Seed banks alter the rate and direction of molecular evolution of Bacillus subtilis

Nature is rarely static. Fluctuations in the availability of resources constrains the growth and reproduction of individuals, which in turn alters evolution at the population scale. Many organisms respond to such fluctuations by entering a reversible state of reduced metabolic activity, a phenomenon known as dormancy. This pool of dormant individuals (i.e., a seed bank) does not reproduce and is expected to act as an evolutionary buffer, though it is difficult to observe this effect directly over an extended evolutionary timescale. Through genetic manipulation, we analyze the molecular evolutionary dynamics of Bacillus subtilis populations in the presence and absence of a seed bank over 700 days. We find that the ability to enter a dormant state increases the accumulation of genetic diversity over time and alters the trajectory of mutations, findings that are recapitulated using simulations based on a simple mathematical model. Remarkably, the removal of a seed bank also altered the direction of molecular evolution across the genome, suggesting that the presence of a life-history strategy can alter the evolutionary trajectory of a population in addition to its rate.