On the evolution of host specificity: a case study of helminths

The significant variation in host specificity exhibited by parasites has been separately linked to evolutionary history and ecological factors in specific host-parasite associations. Yet, whether there are any general patterns in the factors that shape host specificity across parasites more broadly is unknown. Here we constructed a molecular phylogeny for 249 helminth species infecting free-range mammals and find that the influence of ecological factors and evolutionary history varies across different measures of host specificity. Whereas the phylogenetic range of hosts a parasite can infect shows a strong signal of evolutionary constraint, the number of hosts a parasite infects does not. Our results shed new light on the evolution of host specificity in parasites, suggesting that phylogenetic breadth may capture the evolutionary potential of a parasite to jump between hosts, whereas the number of hosts may reflect ecological opportunity. Finally, we show parasite phylogenies can also provide an alternative perspective on zoonosis by identifying which hosts are infected by a broad phylogenetic range of parasites. ### Competing Interest Statement The authors have declared no competing interest.