength or survival alone.
- Selective pressures include predation, competition, disease, climate, food availability, and antibiotics.
- The environment determines which phenotypes are favorable; if the environment changes, the favored phenotype may also change.
- Individuals do not evolve during their lifetimes, and organisms do not develop traits because they need them.
- Strong AP answers explain variation, heritability, selection pressure, reproductive success, and population change over time.

## Key Terms and Concepts

- evolution
- natural selection
- heritable variation
- adaptation
- selective pressure
- fitness
- differential reproductive success
- phenotype
- genotype
- artificial selection


Introduction to natural selection - Darwin and Wallace