An ecological mechanism determining the direction of species evolution

Each species possesses a specific adaptive evolutionary direction, yet the mechanism determining this direction remains less clear. Here, we investigated this mechanism by conducting experimental evolution on Escherichia coli. We introduced an E. coli strain (lac-), initially unable to utilize lactose due to a frameshift mutation, into two different culture media: one medium (L) containing ample sodium acetate and lactose as carbon sources, and the other medium (G) containing abundant glucose and lactose as carbon sources. After 20 days of experimental evolution, our findings revealed that all L-populations underwent parallel evolution through reverse mutation to utilize lactose (lac+), resulting in relatively high fitness gain, while all G-populations did not transition towards lactose utilization but instead maintained the utilization of glucose, which provides higher fitness gain. When lac+ (lactose-eater) and lac- (acetate-eater) were co-cultured in L medium, all lac- individuals were eventually eliminated, while lac+ individuals were consistently selected and retained, indicating that adaptive mutations favoring the evolutionary direction with higher fitness gains competitively exclude those adapted to relatively lower fitness-gain directions. Our findings demonstrate that natural selection, operating between mutations in different adaptive directions, promotes the retention of high-fitness gain directions while eliminating lower-fitness gain directions. This selective pressure compels species to evolve exclusively towards directions offering the highest relative fitness gains, serving as a crucial ecological mechanism in determining the direction of species evolution.