Population Genetics
Describe allele frequencies, genetic drift, gene flow, mutation, nonrandom mating, and selection as mechanisms that change populations.
A lesson of Biology, in Evolution and Diversity of Life, about 10 minutes.
By the end of this lesson, you will be able to define allele frequency and population genetics, state Hardy-Weinberg equilibrium conditions, calculate allele and genotype frequencies, and identify forces that cause allele frequency changes.
The Big Idea
Evolution, at its core, is a change in allele frequencies in a population over time. Population genetics provides the mathematical framework to track these changes, determine whether a population is evolving, and identify the forces driving evolution.
Allele and Genotype Frequencies
Allele frequency: proportion of all alleles at a locus in the population that are of a specific type.
If 60% of alleles at a locus are A and 40% are a: $p = 0.60$, $q = 0.40$; $p + q = 1$.
Genotype frequency: proportion of individuals with each genotype.
Hardy-Weinberg Equilibrium
The Hardy-Weinberg principle states that allele and genotype frequencies in a population will remain constant from generation to generation IF:
- Large population (no genetic drift)
- Random mating
- No mutation
- No migration (gene flow)
- No natural selection
Hardy-Weinberg equations:
$$p + q = 1$$
$$p^2 + 2pq + q^2 = 1$$
Where: $p^2$ = frequency of homozygous dominant, $2pq$ = frequency of heterozygotes, $q^2$ = frequency of homozygous recessive.
Using Hardy-Weinberg
If a population is in H-W equilibrium, it is NOT evolving. The value of the equilibrium is in identifying when it is violated — and therefore, which force is driving evolution.
Example: In a population, 9% have the genotype aa (recessive disease).
$q^2 = 0.09 \implies q = 0.30$; $p = 1 - 0.30 = 0.70$
Carrier frequency: $2pq = 2(0.70)(0.30) = 0.42 = 42%$
Forces That Change Allele Frequencies (Microevolution)
Natural selection: differential reproductive success → changes allele frequencies directionally.
Genetic drift: random changes in allele frequency due to chance. More significant in small populations.
- Bottleneck effect: population drastically reduced → survivors may not represent original allele frequencies (e.g., cheetahs, northern elephant seals)
- Founder effect: small group colonizes a new area → limited genetic diversity (e.g., Amish community, isolated islands)
Gene flow (migration): movement of alleles between populations → reduces genetic differences between populations.