population genetics

Course Information

• Material distributed by Dr. Abby Drake from Cornell University.

• For students enrolled in BioEE1780.

• Prohibition on distribution or posting of this material in any context.

Gene Flow

• Definition of Gene Flow: The movement and incorporation of genes from one population into the gene pool of another population.

• Importance of Migrants: Migrants must have reproductive success for gene flow to affect the populations.

• Consequences of Migration:

  • Migratory individuals lead to genetic mixing among populations.

  • Homogenizing Effect: Gene flow acts as a homogenizing force, making populations more similar.

  • A high migration rate decreases differences among subpopulations, leading them to behave like a single, mixed population.

Genetic Drift

• Definition: Independent evolutionary changes in allele frequencies due to random sampling processes.

• Major Points:

  • Occurs independently in each subpopulation, leading to divergence in allele frequencies.

  • Can result in the extinction or fixation of alleles based on chance events.

• Formula for Fst:

  • $F<em>{st} = \frac{V</em>{a}}{V<em>{a} + V</em>{s}}$

  • Where:

  • $F_{st}$ = Variance in allele frequency among subpopulations.

  • $V_{a}$ = Variance among populations.

  • $V_{s}$ = Variance within populations.

Migration and Population Genetics

• Effect of Migration:

  • Small migration rates lead to increased divergence among populations due to genetic drift.

  • Larger rates allow populations to converge toward a single genetically mixed group.

• Example of Allele Frequencies:

  • If populations A and B each have an allele frequency of 50%, and only 1 offspring is produced, there is a 25% chance either allele can be lost, affecting overall allele frequencies.

Bottleneck Effect

• Definition: An event that drastically reduces population size, leading to a loss of genetic diversity.

• Founder Effect:

  • Occurs when a small group starts a new population.

  • May lead to increased allele frequencies if certain traits are overrepresented in the founding individuals.

Conservation Genetics

• Microevolutionary Forces:

  • Forces:

  • Mutation

  • Natural Selection

  • Genetic Drift

  • Gene Flow

  • Consequences of Microevolution:

  • Can increase the frequency of negative traits in a population, like inbreeding depression.

  • Inbreeding exposes recessive traits due to limited genetic variation.

Quantitative Genetics

• Definition: The study of traits that are influenced by multiple genes and their associated interactions.

• Key Concepts:

  • Trait Response:

  • Traits respond differently to selection pressures, categorized into:

    • Stabilizing Selection: Favors intermediate phenotypes.

    • Directional Selection: Favors one extreme phenotype.

    • Disruptive Selection: Favors both extreme phenotypes over intermediate ones.

  • Heritability:

  • The ability of a trait to be passed down from one generation to the next.

• Breeder's Equation:

  • $R = b \cdot S$

  • Where:

  • $R$ = Response to selection.

  • $b$ = Narrow-sense heritability.

  • $S$ = Selection differential.

Allele Characteristics in Genetics

• A₁ Allele Characteristics:

  • Dominant: Expressed in both homozygous dominant (AA) and heterozygous (A1A2) individuals.

  • Recessive: Expressed only in homozygous recessive individuals (A2A2).

  • Additive and Heterozygote Advantage: A degree of fitness associated with heterozygotes that may lead to a benefit in mixed genotype populations.

• Expressed Fitness Values:

  • Varies with genotype:

  • Fitness of A1:

    • Rapid increase if dominant.

    • Eventually may become fixed in a population.

  • Fitness of Recessive:

    • Slow increase and often sheltered by dominant as they share the same phenotype as heterozygotes.

  • Effect on Population:

  • A₁ eventually may fix, with different rates of increase depending on dominance and initial frequencies.