The modern synthesis merges Darwin's evolution theory with Mendel's genetics, explaining how populations change over time. It shows how genetic variations within groups drive evolution through natural selection, mutation, genetic drift, and gene flow.

Population genetics dives into the math behind evolution, tracking how gene versions (alleles) change in populations. It connects small genetic shifts to big evolutionary leaps, like how bacteria become antibiotic-resistant or how bird beaks adapt to different foods.

Modern Synthesis of Evolution

Integration of Darwinian Evolution and Mendelian Genetics

• Modern synthesis reconciles Darwin's theory of evolution by natural selection with Mendel's laws of inheritance
• Developed in early 20th century by key contributors (Theodosius Dobzhansky, Ernst Mayr, George Gaylord Simpson, G. Ledyard Stebbins)
• Emphasizes genetic variation within populations as raw material for evolution
• Natural selection acts on phenotypes while underlying genotypes shape evolutionary trajectories
• Incorporates additional mechanisms of evolutionary change
  • Mutation introduces new genetic variations
  • Genetic drift alters allele frequencies through random chance
  • Gene flow transfers genetic material between populations

Bridging Micro and Macroevolution

• Population genetics provides mathematical framework for understanding allele frequency changes over time
• Explains how small genetic changes lead to large-scale evolutionary changes
  • Microevolution (changes within species) accumulates to produce macroevolution (formation of new species)
• Demonstrates connection between molecular-level processes and observable evolutionary patterns
  • Example: Antibiotic resistance in bacteria evolves through accumulation of genetic mutations
  • Example: Beak shape changes in Galápagos finches result from selection on genes affecting beak development

Key Concepts in Population Genetics

Allele Frequencies and Hardy-Weinberg Equilibrium

• Population genetics focuses on distribution and change of allele frequencies within populations
• Gene frequency (allele frequency) represents proportion of particular allele relative to all alleles at locus
• Hardy-Weinberg equilibrium describes theoretical stability of allele frequencies under specific conditions
  • No mutation
  • No migration
  • Random mating
  • Large population size
  • No selection
• Hardy-Weinberg equation calculates expected genotype frequencies in equilibrium population
  • Formula: $p^2 + 2pq + q^2 = 1$
  • p represents frequency of dominant allele
  • q represents frequency of recessive allele
• Deviations from Hardy-Weinberg equilibrium indicate evolutionary forces at work

Genetic Drift and Population Size

• Genetic drift causes random changes in allele frequencies due to chance events
  • More pronounced in small populations
• Special cases of genetic drift lead to rapid allele frequency changes
  • Founder effect occurs when small group establishes new population (Hawaiian Drosophila)
  • Population bottleneck results from drastic reduction in population size (Northern elephant seals)
• Effective population size (Ne) measures genetic diversity
  • Often smaller than census population size
  • Influenced by factors like sex ratio, mating system, and population structure
• Small effective population size increases vulnerability to genetic drift and loss of genetic variation

Factors Influencing Allele Frequencies

Mutation and Migration

• Mutation introduces new alleles into population
  • Serves as ultimate source of genetic variation
  • Can be beneficial, neutral, or deleterious
• Migration (gene flow) alters allele frequencies through movement of individuals between populations
  • Introduces new alleles or changes existing allele frequencies
  • Can increase genetic diversity in recipient population
  • Example: Human migration out of Africa introduced new genetic variants to other continents

Natural and Sexual Selection

• Natural selection acts on phenotypes, changing allele frequencies based on differential survival and reproduction
• Types of selection affect allele frequencies differently
  • Directional selection favors one extreme of trait distribution (giraffe neck length)
  • Stabilizing selection favors intermediate phenotypes (human birth weight)
  • Disruptive selection favors both extremes of trait distribution (beak size in Galápagos finches)
• Sexual selection, a form of natural selection, leads to rapid changes in allele frequencies for mating-related traits
  • Example: Peacock tail feathers evolved through female preference
• Balancing selection maintains genetic variation in populations
  • Heterozygote advantage (sickle cell trait in malaria-endemic regions)
  • Frequency-dependent selection (predator avoidance in guppies)

Non-random Mating and Other Factors

• Non-random mating affects genotype frequencies without changing allele frequencies
  • Assortative mating based on phenotypic similarities (human height)
• Other factors influencing allele frequencies
  • Genetic hitchhiking: neutral alleles change frequency due to linkage with selected alleles
  • Meiotic drive: certain alleles are preferentially transmitted to offspring
  • Epigenetic modifications: heritable changes in gene expression without DNA sequence changes

Population Genetics and Human Variation

Genetic Basis of Human Diversity

• Population genetics framework studies genetic basis of human diversity and trait distribution
• Genome-wide association studies (GWAS) identify genetic variants associated with complex traits and diseases
  • Example: Identifying genes associated with type 2 diabetes susceptibility
• Human population genetics reveals patterns of migration, admixture, and adaptation throughout evolutionary history
  • Example: Genetic evidence of Neanderthal admixture in non-African populations

Complex Traits and Personalized Medicine

• Quantitative genetics combines population genetics with statistical analysis to understand inheritance of complex traits
  • Influenced by multiple genes and environmental factors (height, intelligence)
• Population genetics explains persistence of genetic disorders through concepts like heterozygote advantage
  • Example: Cystic fibrosis carrier status may provide resistance to cholera
• Analysis of genetic variation informs understanding of recent human evolution
  • Adaptations to different environments (lactase persistence in dairy-consuming populations)
  • Dietary adaptations (amylase gene copy number variation in high-starch diets)
• Contributes to development of personalized medicine
  • Elucidates genetic basis of drug responses (warfarin dosing based on genetic variants)
  • Identifies disease susceptibility among different populations (BRCA1/2 mutations in Ashkenazi Jewish populations)