3- genetic drift

Randomness is inherent to evolution

• In a finite population, allele frequency of the next generation is affected by differences in offspring numbers

  • An animal that has 10 different offspring contributes more than one that has 5

    • Effect is stronger the smaller the population

• Genetic drift

The founder effect - humans in Finland

• Migration into Finland

  • Only inhabited coastal regions

• Internal migration = settlement of inland

• Each village established by a few settlers

  • 36 mostly autosomal recessive diseases = common in Finland but rare/absent elsewhere

    • Different pattern of diseases than the rest of the world

• Diseases have distinct geographical distributions

Long term small populations in mountain gorillas

• 2 gorilla species - eastern and western

  • 4 subspecies

• Population size threatened by

  • Habitat loss

  • Poaching

  • Ebola virus

• Population decreases

  • Evidence of syndactyly (fused digits) —> suggests inbreeding

• Whole eastern species = low genetic diversity

Effective population size

• The size of an idealised population that would love genetic diversity via drift at the same rate as the actual population

What factors would reduce effective population size

• Not large

• No random mating

• No equal sex ratio

• Non-equal family sizes

• Population size fluctuates

• Generations overlap

  • Each individual doesn’t have an equal probability of contributing genes to the next generation

How does genetic drift affect generations

• Erases genetic variation within populations

• Causes population differentiation

• Strength depends on population census size

• The behaviour and demographic history of populations affects patterns of genetic variation and effective population size