Evolutionary Trade-Offs Under Conditions of Resource Abundance and Scarcity: Experiments With Bacteria

It is often hypothesized that bacteria that are superior competitors when resources are abundant must be inferior competitors when resources are scarce, and vice versa. Most previous studies that sought to test this trade-off hypothesis compared kinetic parameters of extant strains of bacteria, with mixed results. We employed an experimental approach in which bacterial populations were propagated for many generations under two distinct regimes and their evolutionary responses were monitored. Thirty-six populations of bacteria were allowed to adapt evolutionarily to either abundant (batch culture) or scarce (chemostat culture) resource regimes. The competitive fitness of each derived line, relative to its ancestor, was then measured under both regimes. The trade-off hypothesis predicts that adaptation to either selective regime causes a concomitant loss of fitness under the alternative regime. Overall, our findings failed to support this hypothesis, and several cases contradict it. Only two derived lines showed clear trade-offs, having significantly adapted to the selective regime while becoming significantly less fit in the alternative regime. By contrast, five derived lines significantly improved in the alternative regime even as they adapted to their selective regime. Summing over all 36 derived lines (including those for which the observed fitness changes were nonsignificant under one or both regimes), 15 cases support the trade-off hypothesis, whereas 21 indicate the opposite result. These data therefore refute the necessity, or even general tendency, for evolutionary trade-offs in performance under conditions of resource abundance vs. scarcity. Instead, these data suggest that bacteria are able to adapt to a particular level of resource via multiple evolutionary pathways, which may produce either gains or losses in fitness at some different level of resource.