Genetic Drift and Evolution

Genetic Drift

Overview of Genetic Drift

  • Genetic Drift refers to a non-random process of change in allele frequencies and traits across generations due to differences in fitness (i.e. ability to survive & reproduce).

  • It contrasts with Natural Selection, which is a random process of change in allele frequencies and traits due to chance and is not tied to fitness.

  • Genetic Drift results in a change in heritable traits in a population over generations characterized by:

    • Effect of fitness

    • Effect of chance

Evolution and Processes Driving Changes in Allele Frequencies

  • Evolution can be categorized into:

    1. Natural Selection: Involves processes that are affected by the fitness of traits.

    2. Genetic Drift: Involves random changes in allele frequencies that are not determined by fitness.

The Story of Colorblind Island

  • In 1775, a massive typhoon devastated a Pacific island, significantly reducing its population.

  • The surviving king of the island had a rare type of colorblindness called achromatopsia, which led to a notable change in allele frequency.

  • Current statistics about achromatopsia:

    • Approximately 10% of the island's population has this condition.

    • This translates to 1 in 100 people on the island having achromatopsia, a stark contrast to the worldwide prevalence of 1 in 30,000.

Mechanisms of Genetic Drift

Founder Effect

  • Definition: Initiation of a change in allele frequency in a new subpopulation that does not reflect the original population's allele frequencies.

  • Causes of Founder Effect:

    • Often results from migration or when a subpopulation becomes isolated from genetic exchange with the original population.

    • Frequently associated with new colonization events to new territories.

Bottlenecking Event

  • Definition: A sudden reduction in genetic diversity that affects allele frequency distribution without a change in location.

  • Causes of Bottlenecking Events:

    • Commonly due to catastrophic occurrences, such as natural disasters that significantly reduce population size.

    • This reduction leads to a different allele frequency distribution that becomes evident in today's population.

  • Case Study: Northern Elephant Seals

    • Historical context: Intense hunting in the late 1800s reduced their population size drastically.

    • Result: The surviving population exhibited different allele frequencies and lacked genetic diversity compared to the original population.

Case Study of Founders Effect: Maple Syrup Urine Disease

  • Definition: A genetic disease that results from an autosomal recessive condition that affects the breakdown of branched-chain amino acids due to a deficient enzyme.

  • Statistics related to the disease:

    • Global prevalence is about 1 in every 185,000 individuals.

    • In the U.S., it affects approximately 1 in every 100,000 individuals.

    • In the U.S. Mennonite population, the prevalence is about 1 in 380 individuals.

  • Characteristics of the disease include:

    • Foul metabolic waste can cause severe neurological issues.

    • Typically diagnosed in early childhood but may occasionally be discovered in the 20s to early 30s.

    • Notable for causing a sweet urine smell.

Characteristics of Populations at Risk for Genetic Drift

  • Common traits that predispose populations to genetic drift include:

    • Low genetic diversity.

    • Presence of alleles that are not lethal but also not particularly fit.

    • ISOLATION of populations is a major factor.

    • Small population sizes increase vulnerability to genetic drift.

Key Big Ideas about Genetic Drift

Concept of Fitness Effects and Allele Fate

  • Under Genetic Drift, the fitness effects of an allele do not determine its ultimate fate:

    • Large, stable populations usually experience minimal genetic drift.

    • Small populations are influenced more readily by genetic drift, which can cause significant random changes in allele frequencies.

    • Example: Comparing sizes of populations and responses to genetic drift can lead to either:

    • Fixation of an allele, regardless of its fitness effect.

    • Loss of an allele independent of its fitness.

Simulations of Genetic Drift

  • Simulation insights:

    • In larger populations, genetic drift might preserve allele diversity.

    • In smaller populations, genetic drift is more likely to lead to accidental loss or fixation of alleles, irrespective of their relationship to fitness effects.

Summary of Evolution Through Genetic Drift

  • Evolution driven by natural selection allows predictions about allele fate based on fitness effects:

    • Advantageous alleles are likely to become fixed (selected for).

    • Deleterious alleles are generally lost (selected against).

    • Neutral alleles may experience fluctuations in frequency.

  • In contrast, when evolution is governed by genetic drift:

    • Fitness effects do not provide any reliable indication of allele fate.

    • Any allele can potentially be lost or fixed, regardless of its advantageous or deleterious nature, assuming a constant environment.