Genetic drift | Khan Academy

Genetic Drift Overview

  • Definition: A mechanism of evolution where allele frequencies change due to chance (sampling error).

  • Impact: Affects all populations but is strongest in small populations.

  • Consequences: Can lead to:

    • Loss of alleles (even beneficial ones)

    • Fixation (100% frequency) of some alleles

How Genetic Drift Works

  • Mechanism: Occurs due to random sampling of alleles in each generation.

  • Effectiveness: Stronger effects observed in smaller populations where chance events can significantly alter allele frequencies.

Example of Genetic Drift: Rabbits

  • Initial Population: A small rabbit population starts with equal B and b alleles.

  • Reproductive Sampling: If only a few individuals reproduce by chance, allele frequencies will shift over time.

  • Long-term Effect: Over generations, one allele may become completely lost from the population.

Population Size Matters

  • Larger Populations: Provide a buffer against genetic drift, as allele frequencies tend to be more stable.

  • Smaller Populations: More prone to drastic changes in allele frequencies due to genetic drift, leading to greater fluctuations in genetic makeup.

Natural Selection vs. Genetic Drift

  • Natural Selection:

    • Mechanism favoring traits that enhance survival and reproduction.

    • Leads to adaptive changes in the population.

  • Genetic Drift:

    • Involves random changes in allele frequencies.

    • May cause loss of beneficial alleles or fixation of harmful ones.

Bottleneck Effect

  • Definition: A drastic reduction in population size due to events like natural disasters, overhunting, etc.

  • Genetic Diversity: Survivors represent a random subset of the original genetic diversity, potentially leading to reduced variability.

  • Example: Northern elephant seals experienced significant genetic diversity loss after being nearly hunted to extinction.

Founder Effect

  • Definition: Occurs when a small group establishes a new colony, carrying only a fraction of the original population’s genetic diversity.

  • Implications: Can increase the frequency of rare traits or genetic disorders within the new colony.

  • Example: Amish populations demonstrate a higher prevalence of Ellis-Van Creveld syndrome due to their small founding group.

Key Takeaways

  • Genetic drift is a random process that does not factor in an allele's benefit or harm.

  • Smaller populations are more susceptible to the effects of genetic drift.

  • Both founder and bottleneck effects can amplify genetic drift by decreasing genetic diversity in a population.