AP Biology / Natural Selection
Lesson
What natural selection actually requires
Need is not a mechanism. Differential reproduction is.
Learning goals
- List the conditions for natural selection.
- Explain why individuals do not evolve in the population-genetics sense.
Explanation
In a population, individuals vary. Some of that variation is heritable. If bearers of one version leave more offspring, the next generation’s mix of alleles can shift. That is natural selection. A giraffe does not stretch its neck because it wishes; longer-necked ancestors, if they reproduced more, became more common.
Fitness here means reproductive success in an environment, not gym time. Environments change, so a winning trait can later become a liability. Drift, gene flow, and mutation also change allele frequencies. Selection is one process among several.
Key terms
- Natural selection. A process in which heritable trait differences cause differences in survival or reproduction, shifting a population’s makeup.
- Fitness. An individual’s contribution of offspring to the next generation in a given environment.
Common mistakes
- Writing that organisms adapt because they need a trait.
- Saying a population evolves because one individual learned a skill.
Practice
Original Marlow Works items. Check the answer explanation after you try.
A beetle population includes both green and brown individuals. After several dry years, brown beetles become more common on exposed soil. In a short response, explain how mutation, variation, and selection together account for the change, and identify one condition that would prevent the population from evolving further in this trait.
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
A weightlifter grows larger muscles. Why is that not evolution of the population?
Answer. The change is not a heritable shift in the gene pool; it is one body’s response to training.
Evolution in this course is change in a population’s genetic makeup across generations.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Biology course page
Official eight-unit framework, science practices, and lab resources.
College Board · All rights reserved · accessed 2026-10-01
Open sourceReference only — do not copy
OpenStax Biology 2e
Biology reference with commercial-use restrictions on the current publisher page.
OpenStax · CC BY-NC-SA 4.0 with additional restrictions · accessed 2026-10-01
Link only unless permission is obtained.
Open source