GENETIC DRIFT

Drift and Selection: Key Concepts

Hardy-Weinberg Theorem

  • Definition: A mathematical formula that allows calculation of genotype frequencies from allele frequencies and vice versa.
  • Equilibrium: Indicates when evolution does not occur, given certain conditions are met.
Assumptions of Hardy-Weinberg Equilibrium:
  • Population is infinitely large.
  • No migration into or out of the population.
  • No mutations occurring.
  • No natural selection (i.e., all individuals have equal survival chances).

Genetic Drift

  • Definition: A process where allele frequencies fluctuate purely by chance within a population.
  • Mechanism: Occurs when a random, non-representative sample produces the next generation; outcomes are less predictable in smaller populations.
  • Significance of Population Size:
  • Smaller populations experience larger fluctuations.
  • Genetic drift can lead to a quicker loss of diversity.
Key Points of Genetic Drift:
  • Arises from random sampling in finite populations.
  • Leads to a reduction in genetic diversity as time progresses.
  • Has a more pronounced effect in smaller populations.

Importance of Genetic Drift

  • In the absence of mutations or migrations, all alleles but one can eventually be lost from a population.
  • Probability of fixation (where an allele becomes the only allele present in a population) is equivalent to the allele's frequency in the population.
  • Example scenario:
  • In a population of N diploid individuals (2N total alleles), if no mutations introduce new alleles, genetic drift can lead to loss of diversity.
Probability of Fixation:
  • For a given allele, the fixation probability is the same as its allele frequency (p).
  • New mutations have a very small chance of becoming fixed in large populations, with the probability of loss being 1 - (1/2N).

Special Cases of Genetic Drift

Genetic Bottleneck:
  • Definition: A sharp reduction in population size leading to loss of genetic variation.
  • Example: Northern elephant seals were hunted to near extinction, leading to low genetic diversity even after population recovery.
Founder Effect:
  • Occurs when a new population is started by a small number of individuals from a larger population.
  • This can lead to reduced genetic variation compared to the original population.
Overall Significance of Genetic Drift:
  • Genetic drift is a random mechanism of evolution, distinct from selection.
  • Particularly impacts smaller populations, rare alleles, and new mutations.
  • Effects of genetic drift include genetic bottlenecks and founder effects, important for understanding population genetics.