TRADE-OFFS AND THE EVOLUTION OF HOST SPECIALIZATION

Trade-offs in performance on different hosts are thought to promote th e evolution of host specificity by blocking host shifts. Yet, in contr ast, most experiments using phytophagous insects have shown performanc e on alternative hosts to be uncorrelated or positively correlated. Re cent quantitative genetic models based on mutation-selection balance i ndicate that underlying constraints on the simultaneous maximization o f different components of fitness may not always generate negative gen etic correlations. We suggest an alternative or additional explanation for the lack of observed negative genetic correlations. If performanc e is polygenically controlled and some performance loci possess only a ntagonistically pleiotropic alleles, then the expression of trade-offs in performance will vary over time in populations. Consequently, a tr ade-off will be seen only in populations that have adapted to two host s and are at or close to genetic equilibrium. Therefore, studies testi ng performance on a novel as compared with a normal host will generall y yield non-negative genetic correlations between performance on the t wo hosts. The results of published studies are consistent with the pre dictions of this hypothesis.