Genetic Drift: Random Evolutionary Change

Overview

• Topic: Genetic drift, a form of random evolutionary change.
• Focus: Changes in a gene pool due to random chance.

Key Concepts

Gene Pool

• Represents the collective alleles in a population.
• Allele Frequency: Proportion of different alleles, often shown as P and Q values.
  • P value: Frequency of the dominant allele.
  • Q value: Frequency of the recessive allele.

Process of Genetic Drift

• Random sampling of alleles leads to changes in allele frequencies over generations.
• Example of Drift:
  • Initial P and Q both at 0.5.
  • Over generations, P can drop to 0 (dominant allele eliminated) and Q can rise to 1.0.

Graphical Representation

• P values decrease and Q values increase over time due to chance.
• This drift can be visualized in a graph showing the changes over time.

Factors Affecting Genetic Drift

Population Size

• Smaller populations experience more drastic drift due to fewer alleles being sampled.
• Simulator Example:
  • Population of 50 shows significant drift.
  • Increasing size to 500 or 5,000,000 reduces drift due to the law of large numbers.

Real-World Examples

Bottleneck Effect

• Definition: Occurs when a large population is dramatically reduced in size.
• Example: Northern elephant seals' population dropped to 50 due to hunting. This led to reduced genetic diversity.
  • Pre-Bottleneck: High genetic diversity.
  • Post-Bottleneck: Low genetic diversity, asymmetry in skulls.

Founder Effect

• Definition: Happens when a new population is established by a small number of individuals.
• Example: Pingelap Island post-typhoon.
  • Population reduced to 20, founder carried a rare form of colorblindness.
  • Resulted in high incidence of colorblindness in descendants.

Implications of Genetic Drift

• Randomness: Genetic drift is driven by chance events.
• Smaller Populations: More susceptible to changes due to genetic drift.
• New Species Formation: Through isolated populations changing over time.

Conclusion

• Genetic drift is a crucial concept in understanding evolutionary biology.
• It emphasizes the role of chance in the evolution of populations, especially small ones.