Population Genetics

• Genetic Drift: The random change in allele frequencies in a population due to random mating and random environmental factors.


  An example: there is a small rabbit population: 8 brown and 2 white. Brown is dominant over white. Initially, the frequencies of the B & b alleles are equal. Now let's imagine a scenario where only brown rabbits (BB or Bb) are successful in reproduction.

  First Generation: B gene frequency = 0.5 & b gene frequency = 0.5

  Second Generation: B = 0.7 & b = 0.3

  Third Generation: B = 1.0 & b = 0.0
• The change in allele frequency is random.
• What matters in genetic drift and what doesn't?

  1. Population size matters. Larger populations take longer and are less likely to change drastically due to genetic drift.
  2. Benefit and harm do NOT matter. Unlike natural selection, genetic drift occurs independently of the benefits or harms certain alleles have. In other words, fitness is not a concern for genetic drift.
• Founder Effect: An extreme type of genetic drift that happens when a new colony/population forms from a very small number of individuals. This can lead to an extreme loss of variation within the new population. Note that the founding individuals may NOT represent the full genetic diversity of the original population.
• Bottleneck effect: it is an extreme case of genetic drift, where a large population suddenly decreases in size due to some catastrophic event such as disease, fire, flood etc. This results in a decrease in the amount of genetic diversity present in the remaining population. Smaller populations are more susceptible to the impact, as their genetic diversity is already low.