Interplay of population size and environmental fluctuations: a new explanation for fitness cost rarity in asexuals

Theoretical models of ecological specialization commonly assume that
adaptation to one environment leads to fitness reductions (costs) in
others. However, experiments often fail to detect such costs. We addressed
this conundrum using experimental evolution with Escherichia coli in
several constant and fluctuating environments at multiple population
sizes. We found that in fluctuating environments, smaller populations paid
significant costs, but larger ones avoided them altogether. Contrastingly,
in constant environments, larger populations paid more costs than the
smaller ones. Overall, large population sizes and fluctuating environments
led to cost avoidance only when present together. Mutational frequency
distributions obtained from whole-genome whole-population sequencing
revealed that the primary mechanism of cost avoidance was the enrichment
of multiple beneficial mutations within the same lineage. Since the
conditions revealed by our study for avoiding costs are widespread, it
provides a novel explanation of the conundrum why the costs expected in
theory are rarely detected in experiments.