Biology for AP Courses · The Evolution of Populations
Population Evolution
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In 30 seconds
Evolution is often pictured as an individual animal changing — a giraffe stretching its neck — but that picture is wrong. Evolution happens to populations, not individuals. A Population All interbreeding individuals of one species in a given area Full entry → is all the individuals of one species living in a given area that can interbreed. Every population carries a Gene pool All alleles at all loci present in a population Full entry →: the complete set of alleles at all gene loci present in all its members. The population's genetic character is described by allele frequencies — the proportion of each allele at a locus — and genotype frequencies — the proportion of each genotype among individuals. Microevolution Change in allele frequencies across generations Full entry → is simply a change in allele frequencies from one generation to the next. For that change to occur, the population needs raw material: genetic variation, supplied by Mutation A change in DNA sequence Full entry →, Gene flow Movement of alleles between populations via individuals or gametes Full entry →, and sexual reproduction. This topic builds the vocabulary and the counting skills that the rest of Chapter 19 — Hardy-Weinberg equilibrium and adaptive evolution — depends on.
Why this matters
The population view of evolution is not an abstraction — it is the framework for some of the most urgent real-world biology. Antibiotic resistance in bacteria is population evolution: a bacterial population's allele frequencies shift as resistant strains out-reproduce susceptible ones under drug pressure. The same logic explains pesticide resistance in insects and antiviral resistance in viruses. Conservation biologists use allele frequencies to assess whether small, isolated populations are losing genetic diversity. Breeders and farmers manage gene pools deliberately. And on the AP® exam, computing allele frequencies from observed phenotypes is a staple free-response skill — if you can count alleles correctly here, the Hardy-Weinberg problems in the next topic become straightforward.
The college version
Core Concepts
Populations and gene pools
A population is a group of interbreeding individuals of the same species in a defined area. The gene pool is the sum of all alleles in that population — think of it as the population's full deck of genetic cards. Two different frequencies describe it:
- Allele frequency Proportion of all copies of a gene that are a specific allele (p or q) Full entry →: the fraction of all copies of a gene at a locus that are a particular allele. If a locus has two alleles, A and a, and p = frequency of A, q = frequency of a, then p + q = 1.
- Genotype frequency Proportion of individuals with a given genotype (AA, Aa, aa) Full entry →: the fraction of individuals carrying a particular genotype (AA, Aa, or aa). These three fractions also sum to 1.
To count: if a population of 100 plants has 50 with genotype AA, 40 with Aa, and 10 with aa, then there are 200 alleles total (two per diploid individual). The A allele appears twice in each AA (50 × 2 = 100) and once in each Aa (40), so A = 140 of 200 alleles → p = 0.70; a = 40 + 20 = 60 of 200 → q = 0.30.
Evolution = change in allele frequencies
Microevolution is a change in allele frequencies in a population across generations. The critical implication: an individual does not evolve — an individual's genotype is fixed at conception. What changes over time is the population's genetic composition, as individuals with different genotypes survive and reproduce at different rates. A shift in the average phenotype that is purely environmental — a well-fed plant growing taller — is not evolution, because no allele frequencies changed. Only heritable change in the population's genetic makeup counts.
Sources of genetic variation
Variation is the fuel of evolution; without it, selection has nothing to act on. Three processes supply it:
- Mutation: a change in the DNA sequence. Mutation is the ultimate source of new alleles — no other process creates genuinely new genetic information. Individual mutations are rare and usually neutral or harmful, but across large genomes and many individuals they supply a steady trickle of novelty.
- Gene flow (also called gene migration): the movement of alleles between populations, through migrating individuals or moving gametes (pollen, sperm). Gene flow can introduce alleles a population lacks and tends to make neighboring populations more similar — it counteracts divergence.
- Sexual reproduction: meiosis and fertilization reshuffle existing alleles into new combinations. Crossing over and independent assortment (Chapter 11) plus random fertilization create new genotypes every generation. Sexual reproduction does not change allele frequencies by itself, but it generates the genotypic variety that selection and drift act upon.
