Chapter 21: Population and Evolutionary Genetics

Population genetics bridges Mendelian inheritance and evolutionary theory by examining how allele frequencies change within populations across generations. The chapter introduces foundational concepts including the gene pool, allele frequency, genotype frequency, and phenotypic frequency as basic units of analysis. The Hardy-Weinberg equilibrium model serves as a critical null hypothesis in population genetics, establishing conditions under which allele frequencies remain stable in the absence of evolutionary forces. The model assumes no mutation, migration, selection, genetic drift, or nonrandom mating, and uses the algebraic relationship p² + 2pq + q² = 1 to predict expected genotype frequencies from known allele frequencies. Deviations from Hardy-Weinberg predictions indicate that one or more evolutionary mechanisms are operating. Chi-square testing provides a statistical method to determine whether observed genotype frequencies significantly differ from theoretical expectations. The chapter then examines five primary evolutionary forces that alter allele frequencies. Mutation represents the ultimate source of new genetic variation, while gene flow introduces alleles from other populations through migration. Genetic drift describes random fluctuations in allele frequency, with particularly strong effects in small populations through bottleneck events and founder effects. Natural selection increases the frequency of beneficial alleles through three distinct patterns: directional selection favoring one extreme phenotype, stabilizing selection maintaining intermediate phenotypes, and disruptive selection promoting extreme phenotypes. Nonrandom mating, particularly inbreeding, increases homozygosity and can reveal harmful recessive alleles previously masked in heterozygotes. Fitness and selection coefficients quantify reproductive success and selection intensity respectively. The chapter explores balanced polymorphisms as cases where multiple alleles persist in populations, exemplified by sickle-cell trait in malaria-endemic regions where heterozygotes possess superior fitness. Molecular evolution discusses how DNA and protein sequences diverge over time, with the neutral theory proposing that most molecular variation is selectively neutral and fixed through genetic drift rather than selection. Molecular clocks estimate evolutionary divergence times based on sequence accumulation rates. The final sections address genetic variation measurement through nucleotide diversity and haplotype analysis, referencing large collaborative projects that map human genetic variation globally.