AP Biology / Natural Selection
Lesson
Populations, species, and leftover evidence
Math and fossils are two ways to catch change in the act or after the fact.
Learning goals
- State what Hardy-Weinberg equilibrium assumes.
- Give one morphological and one molecular piece of evidence for common ancestry.
Explanation
Hardy-Weinberg math describes a population that is not evolving: no selection, no mutation, no migration, random mating, and a large size. Real populations break those rules. The model is useful as a comparison, like a silent room you use to hear noise.
Homologous bones, shared DNA sequences, and fossils in dated layers can all support common ancestry. Analogous traits, such as wings in insects and bats, can show similar selection on different starting bodies. Speciation is the splitting of lineages; reproductive isolation is one common classroom marker, not the only possible definition.
Key terms
- Gene pool. All the alleles in a population.
- Speciation. The process by which one lineage splits into separate species.
Common mistakes
- Treating Hardy-Weinberg as a description of nature rather than a null model.
- Calling every similar trait homology.
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
Why are insect wings and bat wings a weak example of homology?
Answer. They perform a similar job but arise from different ancestral structures; they are analogous as wings.
Homology is about shared ancestry of the structure, not shared function alone.
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