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Genetic Drift

• Genetic drift: A mechanism of evolution that leads to changes in allele frequencies due to random sampling effects. This is especially pronounced in smaller populations where chance events can have significant impacts.

Learning Objectives

• Understand the adaptive landscape model as it applies to natural selection and genetic drift

• Explain the concept of genetic drift, its calculations, and its impact on allele frequencies, particularly in small populations

• Discuss the bottleneck and founder effects and their influence on genetic diversity

Quotes on Natural Selection

• Charles Darwin (1859): "...can we doubt that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating their kind?"

• Variations present neither useful nor injurious are not influenced by natural selection; they remain as fluctuating elements.

Overview of Genetic Drift

• Genetic drift results in reduced genetic variation, with the founder and bottleneck effects serving as extreme cases of population sampling.

• Sewall Wright proposed that random fluctuations in allele frequencies are vital to evolutionary changes, emphasizing population splits into subpopulations which reduce effective population sizes.

Adaptive Landscape Model

• Developed by Sewall Wright, this model is a metaphor for visualizing how evolutionary changes in fitness occur.

• Fitness Landscape: Populations often shift towards higher fitness, which is visualized as peaks within the landscape representing combinations of genes.

Population Size and Its Effects on Drift

Key Findings:

• Small populations (N = 20): quick fluctuations in neutral allele frequencies lead to rapid fixation or loss.

• Medium populations (N = 200): allele frequencies vary less rapidly than in smaller populations.

• Large populations (N = 2000): allele frequencies remain stable, showing minimal drift effects.

Summary of Genetic Drift Effects

• Genetic drift can lead to allele fixation (where only one allele remains) or extinction (loss of alleles).

• Alleles have an equal probability of drifting in either direction, but over generations, one allele may dominate.

Calculating Genetic Drift

• Variation in allele frequency after one generation can be found using: [ Var(p) = \frac{p(1 - p)}{2N_e} ]

• Where "p" is the current frequency and "N_e" is the effective population size.

Example Calculation

• For a mutant MC1r with a frequency of 0.5 in a population of 300:

  • [ Var(p) = \frac{0.5(1 - 0.5)}{2(300)} = 0.0004 ]

Rare Alleles and Extinction

• The probability of an allele's extinction is inversely related to its frequency. For instance, an allele at frequency 0.01 has a 99% chance of extinction.

Bottleneck and Founder Effects

• Bottleneck Effect: A significant reduction in population size resulting in decreased genetic diversity, typically following environmental disturbances.

  • Example: Cheetahs have endured genetic bottlenecks due to past events.

• Founder Effect: A reduced diversity resulting from a small number of individuals establishing a new population, often leading to increased prevalence of rare traits.

  • Example: The Amish population, founded by around 200 individuals, exhibits higher occurrences of recessive diseases.

Conclusion

• It is critical to understand that genetic drift is unpredictable and varies significantly with population size, influencing the genetic dynamics of populations over time. Selection and drift are inherently linked, with drift affecting alleles both under selection and neutral processes.