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HOST RANGE EVOLUTION: THE SHIFT FROM NATIVE LEGUME HOSTS TO ALFALFA BY THE BUTTERFLY, COLIAS PHILODICE ERIPHYLE

Many insects have expanded their host range to include crop plants, and thereby have become major pests. Host plant shifts to crops are not only economically important, but also present an excellent opportunity to study the evolutionary relationships between herbivorous insects and their food plants. "Host plant shift" is defined here as the process by which one or more formerly used host plant species are abandoned in favor of one or more new host plant species. If some populations of an herbivorous insect shift hosts while other populations do not, the net result is "host range expansion," the addition of one or more host plant species to the total number used by the herbivore species. Genetic changes in host utilization may be required for some host shifts (Ehrlich and Raven, 1964). For example, mutations affecting host choice might be necessary for oviposition on a novel host, or mutations which improve detoxification capability might enable an herbivore to exploit a previously toxic host species. In some cases, changes in extrinsic factors, such as the availability of host plant species, may lead to host shifts without requiring genetic changes in the herbivore. Some shifts to crop plants may fit this pattern. Even when genetic changes are not required for the initial shift to a new host, differential host use may lead to evolutionary divergence between populations. Evolutionary theory predicts that popu-