Abstract
Bet hedging—stochastic switching between phenotypic states1–3 — is a canonical example of an evolutionary adaptationthat facilitates persistence in the face of fluctuating environmental conditions. Although bet hedging is found in organisms ranging frombacteria to humans4–10 viour is lacking11 , direct evidence for an adaptive origin of this beha- . Here we report the de novo evolution of bet hedging in experimental bacterial populations. Bacteria were sub- jected toanenvironment that continually favourednewphenotypic states. Initially, our regime drove the successive evolution of novel phenotypes by mutation and selection; however, in two (of 12) replicates this trend was broken by the evolution of bet-hedging genotypes that persisted because of rapid stochastic phenotype switching. Genome re-sequencing of one of these switching types revealedninemutations that distinguishedit fromtheancestor.The final mutation was both necessary and sufficient for rapid pheno- type switching; nonetheless, the evolution of bet hedging was con- tingent uponearliermutations that altered the relative fitness effect of the finalmutation.These findings capture the adaptive evolution of bet hedging in the simplest of organisms, and suggest that risk- spreading strategiesmay have beenamong the earliest evolutionary solutions to life in fluctuating environments.
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