Measured fitness variance is too weak to explain the missing diversity

Heritable differences in fitness lower a population’s effective size and reduce neutral diversity across the whole genome, so each observed diversity deficit implies a certain amount of selected fitness variance.

Published data from 19 monitored vertebrate populations present a paradox: the reported fitness variances are too small to explain the observed deficits, and at the same time too large to be maintained under mutation–selection–drift balance. Measurement bias cannot resolve it — adjusting the estimates in either direction closes one gap and widens the other. The implication is that these populations are far from equilibrium.