Repeated rapid evolution of increased resistance to parasites after experimental elimination of parasites in natural environments

Background/Question/Methods

It is commonly assumed that reducing the strength of selection favoring a particular trait should cause the evolution of reduced expression of that trait. We tested this prediction in the wild by assessing evolution of the trait “resistance against parasites.” Guppies (Poecilia reticulata) from a Trinidadian stream were translocated to four replicate, previously guppy-free streams where they were allowed to evolve in the absence of a key parasite (Gyrodactylus spp). After one and two years of selection guppies were collected from the tanslocation sites and from the original site, where Gyrodactylus remained present, and were reared under common garden in the lab for two generations. We performed individual infections with Gyrodactylus turnbulli to assess the expression of resistance of these fish. 

Results/Conclusions

Contrary to current theory we document the rapid evolution of increased resistance to the now-absent parasite, a result replicated through time and in three out of four translocation sites (no significant change was evident in the fourth site). These results could not be explained by resistance-tolerance-tradeoffs, differences in fish vigor, or differences in resource levels. This pattern of increased expression of a trait under relaxed selection invites reconsideration of simple expectations and calls for consideration of more nuanced explanations for the evolution of traits under natural conditions where multiple sources of selection act simultaneously.