Natural Selection and Gene Flow (Lecture 10)

• Continuation of population genetics focuses on natural selection and gene flow.
• The lecture aims to explain effects of selection on allele frequencies, mechanisms maintaining genetic variation, gene flow in population differentiation, and relationship between population size and effectiveness of selection.

Key Concepts

• Selection is the process where differences in genotype fitness lead to changes in allele frequencies.
• Balancing selection refers to mechanisms that maintain genetic variation in populations.
• Pleiotropy occurs when one gene affects multiple phenotypic traits, potentially constraining evolution.
• Inbreeding is mating between related individuals, increasing homozygosity.
• Gene flow is the movement of alleles between populations, affecting genetic differentiation.

Hardy-Weinberg and Fitness

• Fitness serves as the currency of natural selection.
• Hardy-Weinberg equilibrium is when allele frequencies don't change across generations.
• Biological fitness is the survival and reproductive success of individuals with particular phenotypes.
• Relative fitness (w) compares individuals with one genotype to the average or most fit genotype in the population.

Population Size and Selection

• Population size affects both drift and selection power.
• Beneficial alleles increase more steadily in large populations, according to simulations.
• Drift has more power in small populations, and selection in large populations.
• Drift can counteract selection in small populations.

Pleiotropy and Trade-Offs

• Genes often affect multiple traits, creating evolutionary constraints.
• Pleiotropy means most genes are "multitaskers" affecting multiple traits.
• Antagonistic pleiotropy occurs when beneficial effects on one trait negatively affect another.
• The Ester1 gene in mosquitoes conferred insecticide resistance but reduced predator avoidance ability.

Allelic Relationships and Selection

• The relationship between alleles affects how selection functions.
• Additivity: Each allele contributes a measurable amount to the phenotype (e.g., one unit of color per allele).
• Dominance: Heterozygotes have the same phenotype as one homozygote.
• Recessive alleles can be "hidden" from selection as carriers in dominant scenarios.
• Additivity allows selection to act most effectively, leading to fixation of beneficial alleles.

Balancing Selection

• Specific selection mechanisms maintain genetic variation.
• Negative frequency-dependent selection: Rare phenotypes have higher fitness.
• Rover and sitter behaviors in fruit fly larvae are maintained through competition. When rovers are rare, they have higher fitness, and vice versa.
• Heterozygote advantage (overdominance): Heterozygotes have higher fitness than either homozygote.
• Cystic fibrosis (CFTR mutation) carriers have resistance to cholera, typhoid, and tuberculosis.

Inbreeding

• Inbreeding increases homozygosity without changing allele frequencies.
• Inbreeding results in more homozygotes and fewer heterozygotes.
• Inbreeding can lead to inbreeding depression by exposing deleterious recessive alleles.
• Charles II of Spain had severe health issues due to generations of inbreeding.
• His inbreeding coefficient was higher than if he had mated with his own sister.

Population Subdivision and Gene Flow

• Populations can become genetically differentiated, which is counteracted by gene flow.
• FST measures population differentiation at the genetic level.
• Drift makes subpopulations different, whereas gene flow makes them more similar.
• Lynx populations across 4,000 kilometres maintain genetic similarity due to high movement.
• Bighorn sheep populations just 200 metres apart can be genetically distinct due to limited movement.

Summary

• Selection occurs when genotypes differ in fitness.
• Population size influences both drift and selection.
• Pleiotropy can constrain evolutionary trajectories.
• Balancing selection mechanisms maintain genetic variation.
• Inbreeding increases homozygosity without changing allele frequencies.
• Gene flow counteracts population differentiation.