Adaptation, Quantitative Genetics, and the Evolution of Complex Traits

Manual: General · Subject: Evolutionary Biology

This lesson integrates quantitative genetics, G-matrix evolution, selection gradients, and polygenic adaptation.

From alleles to phenotypes

Quantitative trait variation

Complex traits are shaped by many loci, environmental effects, and gene-by-environment interactions. Quantitative genetics provides a way to study the response to selection without requiring complete knowledge of every causal variant. The breeder's equation, $R = h^2 S$ , captures the short-term response to selection under simplifying assumptions.

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Modeling caution

The breeder's equation can fail when genetic architecture changes, selection is nonlinear, or trait variance is not constant across generations.

Selection gradients and multivariate evolution

For multiple traits, response to selection is often approximated by $\Delta \mathbf{z} = \mathbf{G}\boldsymbol{\beta}$ , where $\mathbf{G}$ is the additive genetic covariance matrix and $\boldsymbol{\beta}$ is the vector of selection gradients. Constraints arise when covariance structure channels responses along some directions more than others.

Two views of adaptation

Single-locus adaptation

Useful for large-effect alleles
Often yields classic sweep signatures

Polygenic adaptation

Many small shifts across loci
Often hard to detect without large samples or temporal data

What does $h^2$ represent in the breeder's equation?

Heritability Mutation rate Migration rate Recombination fraction

What is the G-matrix?

Sources of trait variation

Source	Example
Additive genetic variance	Inherited resemblance among relatives
Environmental variance	Nutrition, temperature, or stress effects
Dominance variance	Interactions between alleles at a locus
Epistasis	Interactions among loci

Analyzing a trait-evolution dataset

1

Define the phenotype and its measurement error.
2

Estimate heritability or use genomic prediction if pedigree data are unavailable.
3

Test whether trait means change under selection or drift.
4

Model correlated responses among traits using multivariate methods.
5

Interpret results in light of ecological context and genetic architecture.

Why can polygenic adaptation be difficult to detect using sweep scans alone?

Because it produces no genetic change Because selection is spread across many loci with modest effects Because it only affects noncoding DNA Because it destroys all linkage disequilibrium

State one open problem in quantitative genetics relevant to evolution.

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