Chapter 14 - Violation of HWE part 2

• Migration (gene flow)


  The movement of alleles among populations
• One-island model of migration


  • Alleles arriving on the island from the continent represent a relatively large fraction of the island gene pool
  • Alleles arriving on the continent from the island represent a relatively small fraction of the continental gene pool
• If migration rate, m, = 0


  Frequency of A1 will be constant
• Migration of any new alleles

  Can potentially, and significantly, change allele frequencies in just one generation
• All zygotes will be A1A1 as is
• Mainland snakes


  All have bands

  
• Islands


  Many snakes lack bands
• Banded snakes on islands

  Some migrate every generation
• Selection


  Acts in opposition to migration, prevents the island from being driven to the same allele frequency as on the mainland
• Migration


  • Can cause allele frequencies of populations to change
  • Can be a powerful mechanism for small populations receiving migrants from a large source
  • Can lead to gene flow which tends to homogenize allele frequencies
  • Prevents evolutionary divergence of populations
  • Can introduce alleles with selective advantage

  
• Genetic drift


  Alteration of gene frequencies due to chance (stochastic) effects

  
• Genetic drift


  • The result of finite population size (violates H-W assumption)
  • Smaller the population, the greater the effect of drift
  • The smaller the population, the greater the sampling error

  
• Selection


  Differential reproductive success that is due to a trait phenotype that confers higher fitness
• Genetic drift


  Differential reproductive success that is due to sampling error

  
• Calculating new allele frequencies after genetic drift

  Randomly select 20 gametes to make 10 zygotes
• Genetic drift is exceptionally sensitive to population size

  
• Founder effect


  Change in allele frequency in newly founded population due to the colonization of a few founders (i.e., drift)
• Founder effect


  • The Amish community
  • Polydactyly being more prevalent in Amish communities
• Bottleneck effect


  Change in allele frequency leading to a sharp and significant reduction in population size due to an environmental catastrophe
• Bottleneck effect


  • Typhoon nearly eradicating the Pingelapese people
  • Achromatopsia (color blindness) being more prevalent in Pingelap Atoll
• Genetic drift causes the loss of heterozygosity by random fixation of alleles

  
• Smaller populations have more sampling error and drift has a faster and more significant effect
• Every population follows unique evolutionary path because drift is random

  
• Drift has more rapid and dramatic effect on allele frequencies (and thus heterozygosity) in small populations than in large populations
• Given enough time, drift can be important evolutionary mechanism even in large populations
• In the absence of any other evolutionary mechanism, an allele will become fixed (frequency is 1) or lost (frequency is 0)

  
• Genetic effective population size (Ne)


  The number of individuals in an ideal population (one meeting every HWE assumption) where the rate of genetic drift is the same as it is in the actual population

  
• Ne is often much less than the actual census size because of things like highly skewed reproductive success, skewed sex ratios, and fluctuating population size
• Effective size falls off rapidly in populations with a skewed sex ratio
• Consequences of drift on the selection process


  • A population may not be exactly at the frequencies expected under selection alone, because drift can affect frequencies
  • Selection is more efficient in larger populations
  • A population bottleneck can cause a deleterious allele to increase in frequency
  • Drift and selection together can move a population to a new frequencies if there are multiple stable equilibria

  
• Drift reduces genetic variation as the result of the extinction (removal) of alleles

  
• Drift, generally, does not produce a fit between organism and environment
• Loss of alleles entails increase in homozygosity, which can expose deleterious alleles
• Gene flow


  Movement (flow) of alleles in and out of a population due to the migration of individuals or gametes
• Genetic drift


  Change in existing allele frequency of a population because of random sampling of organisms

  
• Mating systems


  • Random mating (Hardy-Weinberg equilibrium)
  • Inbreeding (mating between biological relatives)
  • Assortative mating (preferential mating between phenotypically similar individuals)
  • Disassortative mating (preferential mating between phenotypically dissimilar individuals)
• Inbreeding


  The genetic consequences of consanguineous relationships - mating between close relatives
• As the inbreeding coefficient (F) increases


  Fitness often decreases (inbreeding depression)
• Inbreeding depression in humans is likely why we have current day marriage laws (no kissing cousins)
• Inbreeding is caused by non-random mating and leads to changes in genotype frequencies but not allele frequencies