Evolution and Natural Selection

Suppose pale beetles leave more offspring than dark beetles in a sandy habitat, and their offspring inherit the color difference. Over generations, pale beetles may become common. That is natural selection. Evolution includes this process and other changes in the inherited characteristics of populations, including changes caused by mutation, genetic drift, and gene flow.

The Logic of Natural Selection

Some differences among beetles are heritable. If these differences affect the number of surviving offspring, the associated traits can change in frequency across generations. A beetle that survives for years but leaves no offspring makes no direct genetic contribution to the next generation. Fitness measures relative reproductive success in the environment being studied.

In the sandy habitat, hiding from birds could give pale beetles a reproductive advantage. Test that connection. Measure survival and offspring production for each inherited color, because a change in color frequency alone cannot establish its cause.

Where Variation Comes From

Natural selection needs variation to work on, and that variation comes mainly from mutations (random changes in DNA) and from the mixing of genes during reproduction. Selection does not create traits on purpose. It simply favors whichever existing traits happen to help. Conditions and interactions affect survival, mating, and offspring success, producing differences in reproductive contribution.

Adaptation and Common Misunderstandings

An adaptation is a heritable feature shaped by natural selection because it improved reproductive success in a particular environment. An individual does not acquire an inherited adaptation simply by needing it. In a selection explanation, identify the preexisting variation, its inheritance, and the resulting difference in reproduction.

Can chance produce the same pattern?

Suppose pale beetles rise from 40% to 70% of a small population. That observation alone does not prove selection. A storm could remove beetles regardless of color, changing frequencies by genetic drift. Immigration could also introduce more pale beetles. To support a selection explanation, look for inherited color differences that are associated with unequal reproductive success under the conditions studied.

Watch: A Short Video Lesson

Use this Natural History Museum lesson to check the sequence from inherited variation to differences in reproduction. Keep individual survival separate from population change.

A Routine for Evolution Questions

1. Identify the heritable variation present before selection.
2. Use the evidence to connect that variation with unequal reproductive success.
3. Those traits are passed on and become more common over generations.
4. A beetle’s need for camouflage does not cause its offspring to inherit a new color.

Practice

1. What is evolution?
2. What does “fittest” mean in natural selection?
3. Where does the variation that selection acts on come from?
4. What is an adaptation?
5. Does an individual evolve during its lifetime?
6. Why can the same trait be favored in one habitat but not another?

Answers

1. Change in heritable characteristics of populations across generations.
2. Higher relative reproductive success in the relevant environment.
3. Mutations and the mixing of genes during reproduction.
4. A heritable feature shaped by natural selection because it improved reproductive success in a particular environment.
5. No. Evolution acts on populations over generations.
6. Environmental conditions and interactions affect reproductive success, so the advantage of a heritable trait can change between habitats.

Where This Fits in Your Science Prep

Natural selection builds on heredity and leads into genetic variation, common ancestry, and cladograms. See all topics on the Science Topics Hub.

For the AP course sequence and practice tests, use the AP Biology study hub.

Further reading: OpenStax Biology 2e: Adaptive Evolution