Notes on Genetic Drift

Genetic Drift as an Evolutionary Force

• Definition of Genetic Drift:

  • Genetic drift refers to the random alteration of allele frequencies from one generation to the next.
  • Most significantly impacts small populations.
  • Tends to decrease genetic variation through the extinction of alleles.
  • Does not favor adaptive traits, possibly leading to maladaptive changes.

• Mechanisms Causing Genetic Drift:

  • Random sampling during gamete formation due to Mendel’s Law of Segregation (aGametes are formed randomly regarding alleles during meiosis).
  • Demographic stochasticity: Some parents may have fewer or no offspring, affecting allele frequencies randomly.

• Neutral Evolution:

  • Fitness is often equated to reproductive success, with neutral genotypes implying no selection pressure impacting those phenotypes.
  • Neutrality arises from:
  • Redundancy in the genetic code.
  • Certain amino acid substitutions being neutral to protein function.
  • Parts of the genome that do not get expressed.
  • Trade-offs in fitness wherein the effect of one trait's variation is compensated by another.

• Consequences of Genetic Drift:

  1. Random Change: Allele frequencies for neutral alleles change unpredictably (random walk). Fixation of alleles resembles radioactive decay.
  2. Decreased Heterozygosity: Increases frequency of homozygotes while reducing heterozygotes.
  3. Population Differentiation: Independent genetic drift in different populations can lead to varied genetic profiles among them.
  4. Uniformity Within Populations: A steady reduction in genetic variation with the population over time.

• Quantifying Genetic Drift:

  • Variance in allele frequency due to one generation of drift can be estimated as: $ext{Variance} = \frac{pq}{2N}$
  • Where $p$ and $q$ are allele frequencies and $N$ is population size.
  • Standard deviation of average change in allele frequency is given by: $\sqrt{\frac{pq}{2N}}$.

• Drift Impact on Population Size:

  • As population size increases, genetic drift occurs slower.
  • Drift is more pronounced in smaller populations (e.g., in populations of size 4, alleles can become fixed or lost quickly).

• Models of Genetic Drift:

  1. Random Walk Model:
  • Projects future allele fates based on current frequencies and estimates changes in each generation.
  1. Coalescent Theory:
  • Looks back in time to trace genealogical lineage of alleles, revealing the extinction of alleles over generations, leading to a single common ancestor (Most Recent Common Ancestor, MRCA).

• Effective Population Size (Ne):

  • Ne reflects the idealized size where genetic drift would occur at a rate identical to the real population. Typically smaller than the actual census size due to differential contributions from individuals (e.g., unequal sex ratios affect Ne).

• Key Takeaways:

  • Drift's effects are more pronounced in smaller populations, with greater fluctuation in allele frequencies.
  • Drift can cause significant changes to allele frequencies over time, with a trend towards fixation or loss of alleles.
  • Random genetic drift exemplifies how evolutionary changes can occur independently of natural selection, shaping genetic diversity in populations.