PREFERENCE HIERARCHIES AND THE ORIGIN OF GEOGRAPHIC SPECIALIZATION INHOST USE IN SWALLOWTAIL BUTTERFLIES

Four allopatric populations of the widely distributed western anise sw allowtail butterfly, Papilio zelicaon, use different plant genera as h osts, but simultaneous choice experiments showed that these population s have diverged only slightly in oviposition preference. Of the four p opulations-two from southeastern Washington State, one from coastal so uthwestern Washington State, and one from central California-three use hosts that are not available to any of the others. Although variation for the degree of preference for particular plant species occurred wi thin and among populations, all four populations ranked hosts in the s ame overall order. Monophagy on a local, low-ranking host outside the range of high-ranking hosts did not necessarily lead to the loss of pr eference for those high-ranking hosts, thereby indicating that the hig h-ranking hosts would still be accepted, and in some cases even prefer red, if a population encountered them again. Hence, the overall prefer ence hierarchy among P. zelicaon populations appears to be evolutionar ily conservative. Analyses of differences among families within the Ca lifornia population indicated that increased preference for some hosts is inversely correlated, whereas preference for other hosts may be un correlated. Positive correlations may also occur but were not observed among the plant species tested. Overall, the results indicate local m onophagy on different plant species in P. zelicaon has not involved ma jor reorganizations in the preference hierarchy of ovipositing females , even in populations that may have fed on a low-ranking host for many generations. Instead, small increases in preference for local hosts h ave occurred within an evolutionarily conservative preference hierarch y.