Relate variation, changing environments and extinction

Mechanism before a label

Genetic diversity supplies variants on which selection may act, but diversity does not guarantee persistence. Whether any variant helps depends on the environmental challenge, demographic constraints and ecological interactions. Small populations can lose variation through drift even without a directional selective pressure.

Extinction occurs when a lineage loses all living members. Changes in extinction and speciation rates reshape diversity; a mass extinction is not a planned clearing of space for more advanced organisms. Surviving lineages may subsequently diversify when opportunities and constraints change.

An adaptive radiation is diversification associated with new ecological opportunities and lineage traits. A scenario needs evidence of descent, branching and ecological differentiation; several species with different appearances alone do not establish its cause or tempo.

Test heterozygote advantage without confusing dominance

Heterozygote advantage means that the heterozygote has higher relative reproductive success than either homozygote in the specified environment. This can help maintain allelic variation; it is not the same as dominance, which concerns the scored heterozygote phenotype. A changed environment can change the ranking. OpenStax adaptive evolution, checked 31 October 2026.

Original changed case: Equal starting numbers of AA, Aa and aa have relative reproductive contributions 0.8, 1.0 and 0.6. A new environment gives 1.0, 0.7 and 0.9. Which environment shows heterozygote advantage? Can either set alone determine dominance of a separate color phenotype?

Check the fitness ordering

Only the first: Aa exceeds both homozygotes. In the second Aa is below both. Neither reproductive-contribution record states the color of heterozygotes, so neither establishes color dominance. The numbers are fictional relative contributions, not predictions for a real genetic disorder or person.

Predict, compare and calculate

The following record is an original synthetic teaching dataset. Its assigned conditions are not a claim that an actual organism was measured. Write your prediction before revealing the changed condition.

Population variants	Value (count)
Population P allele types	8
Population Q allele types	2
Population P surviving individuals	20
Population Q surviving individuals	40

Synthetic populations after one stipulated environmental challenge.

Your prediction before comparison

This scratch prediction is not saved automatically. Copy it to the lesson notebook to retain it.

Evidence conditionReference comparisonChanged condition / follow-up

Relate variation, changing environments and extinction

Supplied synthetic evidence: Synthetic populations after one stipulated environmental challenge.

Population variants: reference condition

Quantity	Value	Unit
Population P allele types	8	count
Population Q allele types	2	count
Population P surviving individuals	20	count
Population Q surviving individuals	40	count

Task: Propose two measurements beyond allele count needed to evaluate persistence without promising survival.

Allele counts depend on sampling effort and do not measure every aspect of diversity.

Compare with the conditional interpretation

The more allele-rich population has fewer survivors here, illustrating that diversity is not a guarantee.

P exceeds Q by 6 observed allele types, yet reference survivor counts differ20−40=−20. A general claim that more diversity always yields more survivors fails even in this constructed counterexample.

Case index for this investigation

0. Connect heritable variation to relative reproductive success
1. Distinguish selection by people from inheritance and trade-offs
2. Separate drift, gene flow and mutation from selection
3. Calculate allele frequencies and test equilibrium claims
4. Combine fossils, homology and molecular evidence
5. Read a longitudinal evolution study without inventing observations
6. Read branching relationships rather than tip order
7. Explain reproductive barriers and divergence
8. Relate variation, changing environments and extinction
9. Separate origin-of-life hypotheses from ancestry evidence

No numerical confidence, laboratory credit or independent mastery is inferred from opening a supplied case. Save your first prediction and comparison in the notebook.

Reset this unsaved comparison

Worked reasoning

P exceeds Q by 6 observed allele types, yet reference survivor counts differ20−40=−20. A general claim that more diversity always yields more survivors fails even in this constructed counterexample.

The more allele-rich population has fewer survivors here, illustrating that diversity is not a guarantee.

Apply to a new case

Propose two measurements beyond allele count needed to evaluate persistence without promising survival.

Compare your reasoning with the worked response

Measure population size/age structure and variant-associated reproductive performance under the actual challenge, with uncertainty and ecological context. Sampling completeness and connectivity are also relevant.

This public response supplies feedback, not a score for independent reasoning. Record which part of your prediction changed and why.

Evidence and investigation boundary

Allele counts depend on sampling effort and do not measure every aspect of diversity. These outcomes do not establish which variants cause survival or subsequent reproduction.

For a supervised investigation, predeclare the question, biological unit of replication, treatment/comparison assignment, quantities and units, exclusion rules, graph and uncertainty method. Keep the raw records and the first prediction separate from later revisions. A simulated case does not supply laboratory hours or establish AP authorization. No physical manipulation is required by this lesson.

Source and continuation

Framework topics: 7.11. Current AP Biology framework; supporting scientific reference. Sources checked 30 October 2026; all teaching wording, questions and numbers here are original. Return to AP Biology to compare this mechanism with another scale of organization.