Despite the fact that parasites are highly specialized with respect to their hosts, empirical evidence demonstrates that host switching rather than co-speciation is the dominant factor influencing the diversification of host-parasite associations. Ecological fitting in sloppy fitness space has been proposed as a mechanism allowing ecological specialists to host-switch readily. That proposal is tested herein using an individual-based model of host switching. The model considers a parasite species exposed to multiple host resources. Through time host range expansion can occur readily without the prior evolution of novel genetic capacities. It also produces non-linear variation in the size of the fitness space. The capacity for host colonization is strongly influenced by propagule pressure early in the process and by the size of the fitness space later. The simulations suggest that co-adaptation may be initiated by the temporary loss of less fit phenotypes. Further, parasites can persist for extended periods in sub-optimal hosts, and thus may colonize distantly related hosts by a "stepping-stone" process.

Using network analysis, we looked for broad patterns of distribution of Monogenoidea gill parasites on Neotropical freshwater fishes within a host phylogenetic framework. We analyzed a database of Monogenoidea parasitizing fishes from Neotropical rivers, from 23 watersheds, based on species descriptions published until 2011. Host-parasite interactions were organized into five matrices grouping species at different taxonomic levels. The network of interactions between host families and parasite genera was significantly modular and revealed that each fish order has a unique composition of parasite genera. Hence, interactions between lower taxa were analyzed separately for the largest fish orders (Perciformes, Siluriformes, and Characiformes). Networks tended to be loosely connected and organized in modules. Despite the putative high host specificity of monogenoids, some have a wider host range that includes distantly related host species. Among the hosts, the clade composed by the piranhas (Serrasalmus spp. and related species, Serrasalmidae) stands out in terms of parasite richness per host species, resulting in a more connected network. The history of the lineages of each host order within Neotropical freshwaters seems to have a great influence on the extent of parasite sharing. The observed modularity was influenced by both spatial structure and phylogenetic relatedness of species. In average, 37 % of modules of networks between host species and parasite genera were associated with a particular river basin and 63 % of modules were associated with a host family. Hence, spatial structure determines the co-occurrence of host and parasite species, but their evolutionary history is the main factor defining which interactions are possible.

Colonization of novel hosts is thought to play an important role in parasite diversification, yet little consensus has been achieved about the macroevolutionary consequences of changes in host use. Here, we offer a mechanistic basis for the origins of parasite diversity by simulating lineages evolved in silico. We describe an individual-based model in which (i) parasites undergo sexual reproduction limited by genetic proximity, (ii) hosts are uniformly distributed along a one-dimensional resource gradient, and (iii) host use is determined by the interaction between the phenotype of the parasite and a heterogeneous fitness landscape. We found two main effects of host use on the evolution of a parasite lineage. First, the colonization of a novel host allowed parasites to explore new areas of the resource space, increasing phenotypic and genotypic variation. Second, hosts produced heterogeneity in the parasite fitness landscape, which led to reproductive isolation and therefore, speciation. As a validation of the model, we analyzed empirical data from Nymphalidae butterflies and their host plants. We then assessed the number of hosts used by parasite lineages and the diversity of resources they encompass. In both simulated and empirical systems, host diversity emerged as the main predictor of parasite species richness.

Despite the great interest to quantify the structure of host–parasite interaction networks, the real influence of some factors such as taxonomy, host body size and ecological opportunity remains poorly understood. In this paper, we investigate the general patterns of organization and structure of interactions in two anuran–parasite networks in the Brazilian Pantanal (seasonally flooded environment) and Atlantic Forest (non-flooded forest). We present theoretical models to test whether the structures of these host–parasite interaction networks are influenced by neutrality, host taxonomy and host body size. Subsequently, we calculated metrics of connectance, nestedness and modularity to characterize the network structure. We demonstrated the structure networks were influenced mainly by body size and taxonomy of the host. Moreover, our results showed that the seasonally flooded environment present networks with higher connectance/nestedness and lower modularity compared to the other environment. The results also suggest that seasonal floods may promote ecological opportunities for new species associations.

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