Abstract
Fisher’s fundamental theorem of natural selection predicts no additive variance of fitness in a natural population. Consistently, observations in a variety of wild populations show virtually no narrow-sense heritability (h2) for traits closely related to fitness. However, counterexamples are also reported, calling for a deeper understanding on the evolution of additive variance. In this study we propose adaptive divergence followed by population mixture as a source of additive variance of fitness-related traits. We experimentally tested the proposal by examining a panel of ∼1,000 yeast segregants produced by a hybrid of two yeast strains subject to adaptive divergence. We measured over 400 yeast cell morphological traits and found a strong positive correlation between their h2 and their relatedness to fitness. This pattern, being a counterexample of the prediction of Fisher’s theorem, well supports our proposal. Because adaptive divergence followed by population mixture could happen constantly, particularly in some species including humans and domesticated animals or crops, the proposal provides a framework for reconciling the availability of abundant additive variances of important traits with Fisher’s fundamental theorem of natural selection.