Microevolution and Population Genetics

Microevolution: Changes Within Populations

• Species within a population may look similar but are not identical
• Phenotypic variation is differences in features
• Phenotypic variation can be caused by genetic differences between individuals or environmental factors
• Scientists can experimentally determine causes (e.g., soil acidity affects hydrangea flower color)

Microevolution and Genetic Makeup

• Microevolution examines changes in the genetic makeup (genotype) of populations over time
• Quantitative variation: Individuals differ incrementally (e.g., toe length, hair count)
  • A broad, low curve indicates high variation among individuals
  • A high, narrow curve indicates low variation among individuals
• Qualitative variation: Exists in distinct states, lacking intermediate forms (e.g., blood types, snow goose feathers)
• Polymorphism: Existence of multiple discrete variants of a character
  • Described quantitatively by calculating trait frequencies

Population Genetics

• Gene pool: The sum of all alleles at all gene loci in a population
• Population genetics focuses on genetic variation within a population and how it changes over time due to evolution
• Evolutionary changes can be caused by four distinct processes

The Hardy-Weinberg Principle

• A null hypothesis that defines a non-evolving population
• Conditions for genetic equilibrium:
  • No mutations
  • No migration
  • Large population size
  • No natural selection
  • Random mating

Research Methods Using the Hardy-Weinberg Principle

• Example research approach using allele (CR, CW) and genotype frequencies in parent and offspring generations
• Methods to assess allele and genotype frequencies
• Applying the Hardy-Weinberg equation (p² + 2pq + q² = 1) to determine genotype frequencies in offspring

Gene Flow

• Movement of organisms or gametes between populations
• Introduces new alleles, altering existing allele and genotype frequencies
• Violates the Hardy-Weinberg equilibrium assumption of closed populations

Genetic Drift

• Random changes in allele frequencies within a population
• More prominent in small populations
• Leads to reduced genetic diversity

Mutations

• DNA sequence alterations
• Sources include radiation, errors in DNA copying, and movement of transposable elements
• Can be neutral, harmful, or beneficial
• Mutations are random and spontaneous events

Natural Selection

• Favors traits that enhance survival and reproduction
• Three main types:
  • Directional selection: shifts the mean phenotype
  • Stabilizing selection: favors intermediate phenotypes
  • Disruptive selection: favors extreme phenotypes

Inbreeding Depression / Sexual Selection

• Inbreeding: Mating between genetically related individuals
• Results in increased proportion of homozygous genotypes and lower fitness
• Sexual selection: Favors traits that increase mating success
• Intrasexual and intersexual selection

Balancing Selection

• Maintains multiple alleles in a population
• Driven by heterozygote advantage or varied environmental pressures.