The duplicated segment contained the gene citT for the citrate transporter protein used in anaerobic growth on citrate. The duplication is tandem, and resulted in copies that were head-to-tail with respect to each other. This new configuration placed a copy of the previously silent, unexpressed citT under the control of the adjacent rnk gene's promoter, which directs expression when oxygen is present. This new rnk-citT module produced a novel regulatory pattern for citT, activating expression of the citrate transporter when oxygen was present, and thereby enabled aerobic growth on citrate. Movement of this rnk-citT module into the genome of a potentiated Cit− clone was shown to be sufficient to produce a Cit+ phenotype. However, the initial Cit+ phenotype conferred by the duplication was very weak, and only granted a ~1% fitness benefit. The researchers found that the number of copies of the rnk-citT module had to be increased to strengthen the Cit+ trait sufficiently to permit the bacteria to grow well on the citrate. Further mutations after the Cit+ bacteria became dominant in the population continued to accumulate improved growth on citrate. The researchers concluded that the evolution of the Cit+ trait occurred in three distinct phases: (1) mutations accumulated that increased the rate of mutation to Cit+, (2) the trait itself appeared in a weak form, and (3) the trait was improved by later mutations. Blount et al. suggested that this pattern might be typical of how novel traits in general evolve, and proposed a three-step model of evolutionary innovation: Potentiation: a genetic background evolves in which a trait is mutationally accessible, making the trait's evolution possible. Actualization: a mutation occurs that produces the trait, making it manifest, albeit likely in a weak form. Refinement: Once the trait exists, if it provides selective benefit, mutations will accumulate that improve the trait, making it effective. This phase is open-ended, and will continue so long as refining mutations arise and the trait remains beneficial. This model has seen acceptance in evolutionary biology. In 2015 paleontologist Douglas Erwin suggested a modification to a four-step model to better reflect a possible distinction between evolutionary novelty and evolutionary innovation, and to highlight the importance of environmental conditions: potentiation, generation of novel phenotypes (actualization), adaptive refinement, and exploitation (conversion of a novelty to an innovation as it becomes important for the ecological establishment of possessing organisms). Investigation of potentiation In 2014, a research team led