Population Genetics Signatures of Selection

Types of Selection

• Positive, negative, balancing, frequency dependent, background, runaway, and heterosis (hybrid vigor).

How Selection Looks

• Examines phenotype and genotype frequency changes.
• Genetic locus is the selection target.

Demography and Selection

• Population stability and expansion influence selection detection.

SNPs vs. Haplotypes

• Haplotypes provide combined information for detecting selection.
• F{st} = (HT - HS) / HT

Fst and Heterozygosity

• Used to identify candidate regions under positive or balancing selection.

Tajima’s D

• Measures differences between nucleotide diversity estimates.
• \Theta{\pi} \approx \ThetaS; D \approx 0 indicates equilibrium.
• \Theta{\pi} < \ThetaS; D < 0 suggests positive selection.
• \Theta{\pi} > \ThetaS; D > 0 suggests balancing selection.

Quantitative Trait Loci (QTL)

• P = G + E (Phenotype = Genotype + Environment).
• Var(P) = Var(G) + Var(E)
• Broad sense heritability: H^2 = Var(G) / Var(P)
• Narrow sense heritability: h^2 = Var(A) / Var(P)

Estimating Heritability

• Crosses within populations selected from phenotypic extremes.
• Significant difference in offspring phenotypic distributions indicates heritability.

Truncation Selection

• Selection differential: S = \mu_s - \mu
• Selection response: R = \mu' - \mu
• Heritability estimation: h^2 = R / S

Mapping QTL

• Using Recombinant Inbred Lines (RILs).

Mapping Designs

• F2, backcrossing, recombinant inbred lines.

Mapping Statistics

• Single locus tests: regression, t-test, ANOVA.
• Interval mapping.

Summary

• Demography confounds selection signatures.
• Positive selection resembles population expansion.
• Haplotypes enhance statistical power to detect selection.
• Narrow-sense heritability measured with molecular markers and experiments.
• RILs facilitate rapid QTL mapping.
• Correlative methods are powerful for detecting markers under selection.