🐾General Biology II Unit 11 – Population and Evolutionary Genetics

Key Concepts

• Population genetics studies the genetic composition and changes in allele frequencies within populations over time
• Allele frequencies represent the proportion of different alleles for a gene in a population
• Hardy-Weinberg equilibrium describes a population with constant allele frequencies across generations in the absence of evolutionary forces
• Evolutionary forces such as natural selection, genetic drift, mutation, and gene flow can alter allele frequencies in a population
• Natural selection favors the survival and reproduction of individuals with advantageous traits, leading to changes in allele frequencies
• Genetic drift causes random changes in allele frequencies, particularly in small populations
• Gene flow introduces new alleles into a population through migration or interbreeding with other populations
• Speciation occurs when populations become reproductively isolated and diverge genetically over time

Population Genetics Basics

• Populations are groups of interbreeding individuals of the same species living in a specific area
• Genes are inherited in different forms called alleles, which can be dominant or recessive
• Genotype refers to an individual's genetic makeup, while phenotype is the observable characteristics resulting from the genotype and environmental factors
• The frequency of an allele in a population is calculated by dividing the number of copies of that allele by the total number of alleles for that gene
• Genotype frequencies represent the proportion of individuals with specific genotypes in a population
• Allele frequencies can change over time due to evolutionary forces such as natural selection, genetic drift, mutation, and gene flow
• Population genetics helps understand the distribution and dynamics of genetic variation within and among populations

Hardy-Weinberg Equilibrium

• Hardy-Weinberg equilibrium (HWE) is a theoretical model describing a population with constant allele frequencies across generations
• HWE assumes no evolutionary forces are acting on the population, such as natural selection, genetic drift, mutation, or gene flow
• The Hardy-Weinberg equation, $p^2 + 2pq + q^2 = 1$, calculates genotype frequencies based on allele frequencies
  • $p$ represents the frequency of the dominant allele, and $q$ represents the frequency of the recessive allele
  • $p^2$ is the frequency of homozygous dominant individuals, $2pq$ is the frequency of heterozygous individuals, and $q^2$ is the frequency of homozygous recessive individuals
• Populations in HWE must meet several assumptions, including random mating, large population size, no migration, no mutation, and no natural selection
• Deviations from HWE can indicate the presence of evolutionary forces or violations of the assumptions
• HWE serves as a null hypothesis to test for the presence of evolutionary forces in a population

Factors Affecting Allele Frequencies

• Natural selection is a major factor influencing allele frequencies, favoring the survival and reproduction of individuals with advantageous traits
  • Directional selection shifts the allele frequency towards one extreme (e.g., antibiotic resistance in bacteria)
  • Stabilizing selection maintains the average phenotype and reduces variation (e.g., human birth weight)
  • Disruptive selection favors extreme phenotypes over intermediate ones (e.g., beak size in African seedcrackers)
• Genetic drift causes random changes in allele frequencies, particularly in small populations
  • Founder effect occurs when a small group of individuals establishes a new population, leading to reduced genetic diversity (e.g., Amish populations)
  • Bottleneck effect happens when a population undergoes a drastic reduction in size, leading to the loss of rare alleles (e.g., cheetahs after the last ice age)
• Mutation introduces new alleles into a population, although at a relatively low rate
• Gene flow occurs when individuals migrate between populations or interbreed, introducing new alleles or changing allele frequencies
• Non-random mating, such as inbreeding or assortative mating, can alter allele frequencies and genotype frequencies in a population

Types of Natural Selection

• Directional selection shifts the allele frequency towards one extreme, favoring individuals with a particular trait
  • Example: Antibiotic resistance in bacteria, where resistant strains survive and reproduce more effectively
• Stabilizing selection maintains the average phenotype and reduces variation, favoring individuals with intermediate traits
  • Example: Human birth weight, where both extremely low and high birth weights are disadvantageous
• Disruptive selection favors extreme phenotypes over intermediate ones, leading to a bimodal distribution of traits
  • Example: Beak size in African seedcrackers, where larger and smaller beaks are advantageous for different food sources
• Frequency-dependent selection occurs when the fitness of a phenotype depends on its frequency in the population
  • Negative frequency-dependent selection favors rare phenotypes (e.g., rare color morphs in prey species)
  • Positive frequency-dependent selection favors common phenotypes (e.g., Mullerian mimicry in butterflies)
• Sexual selection arises from competition for mates or mate choice, leading to the evolution of traits that increase reproductive success
  • Example: Elaborate tail feathers in male peacocks, which are preferred by females

Genetic Drift and Gene Flow

• Genetic drift is the random change in allele frequencies due to chance events, particularly in small populations
• Founder effect occurs when a small group of individuals establishes a new population, leading to reduced genetic diversity
  • Example: Amish populations in the United States, which descended from a small number of founders
• Bottleneck effect happens when a population undergoes a drastic reduction in size, leading to the loss of rare alleles
  • Example: Cheetahs experienced a bottleneck after the last ice age, resulting in low genetic diversity
• Genetic drift can lead to the fixation or loss of alleles in a population over time
• Gene flow is the transfer of alleles between populations through migration or interbreeding
• Gene flow can introduce new alleles into a population or change the frequencies of existing alleles
• High levels of gene flow can homogenize populations, reducing genetic differences between them
• Barriers to gene flow, such as geographic isolation or reproductive isolation, can lead to genetic differentiation between populations

Speciation and Evolutionary Processes

• Speciation is the formation of new species from existing populations through reproductive isolation and genetic divergence
• Allopatric speciation occurs when populations become geographically isolated and diverge over time
  • Example: Darwin's finches on the Galapagos Islands, which evolved into distinct species adapted to different food sources
• Sympatric speciation happens when populations diverge without geographic isolation, often due to ecological or behavioral factors
  • Example: Apple maggot flies, which evolved to specialize on different host plants within the same area
• Reproductive isolation prevents interbreeding between populations, allowing genetic differences to accumulate
  • Prezygotic barriers prevent the formation of a zygote (e.g., habitat isolation, temporal isolation, behavioral isolation)
  • Postzygotic barriers reduce the fitness of hybrids (e.g., hybrid inviability, hybrid sterility)
• Divergent selection can lead to adaptive radiation, where a single ancestral species gives rise to multiple descendant species adapted to different niches
  • Example: Hawaiian honeycreepers, which evolved into a diverse array of species with specialized beak shapes and feeding habits

Real-World Applications

• Population genetics principles are used in conservation biology to manage endangered species and maintain genetic diversity
  • Example: Captive breeding programs for critically endangered species, such as the California condor
• Evolutionary concepts are applied in agriculture to develop crops with desirable traits, such as disease resistance or increased yield
  • Example: Selective breeding of high-yielding wheat varieties during the Green Revolution
• Understanding the evolution of pathogens and pests helps develop effective control strategies and predict the emergence of resistance
  • Example: Monitoring the evolution of insecticide resistance in mosquitoes to inform malaria control efforts
• Evolutionary medicine examines how evolutionary processes have shaped human health and disease susceptibility
  • Example: Investigating the evolutionary origins of lactose tolerance in human populations
• Forensic genetics relies on population genetics principles to analyze DNA evidence and determine the likelihood of a match
  • Example: Using allele frequencies to calculate the probability of a random match in a criminal case
• Phylogenetic analysis, based on evolutionary relationships, is used to study the diversity and history of life on Earth
  • Example: Constructing evolutionary trees to understand the relationships between species and track the spread of viral outbreaks