An evolutionary innovation is contingent on maintaining adaptive potential until competition subsides

After 15 years of the Lenski experiment one of twelve Escherichia coli populations evolved the ability to utilize an abundant but previously untapped carbon source, citrate. Mutations responsible for the appearance of rudimentary citrate utilization (Cit+ phenotype) and for refining this ability have been characterized. However, the complete nature of the genetic and/or ecological events that set the stage for this key innovation remain unknown. We found that there was a slight fitness benefit for introducing an activated citT cassette that mimics the mutation causing Cit+ into the ancestor of the evolution experiment and strains isolated from the population close to when it evolved. However, there was no benefit or even a large deleterious effect in intermediate strains. We conclude that achieving Cit+ was contingent on both an evolutionary trajectory that maintained a potentiated genetic state and the slowing rate of adaptation in this population late in the experiment.