Quantitative Genetics of Evolution

Quantitative genetics studies the inheritance and evolution of continuous traits controlled by many genes, predicting how such traits respond to selection.

Definition

Quantitative genetics is the branch of genetics dealing with traits that vary continuously and are influenced by many loci and the environment. It models phenotypic variation statistically, partitioning it into components to predict the response of populations to selection.

Scope

This topic covers the partitioning of phenotypic variance into genetic and environmental components, the concepts of heritability and additive genetic variance, the breeder's equation for predicting selection response, genetic correlations among traits, and the multivariate framework that describes the evolution of suites of traits.

Core questions

• How is phenotypic variance partitioned into additive genetic, dominance, and environmental components?
• What does heritability measure, and how is it estimated?
• How does the breeder's equation predict a trait's response to selection?
• How do genetic correlations and trade-offs shape the joint evolution of multiple traits?

Key theories

The breeder's equation

The response to selection of a quantitative trait equals its narrow-sense heritability multiplied by the selection differential, connecting heritable variance to the rate of evolutionary change.

Multivariate evolution and the G matrix

Because traits are genetically correlated, their joint response to selection is governed by the additive genetic variance-covariance matrix, which can deflect evolution away from the direction of strongest selection.

Mechanisms

Phenotypic variance is decomposed into additive, dominance, and interaction genetic variances plus environmental variance. Narrow-sense heritability is the ratio of additive genetic variance to total phenotypic variance and determines how faithfully offspring resemble parents. The breeder's equation predicts the per-generation change in a trait mean from heritability and the selection differential. For multiple traits, the genetic variance-covariance matrix combined with the vector of selection gradients predicts multivariate evolution, so genetic correlations and pleiotropy can constrain or redirect adaptation.

Clinical relevance

Quantitative-genetic methods estimate the heritability of complex human traits and disease risk, underpin genomic prediction in animal and plant breeding, and inform the interpretation of polygenic scores.

History

Fisher's 1918 paper reconciled continuous variation with Mendelian inheritance, founding quantitative genetics. The field matured through mid-century work on heritability and selection response, and was synthesized for evolutionary biology by Lande and Arnold's multivariate framework in the 1980s.

Debates

How stable is the genetic variance-covariance matrix?

Whether the G matrix remains constant enough over evolutionary time to predict long-term trait evolution, or shifts as allele frequencies change, is an open question.

Key figures

• Ronald A. Fisher
• Sewall Wright
• Douglas Falconer

Seminal works

• falconerMackay1996
• futuyma2017
• saetreRavinet2019

Frequently asked questions

What does a heritability of 0.5 mean?

It means that about half of the phenotypic variation for that trait in that population and environment is due to additive genetic differences among individuals; it is a population statistic, not a property of an individual.

Can traits evolve against the direction of selection?

Yes. Because of genetic correlations, a trait can change in a direction opposite to the selection acting directly on it when it is genetically correlated with other traits under stronger selection.