A new study by researchers at the University of California, Davis, has shed light on how external ecological communities can influence the coevolution of hosts and their parasites. The study, published in the journal *Ecology Letters*, found that the presence of other species in the environment can alter the selective pressures on hosts and parasites, leading to different evolutionary outcomes.

Coevolution is a process in which two species evolve in response to each other over time. In the case of hosts and parasites, this process can lead to the development of adaptations that allow the host to resist infection or the parasite to evade the host's defenses.

Previous studies of coevolution have typically focused on the interactions between two species in isolation. However, the new study suggests that the presence of other species in the environment can play an important role in shaping the evolutionary trajectory of host-parasite systems.

The researchers conducted a series of experiments using the bacterium *Pseudomonas fluorescens* and its phage, *phi2*. They found that the presence of a third species, *Escherichia coli*, in the environment altered the selective pressures on *P. fluorescens* and *phi2*, leading to different evolutionary outcomes.

In the absence of *E. coli*, *P. fluorescens* evolved resistance to *phi2*. However, in the presence of *E. coli*, *P. fluorescens* evolved to tolerate *phi2* infection. This is because *E. coli* competed with *P. fluorescens* for resources, making it less costly for *P. fluorescens* to tolerate *phi2* infection.

The study's findings suggest that the presence of other species in the environment can play an important role in shaping the coevolution of hosts and their parasites. This has implications for understanding the evolution of infectious diseases and for developing strategies to control them.

"Our study shows that the coevolution of hosts and parasites is not simply a two-species interaction," said study lead author Dr. Jacob Malone. "The presence of other species in the environment can alter the selective pressures on hosts and parasites, leading to different evolutionary outcomes."

"This has implications for understanding the evolution of infectious diseases and for developing strategies to control them," Malone added. "For example, our findings suggest that it may be possible to use other species to manipulate the coevolution of hosts and parasites, making it more difficult for parasites to evolve resistance to treatments."