🙈Evolutionary Biology Unit 8 – Population Genetics: Hardy-Weinberg & Deviations

Key Concepts in Population Genetics

• Population genetics studies the genetic composition of populations and how it changes over time
• Allele frequency represents the proportion of a specific allele in a population
• Genotype frequency indicates the proportion of individuals with a particular genotype in a population
• Gene pool refers to the total collection of alleles present in a population at a given time
• Microevolution describes changes in allele frequencies within a population over generations
  • Can lead to speciation and macroevolution over longer timescales
• Genetic drift, natural selection, mutation, and gene flow are the main forces driving changes in allele frequencies
• Hardy-Weinberg equilibrium serves as a null model for understanding population genetics

The Hardy-Weinberg Principle

• States that allele and genotype frequencies remain constant from generation to generation in the absence of evolutionary forces
• Assumes an infinitely large population, random mating, no mutation, no migration, and no natural selection
• Provides a baseline for measuring changes in allele frequencies over time
• Represented by the equation $p^2 + 2pq + q^2 = 1$, where $p$ and $q$ are allele frequencies
  • $p^2$ represents the frequency of the homozygous dominant genotype (AA)
  • $2pq$ represents the frequency of the heterozygous genotype (Aa)
  • $q^2$ represents the frequency of the homozygous recessive genotype (aa)
• Allows for the prediction of genotype frequencies based on allele frequencies in a population

Calculating Allele and Genotype Frequencies

• Allele frequency is calculated by counting the number of copies of an allele and dividing by the total number of alleles in the population
  • For a biallelic locus with alleles A and a, $p + q = 1$, where $p$ is the frequency of allele A and $q$ is the frequency of allele a
• Genotype frequency is determined by counting the number of individuals with a specific genotype and dividing by the total number of individuals in the population
• Hardy-Weinberg equation ($p^2 + 2pq + q^2 = 1$) can be used to calculate expected genotype frequencies from allele frequencies
• Observed genotype frequencies can be compared to expected frequencies to detect deviations from Hardy-Weinberg equilibrium
• Allele frequencies can be estimated from genotype frequencies using the equations $p = (2AA + Aa) / 2N$ and $q = (2aa + Aa) / 2N$, where $N$ is the total number of individuals

Conditions for Hardy-Weinberg Equilibrium

• Infinitely large population to minimize the effects of genetic drift
• Random mating ensures that alleles are combined independently and genotype frequencies follow Hardy-Weinberg proportions
• No mutation, as new alleles would change allele frequencies over time
• No migration (gene flow) to prevent the introduction or removal of alleles from the population
• No natural selection, ensuring that all genotypes have equal fitness and survival rates
  • Directional, stabilizing, and disruptive selection can alter allele frequencies
• Violation of any of these conditions leads to changes in allele frequencies and deviations from Hardy-Weinberg equilibrium

Deviations from Hardy-Weinberg Equilibrium

• Non-random mating (assortative mating, inbreeding) alters genotype frequencies and can lead to an excess or deficiency of heterozygotes
• Genetic drift causes random fluctuations in allele frequencies, particularly in small populations
  • Bottlenecks and founder effects are extreme examples of genetic drift
• Mutation introduces new alleles into the population, changing allele frequencies over time
• Migration (gene flow) can introduce new alleles or change the frequencies of existing alleles in a population
• Natural selection favors certain alleles or genotypes, leading to changes in allele frequencies
  • Directional selection shifts the mean phenotype in a particular direction
  • Stabilizing selection favors intermediate phenotypes and reduces variation
  • Disruptive selection favors extreme phenotypes and increases variation

Evolutionary Forces and Their Effects

• Genetic drift leads to random changes in allele frequencies, especially in small populations
  • Can result in the fixation or loss of alleles by chance
• Natural selection is a non-random process that favors alleles or genotypes with higher fitness
  • Directional selection increases the frequency of advantageous alleles (antibiotic resistance in bacteria)
  • Stabilizing selection maintains allele frequencies around an optimal value (birth weight in humans)
  • Disruptive selection favors extreme phenotypes over intermediates (beak size in finches)
• Mutation generates new alleles and maintains genetic variation in populations
  • Most mutations are neutral or deleterious, but some can be advantageous
• Gene flow can introduce new alleles or change the frequencies of existing alleles through migration
  • Can counteract the effects of genetic drift and natural selection
• Non-random mating can lead to changes in genotype frequencies and alter the distribution of genetic variation within populations

Applications in Real-World Populations

• Conservation genetics uses population genetic principles to manage and protect endangered species
  • Assessing genetic diversity and inbreeding levels to inform conservation strategies (Florida panther)
• Forensic genetics applies Hardy-Weinberg principles to estimate allele frequencies and match DNA evidence to individuals
• Agricultural breeding programs use population genetics to select for desirable traits and maintain genetic diversity in crops and livestock
  • Marker-assisted selection and genomic selection to improve yield, disease resistance, and other traits (maize, cattle)
• Public health and medical genetics rely on population genetic concepts to understand the distribution and inheritance of genetic disorders
  • Newborn screening programs for recessive disorders (phenylketonuria, cystic fibrosis)
• Evolutionary studies employ population genetics to reconstruct the evolutionary history and relationships among species
  • Phylogenetic analyses and population structure assessments (human migration patterns, domestication events)

Problem-Solving and Data Analysis

• Calculating allele and genotype frequencies from given data sets
  • Using the Hardy-Weinberg equation to determine expected genotype frequencies
• Identifying deviations from Hardy-Weinberg equilibrium and inferring the potential causes
  • Comparing observed and expected genotype frequencies using chi-square tests
• Estimating the strength and direction of evolutionary forces based on changes in allele frequencies over time
  • Calculating selection coefficients, migration rates, and mutation rates
• Interpreting real-world data sets and drawing conclusions about population genetic processes
  • Analyzing DNA sequence data, genotype frequencies, and phenotypic distributions
• Designing experiments and simulations to test hypotheses about population genetic phenomena
  • Modeling the effects of evolutionary forces on allele frequencies using computer simulations