Genetic Diversity and Evolution: Hardy-Weinberg, Migration, Drift, and Non-Random Mating, Exercises of Swedish

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Migration, drift, and non-random

mating

Hardy-Weinberg conditions

• No mutation
• No selection
• No migration
• No genetic drift
• No non-random mating

Migration

• Not seasonal movement
  • E.g. birds
• Movement of alleles form one population to another - Called ‘gene flow’
• Makes populations more similar to each other

Migration

Selection on banding pattern

• Mainland
  • Banded snakes favored (dappled light)
• Islands
  • Unbanded snakes favored
    • Barren limestone basking surfaces
• Banded alleles on island persist due to migration from mainland

Migration of alleles

• Changes allele frequencies
• Can alter genotype frequencies
• Makes populations more similar

Silene dioica Swedish islands

• Colonize young island
  • Genes that get to any specific island mostly a matter of chance
• Pollination by insects
  • Over time, genes get spread from island to island (migration of alleles)
• Die off through ecological succession
  • Old populations survivors stochastic

• Giles and Goulet,

Population size and genetic drift

• Flip a coin, odds are even (50:50) heads or tails
• If you flip the coin 10, 000, 000 times
  • You’d better get really close to 50:
• If you flip the coin only 4 times, you have a good chance of getting either all heads or all tails 12.5% chance, even if the coin is a fair coin

Sampling error in small populations

Drift versus sample size

• 3 runs of a simulation model
• True allele frequency 60:

Drift as an evolutionary force

• Drift not an important evolutionary force in large populations
• Can be important in small populations
  • Founding of new populations
  • Fixation of alleles, loss of heterozygosity

Drift and allele frequency change

• small populations over many generations
• Fixation: an allele is fixed at a locus if it is at a frequency of 100%
• Heterozygosity decreases as alleles becomes rarer (^) Note: 2(p)*(1-p) = 2pq