Rucete ✏ AP Biology In a Nutshell
18. Population Genetics
This chapter introduces population genetics, focusing on the mechanisms that cause changes in allele frequencies—like genetic drift, gene flow, and natural selection—and explains how to mathematically describe populations using Hardy-Weinberg equilibrium.
Population Genetics and Genetic Drift
• Population genetics studies variation within populations and how allele frequencies change over time.
• Three main drivers of allele frequency change:
– Natural selection (nonrandom)
– Gene flow (movement of alleles between populations)
– Genetic drift (random loss of alleles, especially in small populations)
Gene flow:
• Caused by migration of individuals or pollen between populations.
• Introduces new alleles or changes allele frequencies.
Genetic drift:
• More impactful in small populations.
• Can cause alleles to be lost entirely by chance.
• Reduces genetic diversity, which is important in conservation.
Bottleneck Effect
• Occurs when a large population is sharply reduced due to disaster or human activity.
• Survivors carry only a small subset of the original gene pool.
• Example: Northern elephant seals reduced to fewer than 30 individuals due to hunting → lower diversity than southern elephant seals.
Founder Effect
• Occurs when a new population is started by a small group of individuals.
• The new population may have different allele frequencies than the original.
• Example: The Amish population has a higher frequency of Ellis-van Creveld syndrome due to a small founder group.
Hardy-Weinberg Equilibrium
• Describes populations that are not evolving (allele frequencies are stable).
• Five required conditions:
1. Large population (prevents drift)
2. Random mating (prevents sexual selection)
3. No gene flow (no migration)
4. No selection (equal reproductive success)
5. No mutations (prevents new alleles)
• Equation for allele frequency: p + q = 1
• Equation for genotype frequency: p² + 2pq + q² = 1
– p² = frequency of AA
– 2pq = frequency of Aa
– q² = frequency of aa
Using the Hardy-Weinberg Equation
• Example: In a population, 36% are homozygous recessive (aa).
– q² = 0.36 → q = √0.36 = 0.6
– p = 1 – q = 0.4
– p² = 0.16 (AA), 2pq = 0.48 (Aa), q² = 0.36 (aa)
• Hardy-Weinberg helps estimate allele and genotype frequencies in a population, assuming equilibrium.
When Populations Are Not in Hardy-Weinberg
• If observed genotype frequencies differ from expected values, one or more assumptions are violated → evolution is occurring.
• Most real-world populations are not in perfect Hardy-Weinberg equilibrium, but the model provides a useful baseline for comparison.
In a Nutshell
Population genetics examines how evolutionary forces—like selection, genetic drift, and gene flow—affect allele frequencies in populations. Hardy-Weinberg equilibrium provides a mathematical framework to detect whether evolution is occurring and what mechanisms might be responsible for genetic change.