The evolution of multi-antibiotic resistance in bacterial pathogens, often resulting from de novo mutations, is creating a public health crisis. Phages show promise for combating antibiotic-resistant bacteria, the efficacy of which, however, may also be limited by resistance evolution. Here, we suggest that phages may be used as supplements to antibiotics in treating initially sensitive bacteria to prevent resistance evolution, as phages are unaffected by most antibiotics and there should be little cross-resistance to antibiotics and phages. In vitro experiments using the bacterium Pseudomonas fluorescens, a lytic phage, and the antibiotic kanamycin supported this prediction: an antibiotic–phage combination dramatically decreased the chance of bacterial population survival that indicates resistance evolution, compared with antibiotic treatment alone, whereas the phage alone did not affect bacterial survival. This effect of the combined treatment in preventing resistance evolution was robust to immigration of bacteria from an untreated environment, but not to immigration from environment where the bacteria had coevolved with the phage. By contrast, an isogenic hypermutable strain constructed from the wild-type P. fluorescens evolved resistance to all treatments regardless of immigration, but typically suffered very large fitness costs. These results suggest that an antibiotic–phage combination may show promise as an antimicrobial strategy.

Summary Species coexistence can be promoted by both equalizing mechanisms that increase fitness equivalence among species and stabilizing mechanisms that decrease niche overlap among species. Strength of the coexistence mechanisms can be altered by evolution. In particular, populations evolved in sympatry may show niche divergence due to competition‐mediated selection, whereas populations evolved in allopatry have a chance to become ecologically equivalent competitors which, on secondary contacts, may form ‘neutral communities’. We addressed how evolution may change ecological mechanisms of coexistence with experimental microcosms of Escherichia coli strains that reproduce asexually and can be considered as analogues of species. We obtained five pairs of bacterial strains, within each of which the two strains could coexist stably. We then allowed the bacterial strains to evolve in a chemically defined nutrient medium for ˜1100 generations, under sympatric (in two‐strain mixtures) or allopatric scenarios (in monocultures). The strength of coexistence mechanisms was assessed based on reciprocal invasion assays and measurements of environmental carrying capacity. Our data confirmed that coexistence could be weaken by either fitness difference or niche overlap among competitors. In allopatrically evolved strain pairs, fitness difference among competitors became larger or unchanged; and the magnitude of niche overlap often showed an increase rather than a decline. Sympatrically evolved competitors showed no consistent changing trend in the strength of the coexistence mechanisms. Overall, sympatrically evolved competitor pairs did not differ from allopatrically evolved ones in the magnitude of fitness difference, but had lower levels of niche overlap. The results are consistent with the ‘character displacement’ view that allopatric populations of competing species occupy more similar niche space compared with sympatric populations. However, the pattern observed here resulted from allopatrically evolved competitors showing niche convergence, but not sympatrically evolved competitors consistently showing further niche divergence. The results also suggest that convergent evolution in allopatry followed by secondary contacts is an unlikely mechanism for the origin of ‘neutral communities’.

Understanding the conditions under which rapid evolutionary adaptation can prevent population extinction in deteriorating environments (i.e. evolutionary rescue) is a crucial aim in the face of global climate change. Despite a rapidly growing body of work in this area, little attention has been paid to the importance of interspecific coevolutionary interactions. Antagonistic coevolution commonly observed between hosts and parasites is likely to retard evolutionary rescue because it often reduces population sizes, and results in the evolution of costly host defence and parasite counter-defence. We used experimental populations of a bacterium Pseudomonas fluorescens SBW25 and a bacteriophage virus (SBW25Φ2), to study how host-parasite coevolution impacts viral population persistence in the face of gradually increasing temperature, an environmental stress for the virus but not the bacterium. The virus persisted much longer when it evolved in the presence of an evolutionarily constant host genotype (i.e. in the absence of coevolution) than when the bacterium and virus coevolved. Further experiments suggest that both a reduction in population size and costly infectivity strategies contributed to viral extinction as a result of coevolution. The results highlight the importance of interspecific evolutionary interactions for the evolutionary responses of populations to global climate change.

This study presents an updated analysis of the relative generality of invasion mechanisms in invasive plants. We categorised eight invasion mechanisms into three classes, according to the ecological processes behind the invasions: physical environment mechanisms (phenotypic plasticity in environmental tolerance and evolutionary adaptation to physical environment), resource use mechanisms (resource competition, resource utilisation and allelopathy) and enemy release mechanisms (high growth ⁄ reproduction rate, evolutionarily increased competitive ability and phenotypic plasticity in resource allocation). An analysis of 133 invasive plant species in the literature showed the enemy release mechanisms are equally general as resource use mechanisms, while the physical environment mechanisms are less general. Among the eight specific invasion mechanisms, phenotypic plasticity (either in resource allocation or in environmental tolerance), allelopathy, evolutionarily increased competitive ability and high resource-use efficiency are fairly common. Furthermore, chemical defence behaviour is very common in plant invaders. Species invading through enemy release were more likely to originate from Europe and those invading through resource competition ⁄ utilisation were often native to tropical and North America and Asia. Invaders with allelopathy were more likely to be from tropical and North America and those showing evolutionarily increased competitive ability were often native to temperate regions such as Europe and North America. Collectively, phenotypic plasticity, defence strategy and native status can be used not only to predict a plantÕs invasiveness, but also for identifying the potential invasion mechanisms.