Overview
This lecture introduces heritability and the Breeder’s equation—two core concepts in quantitative genetics—explaining their definitions, applications, and common misconceptions.
Heritability in Quantitative Genetics
- Heritability is the proportion of total phenotypic variance (VP) attributed to genetic factors.
- Broad-sense heritability (H²) = total genetic variance (VG) / total phenotypic variance (VP).
- Broad-sense heritability is most relevant for clonal or self-fertilizing species where parents and offspring are genetically similar.
- Narrow-sense heritability (h²) = additive genetic variance (VA) / total phenotypic variance (VP).
- Narrow-sense heritability is crucial for outbreeding species as it predicts potential for evolutionary change.
The Breeder’s Equation and Predicting Response to Selection
- Evolution requires phenotypic variation and that this variation has an impact on fitness.
- Additive genetic variance (heritability) must be greater than zero for evolution to occur.
- The Breeder’s equation: R = h² × S, where R is the response to selection and S is the selection differential.
- Selection differential (S) is the mean difference between selected parents and the whole population.
- Heritability determines how much offspring means shift toward selected parent means with each generation.
- Artificial selection examples show that repeated selection reduces overall phenotypic variance in a population.
Misconceptions and Environmental Effects on Heritability
- Heritability is often mistakenly viewed as the degree of genetic determination, but it is a ratio of variances.
- Traits under strict genetic control (e.g., number of eyes) can have zero heritability if there’s no genetic variation.
- Heritability is not a direct measure of a trait’s evolvability; environmental variance affects both the numerator and denominator.
- Both phenotypic and additive genetic variance can vary by environment, leading to environment-specific heritability values.
Empirical Example: Wild Radish Study
- Wild radish data show total phenotypic variance is higher in field conditions than in the greenhouse due to more environmental variation.
- Additive genetic variance and heritability can also vary significantly between controlled and natural environments.
- Heritability estimates are specific to the environment in which they are measured and may not be broadly generalizable.
Key Terms & Definitions
- Heritability (H², h²) — Proportion of phenotypic variance due to genetic variance.
- Broad-sense heritability (H²) — VG / VP; includes all genetic variance.
- Narrow-sense heritability (h²) — VA / VP; includes only additive genetic variance.
- Phenotypic variance (VP) — Total variance observed in a trait.
- Additive genetic variance (VA) — Component of genetic variance attributable to the sum of effects from individual genes.
- Breeder’s equation — R = h² × S, predicts evolutionary change due to selection.
- Selection differential (S) — Difference in mean trait value between selected parents and the population.
Action Items / Next Steps
- Read "A Primer of Ecological Genetics" by Connor and Hartl for foundational concepts.
- Review Ridley’s "Evolution" for broader context.
- Consult Allendorf and Luikart for conservation-related quantitative genetics.
- Prepare for next lecture: methods for estimating heritability.