Population genetics revolutionized our understanding of evolution by providing mathematical models to explain genetic variation in populations. It became a cornerstone of the modern synthesis, integrating Darwin's natural selection with Mendelian genetics and other fields.

The Hardy-Weinberg equilibrium serves as a null model for non-evolving populations, while factors like natural selection, genetic drift, mutation, and gene flow drive evolutionary change. Key figures like Fisher, Haldane, and Wright laid the groundwork for this unified theory of evolution.

Population Genetics in Evolutionary Theory

Integration of Population Genetics and the Modern Synthesis

• Population genetics studies genetic variation within populations and how it changes over time and space, focusing on the distribution and change of allele frequencies
• The modern synthesis, also known as the modern evolutionary synthesis, integrated Darwinian natural selection with Mendelian genetics and other fields like paleontology and systematics in the early to mid-20th century
• Population genetics played a crucial role in the modern synthesis by providing mathematical models and empirical evidence for how evolutionary processes, especially natural selection, shape genetic variation in populations
• The modern synthesis emphasized the importance of genetic variation as the raw material for evolution and how changes in allele frequencies can lead to adaptations and speciation over time

Importance of Genetic Variation

• Genetic variation is the raw material for evolution, providing the basis for natural selection to act upon
• Changes in allele frequencies within populations can lead to adaptations that increase fitness in specific environments (camouflage in peppered moths)
• Over longer timescales, accumulated changes in allele frequencies can result in the formation of new species through the process of speciation (Darwin's finches)
• Population genetics provides a quantitative framework for understanding how genetic variation is maintained and how it changes in response to evolutionary forces

Hardy-Weinberg Equilibrium and Assumptions

Hardy-Weinberg Equilibrium

• The Hardy-Weinberg equilibrium is a null model in population genetics that describes the expected genotype frequencies in a population that is not evolving, assuming no change in allele frequencies from one generation to the next
• The Hardy-Weinberg equation ($p^2 + 2pq + q^2 = 1$) relates allele frequencies to genotype frequencies, where p and q are the frequencies of two alleles at a biallelic locus, and $p + q = 1$
• Under Hardy-Weinberg equilibrium, the genotype frequencies remain constant across generations, with $p^2$ representing the frequency of the homozygous dominant genotype, $2pq$ representing the frequency of the heterozygous genotype, and $q^2$ representing the frequency of the homozygous recessive genotype
• Deviations from the expected Hardy-Weinberg genotype frequencies can indicate that one or more of the assumptions are violated and that evolutionary processes are occurring in the population

Assumptions of Hardy-Weinberg Equilibrium

• The Hardy-Weinberg equilibrium has five main assumptions:
  1. No mutation: New alleles are not introduced into the population, and existing alleles are not modified
  2. No migration (gene flow): There is no exchange of alleles between the population and other populations
  3. No natural selection: All genotypes have equal fitness, meaning they survive and reproduce at the same rate
  4. Random mating: Individuals mate randomly with respect to their genotypes, without any preferential mating or inbreeding
  5. Large population size (no genetic drift): The population is infinitely large, so sampling error does not cause random fluctuations in allele frequencies
• Violations of these assumptions can lead to changes in allele frequencies and genotype frequencies over time, which are the basis for evolutionary change

Factors Affecting Allele Frequencies

Natural Selection

• Natural selection is the differential survival and reproduction of individuals based on their genotypes, leading to changes in allele frequencies over generations if certain alleles confer higher fitness
• Directional selection favors one extreme phenotype and shifts the allele frequency in one direction (antibiotic resistance in bacteria)
• Stabilizing selection favors intermediate phenotypes and reduces variation in the population (human birth weight)
• Disruptive selection favors extreme phenotypes over intermediates and can lead to bimodal distributions (beak size in African seedcracker finches)
• Natural selection acts on phenotypes, but the underlying genetic variation determines the response to selection and the potential for adaptive evolution

Genetic Drift and Other Factors

• Genetic drift is the random fluctuation of allele frequencies due to sampling error in finite populations, more pronounced in small populations and can lead to the fixation or loss of alleles by chance
  • The founder effect is a type of genetic drift that occurs when a new population is established from a small number of individuals, leading to reduced genetic variation (Amish populations)
  • The bottleneck effect is a type of genetic drift that occurs when a population undergoes a drastic reduction in size, leading to the loss of rare alleles (cheetahs)
• Mutation is the ultimate source of genetic variation and introduces new alleles into populations, with usually low rates but significant effects over long timescales
• Gene flow is the transfer of alleles between populations through migration and interbreeding, introducing new alleles and counteracting the effects of genetic drift and local adaptation (coyote-wolf hybridization)

Key Figures in Modern Synthesis

R.A. Fisher, J.B.S. Haldane, and Sewall Wright

• R.A. Fisher, a British statistician and geneticist, developed many foundational concepts and methods in population genetics, such as the fundamental theorem of natural selection and the analysis of variance
• J.B.S. Haldane, a British geneticist and mathematician, made important contributions to the mathematical theory of natural selection and the study of linkage and recombination
• Sewall Wright, an American geneticist, developed the concept of adaptive landscapes and the shifting balance theory of evolution, emphasizing the importance of genetic drift and population structure in the evolutionary process

Contributions to the Modern Synthesis

• These three figures, along with others like Theodosius Dobzhansky and Ernst Mayr, were instrumental in integrating genetics, natural selection, and other fields to create a unified theory of evolution during the modern synthesis
• Their work laid the foundation for modern evolutionary biology and the study of how genetic variation and evolutionary processes shape the diversity of life on Earth
• The modern synthesis provided a comprehensive framework for understanding the mechanisms of evolution, combining insights from genetics, ecology, paleontology, and systematics
• The integration of population genetics with other disciplines helped to explain patterns of adaptation, speciation, and the distribution of genetic variation in natural populations