Quantitative Genetics, Evolution of Sexual Reproduction

Quantitative traits

• Traits controlled by many genes + environment (e.g., height, beak size).

• Significance: Most evolution involves these traits.

Qualitative traits

• Traits controlled by one or few genes with discrete categories (e.g., flower color).

• Significance: Mendelian inheritance.

Heritability

• Proportion of phenotypic variance due to genetic differences among individuals.

• Significance: Determines how much a trait can evolve by selection.

• 1st step in quantitative genetics

Narrow-sense heritability

• h² = VA/VP (additive genetic variance / total phenotypic variance).

• Significance: Predicts response to selection (R = h²S).

Midparent-offspring regression

• Slope of regression of offspring trait on midparent trait.

• y= midoffspring value x=midparent value

• Significance: Estimates narrow-sense heritability (h²).

Breeder’s Equation

• R = h²S

• Significance: Predicts evolution in one generation.

Selection gradient (β)

• Change in fitness per unit change in trait.

• Significance: Quantifies directional selection strength.

Selection differential (S)

• Difference in mean trait between selected parents and whole population.

• Significance: Measures selection before heritability.

Fitness

• Ability of an individual to survive and reproduce relative to others.

• Significance: Central to natural selection.

Types of variation

• Genetic (DNA differences), environmental (variation in phenotype from environment diff), genotype-by-environment (G×E).

• Significance: Only genetic variation is heritable and subject to evolution.

Reaction Norm

• Pattern of phenotypic expression of a single genotype across a range of environments.

• Significance: Shows how phenotypic plasticity evolves.

Red Queen Hypothesis

• Sex helps hosts escape parasites by constantly creating new genotypes.

• Significance: Explains the maintenance of sex despite two-fold cost.

Advantages and Disadvantages of sexual reproduction

• Advantages: genetic variation, adaptation, removes deleterious mutations.

• Disadvantages: two-fold cost of males, need to find mate.

• Significance: Explains why sex is common despite costs.

Hypotheses of the Evolutionary Maintenance of sex

• Hypotheses: Red Queen (parasites), Muller's ratchet (mutation clearance), changing environments.

• Significance: Must overcome two-fold cost of males.

Muller's ratchet

• In asexual populations, deleterious mutations accumulate irreversibly.

• Significance: Explains advantage of sex.

Phenotypic plasticity

• Single genotype produces different phenotypes in different environments.

• Significance: Adaptation without genetic change.