Extinction vortices are driven more by a shortage of beneficial mutations than by deleterious mutation accumulation

The classical account of the extinction vortex is one of decline: as a population shrinks, deleterious mutations fix, fitness erodes, the population shrinks further, and the cycle tightens. This is mutational meltdown.

We propose and evaluate a second mechanism — mutational drought. A small population does not merely accumulate bad mutations; it also loses access to the supply of beneficial mutations that larger populations use to keep pace with a changing environment and to compensate for accumulated load. Using forward-time simulations over a finite genome with recombination and linkage blocks, we find that this shortage of beneficial mutations dominates the long-term dynamics, accelerating decline more strongly than deleterious accumulation alone.

The distinction matters for conservation: interventions that reduce load are addressing only half of the problem, and the half that matters less over long timescales.

Recommended citation: Mawass, W., Hernández, U., Matheson, J., Berg, J. J., and Masel, J. (2026). "Extinction vortices are driven more by a shortage of beneficial mutations than by deleterious mutation accumulation." Proceedings of the Royal Society B, 293(2072), 20260600.