Population Genetics and Microevolution

1. The gene pool and allele frequencies

A population's gene pool is the complete set of alleles at all loci carried by its members. Allele frequency is how common a particular allele is (for a two-allele gene, p = frequency of one allele, q = frequency of the other, and p + q = 1). Genotype frequency is the proportion of individuals with each genotype (AA, Aa, aa). Microevolution is simply a change in these allele frequencies from one generation to the next.

2. Hardy-Weinberg equilibrium: a null model

The Hardy-Weinberg principle states that allele and genotype frequencies remain constant across generations when five conditions hold: (1) no mutation, (2) random mating, (3) no natural selection, (4) a very large population (so no genetic drift), and (5) no gene flow. Under these conditions, genotype frequencies settle into p² + 2pq + q² = 1, where p² is the frequency of homozygous dominant (AA), 2pq of heterozygotes (Aa), and q² of homozygous recessive (aa).

Because the five conditions are essentially never all met, Hardy-Weinberg is a null model — the baseline expectation if no evolution were occurring. When observed genotype frequencies differ from expected, an evolutionary mechanism is at work.

3. The five mechanisms of microevolution

Five processes change allele frequencies:

• Mutation creates new alleles — the ultimate source of all variation — but changes frequencies slowly.
• Gene flow moves alleles between populations through migration (or pollen and spores), making populations more similar.
• Genetic drift is random change in allele frequencies, strongest in small populations. A bottleneck effect occurs when a population crashes; a founder effect occurs when a few individuals start a new population. Both reduce diversity by chance.
• Natural selection is the only mechanism that consistently produces adaptation. It can be directional (favors one extreme), stabilizing (favors the middle), disruptive (favors both extremes), or balancing (maintains multiple alleles, such as heterozygote advantage).
• Nonrandom mating (such as inbreeding) changes genotype frequencies — increasing homozygotes and decreasing heterozygotes — but does not directly change allele frequencies.

How it works

Using Hardy-Weinberg to test for evolution:

1. Count genotypes. For a two-allele locus, tally the numbers of AA, Aa, and aa individuals.
2. Calculate allele frequencies. p = (2 × AA + Aa) ÷ (2 × total individuals); q = 1 − p.
3. Predict expected genotype counts. Multiply the total by p², 2pq, and q².
4. Compare observed with expected. If observed ≈ expected, the population is consistent with equilibrium (no detectable evolution at that locus). If they differ, an evolutionary mechanism — or nonrandom mating — is operating.

For example, a population of 500 with 200 TT, 200 Tt, and 100 tt gives p = 0.6 and q = 0.4. Expected counts are TT = 0.36 × 500 = 180, Tt = 0.48 × 500 = 240, and tt = 0.16 × 500 = 80. Observed (200, 200, 100) differs from expected (180, 240, 80) — more homozygotes and fewer heterozygotes than predicted — so the population is not in equilibrium, a pattern that suggests inbreeding.

Common confusions

Memory aids

To recall the five mechanisms of microevolution, think "My Favorite Dog Needs Nothing" — Mutation, gene Flow, genetic Drift, Natural selection, Nonrandom mating. And remember the "five NOs" that define equilibrium: no mutation, no gene flow, no selection, no drift (large population), no nonrandom mating.

Quick review

Topic Recap

• Evolution is measured as changes in allele frequencies; p + q = 1 and, at equilibrium, p² + 2pq + q² = 1.
• Hardy-Weinberg equilibrium requires no mutation, random mating, no selection, a large population, and no gene flow — a null model almost never fully met.
• Deviation from Hardy-Weinberg expectations is evidence that evolution is occurring.
• The five mechanisms of microevolution are mutation, gene flow, genetic drift, natural selection, and nonrandom mating.
• Genetic drift (bottleneck and founder effects) is strongest in small populations; natural selection alone consistently produces adaptation.

Knowledge Check

1. In a population at Hardy-Weinberg equilibrium, the recessive phenotype appears in 9% of individuals. What are the allele frequencies p and q?
2. Why does genetic drift affect small populations more strongly than large ones?
3. Which mechanism is the ultimate source of new alleles, and which is the only one that consistently produces adaptation?
4. A small group of birds is blown to a remote island and founds a new population. Which phenomenon does this illustrate, and what is its genetic consequence?
5. Why can a population be at Hardy-Weinberg equilibrium at one locus but not another?

Answers and Rationales

1. Answer: Recessive phenotype = aa = q² = 0.09, so q = 0.3 and p = 1 − 0.3 = 0.7. Why: The recessive phenotype equals the homozygous recessive genotype (q²), so take its square root to get q.
2. Answer: In large populations, chance fluctuations average out across many matings; in small populations, a single random event can shift or eliminate an allele. Why: The random swing in allele frequency from one generation to the next is larger when the population is smaller.
3. Answer: Mutation is the ultimate source of new alleles; natural selection is the only mechanism that consistently produces adaptation. Why: Mutation creates variation; the others reshuffle or remove it without fitting organisms to their environment.
4. Answer: The founder effect — a form of genetic drift. Why: The new population carries only a random sample of the original gene pool, so it starts with reduced genetic diversity.
5. Answer: Hardy-Weinberg is assessed one locus at a time, and different loci experience different forces. Why: Equilibrium is locus-specific, not a property of the whole organism.