7.4Natural Selection

Population Genetics — AP BIOLOGY Revision Notes & Topical Questions

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7.4 Population Genetics — Notes — AP Biology notes: Explain how random occurrences and other mechanisms change the genetic makeup of a population.Population = a group of the same species living in one area and interbreeding.Gene pool = all the alleles in the population (the total…

Allele frequency
Genetic drift
Founder effect
Bottleneck effect
Non-random mating
Balancing selection

Section 1 of 11Populations and allele frequency

Population = a group of the same species living in one area and interbreeding.

Gene pool = all the alleles in the population (the total set of alleles at all loci).

Allele frequency = copies of an allele ÷ total alleles for that gene (e.g. A = 0.7); genotype frequencies are the proportions of AA, Aa and aa individuals.

Evolution — a change in allele frequencies in a population over generations; a population evolves, not an individual.

Causes of change — natural selection and random chance (mutation, drift, migration).

Section 2 of 11Mutation

Origin of variation — mutation is the original, random source of genetic variation and produces new alleles.

Gametes — only mutations in dividing cells of the sex organs pass to the next generation; somatic mutations do not affect selection.

Allele effects — a new allele may be advantageous, disadvantageous or have no effect, and advantageous ones are more likely to be passed on.

Asexual species — mutation is the only source of variation.

Rate — alone it changes frequencies only slowly because rates are low, but it supplies the raw material.

Section 3 of 11Natural selection

Natural selection changes frequencies in a predictable direction, and alleles that raise fitness increase.

Forms — directional, stabilising or disruptive.

Balancing selection — heterozygote advantage and frequency-dependent selection keep multiple alleles.

Adaptation — natural selection is the only mechanism that consistently produces adaptation.

Section 4 of 11Genetic drift

Genetic drift = a non-selective process in which chance events influence which alleles pass to the next generation, especially in small populations.

Mechanism — only half of an individual's alleles are in the gametes,

so some are not passed on through random meiosis and fertilisation; alleles can be lost or fixed.

Fixation — one allele reaches frequency 1 and the other is lost, reducing diversity.

Small population — in 10 individuals (20 alleles, 10 of allele A) the frequency can move to 0.4 or 0.6 in one generation.

Large population — in 1000 individuals (2000 alleles) sampling error is tiny, so the frequency stays near 0.50.

Founder effect — a small group colonises a new area,

so the new population has the founders' alleles only.
- (e.g. white-scaled lizards on an island, Ellis–van Creveld syndrome in the Amish)

Bottleneck effect — a disaster or overhunting sharply cuts population size, leaving survivors that are a small sample of the gene pool with reduced diversity (cheetahs, northern elephant seals); rare alleles can be lost and others become common.

Section 5 of 11Gene flow and non-random mating

Gene flow (migration) = movement of alleles or individuals (or gametes, e.g. pollen) between populations.

Effects — makes populations more similar, introduces new alleles and increases variation, though migration can also cause drift.

Non-random mating — inbreeding and assortative mating change genotype frequencies (more homozygotes) but not allele frequencies by themselves.

Sexual selection — mate choice affects which alleles are passed on.

Section 6 of 11Comparison of the mechanisms

Mechanism	Random?	Effect	Population size	Example	Variation
Natural selection	No	Adaptive change	Any	Antibiotic resistance	Can reduce
Genetic drift	Yes	Random change	Small	Cheetahs	Reduces
Bottleneck	Yes	Loss of alleles	Suddenly small	Northern elephant seals	Reduces
Founder effect	Yes	Different frequencies	Small new group	Amish polydactyly	Reduces
Gene flow	—	Populations become similar	Any	Pollen spread	Increases
Mutation	Yes	New alleles	Any	—	Increases
Non-random mating	—	Changes genotypes, not alleles	Any	Inbreeding	Fewer heterozygotes

Section 7 of 11Conservation and human populations

Small populations — lose diversity through drift and inbreeding,

so reduced variation limits their ability to respond to environmental change and raises extinction risk.

Florida panther — few survivors had heart defects and low fertility; introducing Texas individuals (gene flow) raised diversity and numbers.

Conservation — maintain large effective population sizes, wildlife corridors and managed breeding.

Human groups — migration, the founder effect and selection explain allele frequency differences,

such as lactase persistence or sickle cell frequencies.

Section 8 of 11Evidence and identifying the mechanism

Fruit flies — small replicate populations drift in different directions and lose alleles, while large populations stay stable.

Long-term bacterial experiments — samples frozen over thousands of generations show new alleles arising and rising by selection.

Statistics — tests such as chi-squared show whether observed change differs from chance.

Null model — Hardy–Weinberg equilibrium: if frequencies change, one of the mechanisms is acting.

Identifying the mechanism — selection gives directional change linked to survival, drift gives random change in small populations, gene flow needs arriving individuals with different alleles,

and a brand-new allele is mutation.

Section 9 of 11More from the specification

Section 10 of 11Key terms

Term	Meaning
Population	interbreeding group of one species in one area
Gene pool	all alleles in a population
Allele frequency	proportion of a gene's copies that are a given allele
Genetic drift	random change in allele frequencies
Fixation	an allele reaching frequency 1
Founder effect	reduced diversity in a population started by few individuals
Bottleneck effect	loss of diversity after a sharp population decline
Gene flow	movement of alleles between populations
Non-random mating	mate choice not at random, e.g. inbreeding
Balancing selection	selection that maintains multiple alleles

Section 11 of 11Exam technique

Saying drift is selection — drift is random and not driven by an advantage.

Thinking drift affects large populations equally — it is strongest in small ones.

Saying somatic mutations are inherited — only gamete-line mutations are.

Saying non-random mating changes allele frequencies — it changes genotype frequencies.

Confusing gene flow with mutation — gene flow moves existing alleles.

Last updated: 14 October 2026