Variation is the raw material — but it must be heritable
Natural selection changes allele frequencies only when the trait under selection is at least partly heritable — passed from parents to offspring through genes. Phenotypic variation that is purely environmental (a suntan, a scar) does not contribute to evolution. So when you analyze a scenario, ask two questions: Is there variation? And is that variation heritable? If the answer to either is no, the trait cannot evolve by natural selection.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Individuals evolving | Populations evolving | An individual's genotype is fixed at conception; only the population's allele frequencies change |
| Allele frequency | Genotype frequency | Allele frequency counts gene copies (p, q); genotype frequency counts individuals (p², 2pq, q²) |
| Mutation | Gene flow | Mutation creates new alleles; gene flow moves existing alleles between populations |
| Sexual reproduction | Mutation | Sex reshuffles alleles into new combinations; it does not create new alleles |
| Environmental change in traits | Evolution | A purely environmental change (e.g., suntan) alters no allele frequencies and is not evolution |
| Gene pool | Genome | The genome is one individual's DNA; the gene pool is the sum of alleles across a whole population |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of a population as a big jar of marbles of different colors — the gene pool. Each color is a version of a gene (an allele). Evolution is when the mix of colors in the jar slowly changes over time: maybe blue marbles start making more babies, so next generation there are more blues. No single marble changes color — the whole jar's mixture changes. That's why we say populations evolve, not individuals.
Worked example
Counting a color-shifting population. A meadow contains 100 snapdragons: 50 red (CRCR), 40 pink (CRCW), and 10 white (CWCW). First, count alleles: 200 total. CR appears in 50 × 2 + 40 = 140 copies → p = 0.70. CW appears in 40 + 10 × 2 = 60 copies → q = 0.30. Check: p + q = 1.
Now a new beetle arrives that prefers red flowers, eating their seeds. Over the next generations, white and pink plants contribute more offspring than red plants. Ten years later the population is 20 red, 50 pink, and 30 white. Recount: CR = 20 × 2 + 50 = 90 of 200 → p = 0.45; CW = 50 + 60 = 110 → q = 0.55. The allele frequencies shifted from 0.70/0.30 to 0.45/0.55 — the population has evolved, even though no individual snapdragon changed its genotype. That shift, driven by differential reproduction, is microevolution.
Key takeaways
- Population = interbreeding individuals of one species in an area; gene pool = all their alleles.
- Evolution = a change in allele frequencies across generations; individuals do not evolve.
- Allele frequencies at a two-allele locus: p + q = 1; count alleles, not individuals.
- Mutation is the only source of new alleles; it is random and mostly neutral or harmful.
- Gene flow moves alleles between populations and reduces differences between them.
- Sexual reproduction creates new genotypes (new combinations), not new alleles.
- Variation must be heritable for selection to drive evolution — environmental changes don't count.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Define population, gene pool, and allele frequency.
Show answer
A population is all interbreeding individuals of one species in a given area; the gene pool is all alleles at all loci in that population; allele frequency is the proportion of gene copies at a locus that are a particular allele.
Why is it accurate to say populations evolve but individuals do not?
Show answer
An individual's genotype is fixed at conception — it cannot evolve. Evolution is a change in allele frequencies across generations, which is a property of the population as a whole.
In a population of 200 diploid individuals, 80 are genotype AA, 100 are Aa, and 20 are aa. What are p and q?
Show answer
Total alleles = 400. A = 80 × 2 + 100 = 260 → p = 0.65. a = 100 + 20 × 2 = 140 → q = 0.35. (p + q = 1.)
Name the three sources of genetic variation and state what each contributes.
Show answer
Mutation (creates new alleles), gene flow (moves alleles between populations), and sexual reproduction (reshuffles alleles into new genotypes).
Why does gene flow tend to make two populations more similar?
Show answer
Gene flow moves alleles between populations, so any allele that arises or becomes common in one population can spread to the other — their allele frequencies converge.
A plant population grows taller after a wet year. Is this evolution? Explain.
Show answer
No. The taller growth is an environmental (plastic) response; no allele frequencies changed, so it is not evolutionary change.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Population
- All interbreeding individuals of one species in a given area
- Gene pool
- All alleles at all loci present in a population
- Allele frequency
- Proportion of all copies of a gene that are a specific allele (p or q)
- Genotype frequency
- Proportion of individuals with a given genotype (AA, Aa, aa)
- Microevolution
- Change in allele frequencies across generations
- Mutation
- A change in DNA sequence
- Gene flow
- Movement of alleles between populations via individuals or gametes
- Heritable variation
- Trait differences passed from parents to offspring through genes
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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