Course Overview

Biol 300C: EvolutionFocus on Microevolution: Concepts of Drift and Migration

Fitness and Allele Frequency

• Fitness: Key driver of allele frequency changes.

• Absolute fitness: Measured by offspring produced.

• Relative fitness: Comparison to population mean; e.g., if both phenotypes are equal at 1/2 and the mean is 3:

  • 2 offspring: 2/3 relative fitness

  • 4 offspring: 4/3 relative fitness

• Selection Impact:

  • Relative fitness > 1: allele frequency increases

  • Relative fitness < 1: allele frequency decreases

  • Relative fitness = 1: random allele frequency changes

• Formula: p' = p x w

Types of Natural Selection

• Directional selection: Favors one phenotypic change.

• Stabilizing selection: Favors intermediate phenotypes.

• Disruptive selection: Favors extremes, leading to potential speciation.

• Balancing selection: Maintains multiple alleles.

• Fluctuating selection: Varies with environment.

• Heterozygote advantage: Mixed alleles outperform homozygotes.

• Rarity advantage: Rare alleles favored.

Key Concepts of Selection

• Directional selection reduces genetic diversity (e.g., antibiotic resistance).

• Stabilizing selection maintains average traits (e.g., baby birth weights).

• Disruptive selection allows extremes, preserving diversity over time.

Genetic Drift

• Definition: Random changes in allele frequencies.

• Founder effect: New populations differ from original founders.

• Bottleneck effect: Population size reduction affecting genetic diversity.

• Smaller populations experience more fluctuation in allele frequencies, potentially losing variance over time.

Evolutionary Processes

Four key mechanisms of evolution affecting allele frequencies:

• Natural Selection

• Genetic Drift

• Gene Flow

• MutationNatural selection enhances adaptation; genetic drift and mutation are random.

Historical Constraints

• Platforms for phenotypic traits are influenced by ancestral traits and genetic correlations.

• Fitness trade-offs exist where beneficial traits in one aspect may incur costs in another.

Conclusion

This course emphasizes understanding how genetic variation shapes evolution through mechanisms like selection and drift, highlighting the importance of allele dynamics in populations.