To understand how DFEs change through evolution, Couce, Limdi et al. used samples from the ongoing Long-Term Evolution Experiment (LTEE), which began in 1988 with 12 populations of a single strain of Escherichia coli that have since evolved over more than 75,000 generations in a low-nutrient environment. In two experiments that answered complementary questions, the team selected ancestral and evolved strains and performed high-throughput insertion mutagenesis and fitness measurements to compare DFEs at different points along an adaptive evolutionary trajectory. Notably, the fitness measurements were collected using a sequencing-based approach, which enabled the authors to also investigate the genetic underpinnings of any macroscopic findings on overall fitness effects.

The first experiment focused on mutations with deleterious effects on fitness and compared the DFEs of strains that had been evolving for 50,000 generations with those of their ancestor. At the macroscopic level, the authors observed no differences in the deleterious tail of the DFEs between the ancestral and evolved strains. This finding contributes important parameters to theoretical models that aim to predict whether and how mutations with deleterious effects change over time.