Hardy-Weinberg Model

Purpose

• Establishes conditions for population stability.
• Describes relationship between allele frequencies and genotype frequencies.
• Under model conditions, evolution does not occur.
  • Allele and genotype frequencies remain constant.

Conditions for Hardy-Weinberg Equilibrium

• Large population size to avoid genetic drift.
• No mutations affecting allele frequencies.
• No gene flow (migration of organisms carrying alleles).
• Random mating.
• No natural selection.

Violations Leading to Evolution

• Small Population:
  • Genetic Drift: Chance events affect allele frequencies significantly.
• Mutation:
  • Introduces variation changing allele frequencies.
• Gene Flow:
  • Movement of alleles into or out of a population.
• Non-random Mating:
  • Organisms mate preferentially based on genotype.
  • Results in excess homozygous genotypes.
• Natural Selection:
  • Differential survival and reproduction.

Non-random Mating

• Random Mating:
  • Equal probability of mating among all individuals.
• Inbreeding:
  • Mating with related individuals.
  • Causes excess homozygotes and potential inbreeding depression.
  • Alleles more likely identical by descent.
• Outcrossing:
  • Avoiding mating with related individuals.
• Assortative Mating:
  • Selection based on genotypes.
  • Positive Assortative Mating: Preferential selection of similar genotypes.
  • Negative Assortative Mating: Preferential selection of different genotypes.

Genetic Drift

• More significant in small populations due to sample size effect.
• Can lead to elimination of rare alleles, reducing variation.
• Bottleneck Effect:
  • Population size reduction results in decreased variation.

Genetic Variation

• Polymorphism:
  • Genetic variation within a population.
  • A gene is polymorphic if the common allele frequency is <99%.
  • Greater than 99% frequency implies monomorphic.
• Mendelian Genetics:
  • Focus on phenotypic variation.
  • Wild Type Alleles are common, Mutant Alleles are rare.
• Molecular Techniques:
  • Reveal more genetic variation than phenotypic examination.

Conclusion

• Understanding Hardy-Weinberg provides insights into population genetics.
• Highlights the conditions under which evolution may occur.