Host range evolution. The shift from native legume hosts to alfalfa by the butterfly, Colias philodice eriphyle ( Medicago sativa).

Colias philodice eriphyle has pest populations which feed in the field on alfalfa Medicago sativa and non-pest populations which feed on several legumes, but not on alfalfa. Non-pest females laid a substantial fraction of their eggs on alfalfa in preference tests and non-pest larvae were able to grow on alfalfa without a severe disadvantage relative to their growth on native hosts. Genetic changes in host use characters would thus not be required for a shift to alfalfa. Oviposition choice tests showed no differences between pest and non-pest females. On alfalfa, pest larvae grew better than non-pest larvae; but on native hosts, non-pest larvae grew better than pest larvae. Results suggest that pest C. p. eriphyle have improved their ability to grow on alfalfa, while losing some of their ability to grow on native hosts. Shifts to closely related plant species are not likely to lead to rapid divergence. -from Author