Exponential decay in community similarity as a function of distance is a ubiquitous phenomenon in biogeography. Thus, for parasite communities, pairwise similarity decreases with increasing geographical distance between host populations. This biogeographical rule should also apply along other dimensions characterizing the separation between communities. Since host-switching and phylogenetic affinities among host species affect the evolution of parasite faunas across host phylogenetic space the same way as dispersal and environmental gradients affect the assembly of local communities in geographical space, an exponential decay in similarity of parasite faunas with increasing host phylogenetic distance should be observed. This prediction is tested using data on metazoan parasites of 45 species of Canadian freshwater fishes belonging to 5 families. Across all host species, pairwise similarity in the composition of parasite faunas decayed exponentially, though not strongly, with increasing phylogenetic distance between hosts (measured as the number of substitutions per site along DNA sequences). A meta-analysis of correlations computed for separate fish families indicates only a very weak overall relationship. Data distribution indicates that phylogenetically close host species tend to share many of their parasites, while phylogenetically distant hosts have roughly equal chances of harbouring very similar or very dissimilar parasite faunas. The same pattern was seen when monogenean and trematode parasites were analysed separately, whereas no significant decay was observed for cestodes or nematodes, suggesting different patterns of host-switching and parasite colonization among these taxa. The results show that similarity in species composition decreases, though weakly, with increasing distance in the same manner in phylogenetic space as it does in geographical space.
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