Separate drift, gene flow and mutation from selection

Mechanism before a label

Genetic drift is random sampling of heritable variants between generations; its effects are especially pronounced in small populations. A bottleneck reduces population size, while a founder event begins a new population with a subset of ancestral variation. Neither requires a fitness advantage for the allele that increases.

Gene flow moves alleles between populations through reproductive migrants or gametes. Mutation introduces sequence variants; its occurrence is not directed toward an organism’s future need. Selection can then change the frequencies of variants whose effects influence reproduction.

A frequency change alone does not identify its cause. Compare replicate populations, migration records, population size and genotype-specific reproduction. Nonrandom mating can change genotype proportions without necessarily changing allele frequencies by itself.

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.

Allele copies	Value (allele copies)
A before sampling	10
a before sampling	10
A after small sample	7
a after small sample	3

A synthetic neutral founder sample of ten copies from a pool initially containing equal alleles.

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

Separate drift, gene flow and mutation from selection

Supplied synthetic evidence: A synthetic neutral founder sample of ten copies from a pool initially containing equal alleles.

Allele copies: reference condition

Quantity	Value	Unit
A before sampling	10	allele copies
a before sampling	10	allele copies
A after small sample	7	allele copies
a after small sample	3	allele copies

Task: Would two equal neutral populations necessarily show the same allele-frequency change? Explain what replication reveals.

One sample cannot establish the distribution of drift outcomes.

Compare with the conditional interpretation

A frequency moves from 0.5 to 0.7 without stipulated fitness differences.

The founder frequency is 7/10=0.7; the initial frequency was 10/20=0.5. The migrant example instead combines counts: (20+30)/(100+100)=0.25, not a selection estimate.

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

The founder frequency is 7/10=0.7; the initial frequency was 10/20=0.5. The migrant example instead combines counts: (20+30)/(100+100)=0.25, not a selection estimate.

A frequency moves from 0.5 to 0.7 without stipulated fitness differences.

Apply to a new case

Would two equal neutral populations necessarily show the same allele-frequency change? Explain what replication reveals.

Compare your reasoning with the worked response

No. Independent random samples can move in different directions. Repeated populations estimate the range of drift outcomes; a systematic fitness association requires additional evidence.

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

Evidence and investigation boundary

One sample cannot establish the distribution of drift outcomes. Different reproductive contributions require different weights.

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: 7.4. 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.