At the outset, hybridization ought not be regarded intrinsically as the “grossest blunder” organisms can make (Fisher 1930), and seldom have its effects on evolution been tested using taxonomically broad datasets. In constructing a supertree for the order Caudata for ca. 60 percent of its extant diversity, we test the macroevolutionary impact of hybridism on diversification in salamanders. We use character-mediated diversification analyses (i.e., binary-state speciation and extinction, BiSSE) where we treat hybridizability of lineages as a binary evolutionary character. Results provide an inverse correlation between hybridization and diversification such that hybridism appears to erode diversity. Rate estimates for hybridism-mediated extinction greatly exceed those for hybrid speciation, resulting in a negative rate of net diversification for hybridizing lineages. While we find evidence for the species-level disadvantage of hybridism in an amphibian lineage, we, in a forthcoming paper, discuss circumstances where hybridism may offer macroevolutionary advantage: the relationship between hybridization and diversification may be expected to be temporally inconstant, perhaps dependent upon ecological opportunity available to radiating lineages.
Emerging infectious diseases (EIDs) are often recognized as they increase in virulence, geographic range, or host-species range. When EIDs move into new host populations or are associated with wildlife trade, they are often faced with novel selection pressures. To test for variation in molecular selection among strains of an emerging ranavirus of tiger salamanders in western North America, we assembled a large sequence dataset. We explore whether coarse ecological differences among host populations are predictive of variation in forms of molecular selection. Model-selection analyses provide support for strong positive selection acting globally among strains on four Ambystoma tigrinum virus (ATV) genes, which are generally involved in evasion of host defense and recognition or in cooption of host resources. Further, we show that artificial selection imposed in captive-host populations alters the form of molecular selection among these genes in different ATV strains. In particular, we find elevated strengths of positive selection in a set of ATV strains associated with bait colonies of salamanders or in other captive-host environments. Functional-genomic study is needed to determine the relationship of genotype at these loci to phenotypic variation in performance among ATV strains, particularly with regard to virulence.
To dovetail prior work in the ATV-salamander system, we introduce a novel method for assessing host-pathogen codivergence. Results from these analyses support previous inferences of strong tree concordance in the system. One potential correlate of pathogen emergence is recent elevation in rates of lineage diversification, which we assess with phylogenetic methods. Pathogen emergence in this system appears decoupled from lineage diversification but may be driven by altered selective regimes imposed on ATV in captive-host environments. Given evidence of increased virulence associated with bait-associated strains of ATV (Storfer et al. 2007), we argue that increased frequencies of tiger salamander epizootics in the western US are at least in part attributable to a relaxed density dependence in transmission in bait colonies of salamanders.