Conditions Affecting Allele and Genotype Frequencies

Five Conditions for No Change

1. No Mutations
   • No new alleles added to the gene pool.
2. Random Mating
   • No sexual selection; allele distribution is by chance.
3. No Natural Selection
   • Alleles do not confer any adaptive advantage.
4. Large Population Size
   • Ideally infinitely large to avoid genetic drift.
5. No Immigration or Emigration
   • No gene flow; population remains closed.

• When these conditions are met, no microevolution occurs.

Hardy-Weinberg Model

• Null Model: Predicts no evolution if conditions are met.
• Purpose: Disproving the model shows evolution occurs under certain conditions.

Mechanisms of Evolution

Natural Selection

• Non-random: Adaptive changes in allele frequencies due to differential reproductive success.
  • Example: DDT-resistant insects have higher offspring survival.

Genetic Drift

• Random: Changes in allele frequencies due to sampling artifact.
  • Smaller samples lead to greater deviation from the parent population.
  • Loss of Alleles: Smaller samples can lose alleles entirely.
  • Example: M&M analogy - smaller handfuls less representative of full bowl.

Founder Effect

• New population started from a few individuals from a larger population.
• Sample may not represent original population's allele distribution.

Bottleneck Effect

• Sudden reduction in population size leads to non-representative samples.
• Survivors not necessarily the most adaptive.
• Example: Greater prairie chicken population in Illinois.
  • Habitat loss led to reduced heterozygosity and decreased egg viability.

Gene Flow

• Movement of Alleles: Through immigration/emigration or gamete transfer.
• Effects:
  • Reduces variation among populations over time.
  • Can mix up genetic diversity or homogenize it.
  • Negative Case: Dutch birds receiving less adapted mainland alleles.
  • Positive Case: Spread of pesticide resistance in malarial mosquitoes.

Key Takeaways

• Allele and genotype frequencies are affected by mutation, sexual selection, natural selection, genetic drift, and gene flow.
• Large effects seen in small populations and isolated populations.
• Genetic drift and gene flow can lead to fixation or loss of alleles, impacting genetic variability.