The Origin of Additive Genetic Variance Driven by Positive Selection

Area of Science:

• Evolutionary Biology
• Quantitative Genetics

Background:

• Fisher's fundamental theorem predicts no additive genetic variance for fitness in natural populations.
• Observed low narrow-sense heritability (h2) for fitness traits in wild populations contradicts this prediction.
• Counterexamples necessitate a deeper understanding of additive variance evolution.

Purpose of the Study:

• To propose and test adaptive divergence followed by population admixture as a source of additive genetic variance.
• To investigate the relationship between heritability and evolutionary importance of traits.

Main Methods:

• Experimental evolution using yeast segregants from a hybrid of two diverged strains.
• Measurement of over 400 yeast cell morphological traits.
• Quantification of narrow-sense heritability (h2) for these traits.

Main Results:

• A strong positive correlation was found between narrow-sense heritability (h2) and the evolutionary importance of traits.
• Adaptive divergence followed by admixture can generate significant additive genetic variance.
• Positive selection was shown to promote, not deplete, additive variance.

Conclusions:

• Adaptive divergence and admixture provide a mechanism to generate additive genetic variance for evolutionarily important traits.
• This reconciles Fisher's theorem with abundant heritable variation in natural populations.
• Positive selection's role in maintaining additive variance offers an explanation for its dominance despite epistasis.