Digest: Drift and environmental fluctuations reveal fitness variance under balancing selection

Why natural populations harbor high additive genetic variance for fitness remains an evolutionary puzzle. Connallon and Czuppon (2026) show that balancing selection, long considered irrelevant for additive variance at equilibrium, can maintain substantial fitness variance when realistic features such as genetic drift and environmental fluctuations are considered. Their models demonstrate that even a small number of balanced polymorphisms can rival thousands of loci at mutation-selection balance, reshaping how fitness variation is interpreted across taxa.