McInerney, J.

Prokaryotic pangenomes consist of distributed gene pools that far exceed any individual genome. Though universal across prokaryotic life, a satisfactory evolutionary explanation is lacking. Proposed frameworks describe how genes accumulate but not why this architecture persists. Here I show that pangenomes are bet-hedging devices: strategies that buffer environmental uncertainty at the population level. Because fitness compounds multiplicatively, the geometric mean determines long-term success, creating selection for distributed genetic portfolios. Two thresholds emerge from first principles: a selection direction threshold p*=c/(s+c) determining which genes selection favours, and a complexity threshold E_crit=s/c above which no single genome can afford full environmental coverage. The framework dissolves the core-accessory distinction: both obey the same dynamics, differing only in whether beneficial conditions are constant or intermittent. Predictions accord with empirical observations, including that environment dominates phylogeny in explaining pangenome variation, rare genes persist far longer than neutral drift predicts, and genes appearing neutral under arithmetic fitness models show signatures consistent with variance-reducing strategies. From p* and E_crit, the full phenomenology follows, including the U-shaped distribution, the complexity scaling, the bounded genomes, the rare-gene persistence. No patchwork of mechanisms is required. Analysis of the Escherichia coli pangenome provides direct empirical support: niche-specific genes retain 63% of their home-niche frequency in away environments, gene-environment coupling matches bet-hedging predictions rather than migration-selection balance, and the observed strategy performs within 4% of pure bet-hedging across all environmental switching rates. The pangenome is a unified, obligate solution to prokaryotic life under uncertainty.