Study guides & original practice
Natural selection practice
Natural selection changes populations when heritable differences affect reproductive success in a particular environment. Individuals do not develop a useful inherited trait simply because they need it.
By ExamCat · Original study lesson · Published September 24, 2026
Understand the method
Separate variation, inheritance and differential reproduction. An environmental change can alter which variants leave more descendants. A short-term change within an individual, such as acclimation, is not automatically an evolutionary change in allele frequencies.
Worked example, step by step
A bacterial population contains a rare heritable resistance variant before an antibiotic is applied. After exposure, resistant bacteria make up a larger fraction.
- Identify existing variation: some bacteria are resistant and others susceptible.
- Identify the selective condition: exposure reduces reproduction or survival of susceptible variants.
- Explain inheritance: descendants of resistant survivors can inherit resistance.
- Describe the population outcome: resistance becomes more frequent; the antibiotic did not teach every individual to resist.
A mistake to watch for
An increased fraction does not necessarily mean the total number of resistant bacteria increased immediately. Susceptible bacteria may have declined, changing the denominator.
Try it yourself: three questions
Write an answer before opening the explanation. For calculations, keep your units and intermediate steps; for concepts, state the reason for your choice.
1. Is a trait beneficial in every environment if selection favors it somewhere?
Show answer and explanation
No. Benefits and costs depend on conditions; a variant can be advantageous in one setting and costly in another.
2. A muscle grows after exercise. Is this alone evidence of natural selection?
Show answer and explanation
No. It is an individual change and does not demonstrate heritable differences in reproductive success across a population.
3. Why does heritability matter?
Show answer and explanation
If the relevant difference is not transmitted to descendants, differential reproduction will not propagate that trait through the proposed inheritance pathway.
Build your next practice session
Mark the first step where your reasoning differed from the explanation. Turn that step into a question, answer it without looking, then change one condition in the example and explain what would change. A correct guess is a reason to revisit the method.
Use the free flashcard maker for your own question-and-answer pairs, or bring your course material into PDF-to-flashcards practice.
Further reading
OpenStax Biology 2e provides textbook background. The worked scenario and practice questions on this page were written independently for ExamCat.
Related lessons
Practice your own class material
Generate flashcards, notes and quizzes from your lecture notes in Exam Cat.
Try 7 days free ↗Yearly plan: 7 days free, then $39.99/year. Cancel before the trial ends to avoid a charge.
APUSH study guide · Biology study guide · APUSH practice · All study tools