Lecture 11

• Genetic drift


  A random change in allele frequencies caused by sampling error in drawing gametes from the gene pool
• Genetic drift


  • It reduces genetic variation within a population
  • It causes random differences among populations to accumulate

  
• Genetic drift is the only evolutionary force operating

  One allele will eventually become fixed at a locus and the population will become homozygous at that locus

  
• Heterozygosity


  A measure of genetic diversity in a population

  
• Genetic drift causes loss or fixation of alleles, so it reduces genetic diversity</b>

  
• Genetic bottleneck


  A short, severe reduction in census population size that also reduces effective population size (Ne) and genetic diversity
• Founder effect


  Allele frequencies in a new population differ from those in the source population due to sampling error when the new population is founded by only a few individuals

  
• When populations experience both drift and selection

  Evolution is more deterministic (and predictable) in larger populations, but smaller populations evolve more unpredictably

  
• For neutral mutations that evolve by drift, the substitution rate is equal to the mutation rate

  
• Gene flow


  Movement of genes in space (e.g., from one population to another) due to dispersal of organisms or their gametes
• Migration can substantially influence allele frequencies


  It primarily prevents populations from diverging

  
• How migration homogenizes allele frequencies

  1. The change in allele frequency on the island is: Δp_I = m(p_C - p_I)

  2. The allele frequency on the island in the next generation is: p_I' = p_I + Δp_I = p_I + m(p_C - p_I)

  
• Non-random mating changes genotype frequencies but not allele frequencies

  
• Inbreeding


  Mating among relatives
• Inbreeding


  Decreases heterozygosity and increases homozygosity compared to Hardy-Weinberg expectations
• Inbreeding depression frequently causes the evolution of mechanisms that prevent self-fertilization by hermaphrodites and mating between close relatives