Compensatory adaptation and diversification subsequent to evolutionary rescue in a model adaptive radiation

Biological populations may survive lethal environmental stress through evolutionary rescue.  The rescued populations typically suffer a reduction in growth performance and harbour very low genetic diversity compared with their parental populations.  The present study addresses how population size and within-population diversity may recover through compensatory evolution, using the experimental adaptive radiation of bacterium Pseudomonas fluorescens.  We exposed bacterial populations to an antibiotic treatment; and then imposed a one-individual-size population bottleneck on those surviving the antibiotic stress.  During the subsequent compensatory evolution, population size increased and leveled off very rapidly.  The increase of diversity was of slower paces and persisted longer.  In the very early stage of compensatory evolution, populations of large sizes had a greater chance to diversify; however, this productivity-diversification relationship was not observed in later stages.  Population size and diversity from the end of the compensatory evolution was not contingent on initial population growth performance.  We discussed the possibility that our results be explained by the emergence of a “holey” fitness landscape under the antibiotic stress.