The dynamics of virulence evolution in vector-born plant pathogens can be complex. Here we use individual-based simulations to investigate how virulence evolution depends on genetic trade-offs and population structure in pathogen populations. Although quite generic, the model is inspired by the ecology of the plant-pathogenic bacterium Xylella fastidiosa, and we use it to gain insights into possible modes of evolution of virulence in that group. In particular, we aim to sharpen our intuition about how virulence may evolve over short time scales in response to decreases in vector efficacy. We find that even when pathogens find themselves much more often in hosts than vectors, selection in the vector environment can cause correlational and potentially non-adaptive changes in virulence in the host. The extent on such correlational virulence evolution depends on many system parameters, including the pathogen transmission rate, the relative proportions of the pathogen population occurring in hosts versus vectors, the strengths of selection in host and vector environments, and the extent of virulence per se. But there is a statistical interaction between the strength of selection in vectors and the predominance of pathogens in hosts, such that if within-vector selection is strong enough, the predominance of pathogens within hosts has little effect on the evolution of virulence.

Theoretical models of the evolution of discrete phenotypes show that the most evolvable populations are composed of genotypes with intermediate levels of phenotypic robustness. This has been attributed to a special kind of epistasis, the analog of which for complex quantitative traits might not readily apparent. Here, with simulation models, I show that a variety of plausible kinds of quantitative genetic epistasis will do; as long as it increases cryptic genetic diversity and expected allele effect sizes are not too large. In fact, epistasis is not necessary, since cryptic genetic diversity can also accumulate via phenotypic plasticity. But with phenotypic plasticity, the mapping of phenotypic robustness to evolvability is sensitive to the nature and predictability of environmental variation. So, just as for discrete-traits, the robustness of quantitative traits can have complex effects on evolvability, and this depends on exactly how genetic diversity is hidden and revealed.

Most herbivorous insects are diet specialists in spite of the apparent advantages of being a generalist. This conundrum might be explained by fitness trade-offs on alternative host plants, yet evidence of such trade-offs has been elusive. Another hypothesis is that specialization is non-adaptive, evolving through neutral population genetic processes and within the bounds of historical constraints. Here we report on a striking lack of evidence for the adaptiveness of specificity in tropical canopy communities of armored scale insects. We show that specialists abound and that host-use is phylogenetically conservative, but in comparison to generalists, specialists occur on fewer of their potential hosts, and are no more abundant where they do occur. Of course local communities might not reflect regional diversity patterns. But based on our samples, comprising hundreds of species of hosts and armored scale insects at two widely separated sites, host-use specialists do not appear to outperform generalists.