MAINTENANCE OF ECOLOGICALLY SIGNIFICANT GENETIC-VARIATION IN THE TIGER SWALLOWTAIL BUTTERFLY THROUGH DIFFERENTIAL SELECTION AND GENE FLOW

Differential selection in a heterogeneous environment is thought to pr omote the maintenance of ecologically significant genetic variation. V ariation is maintained when selection is counterbalanced by the homoge nizing effects of gene flow and random mating. In this study, we exami ne the relative importance of differential selection and gene flow in maintaining genetic variation in Papilio glaucus. Differential selecti on on traits contributing to successful use of host plants (ovipositio n preference and larval performance) was assessed by comparing the res ponses of southern Ohio, north central Georgia, and southern Florida p opulations of P. glaucus to three hosts: Liriodendron tulipifera, Magn olia virginiana, and Prunus serotina. Gene flow among populations was estimated using allozyme frequencies from nine polymorphic loci. Signi ficant genetic differentiation was observed among populations for both oviposition preference and larval performance. This differentiation w as interpreted to be the result of selection acting on Florida P. glau cus for enhanced use of Magnolia, the prevalent host in Florida. In co ntrast, no evidence of population differentiation was revealed by allo zyme frequencies. F-ST-values were very small and Nm, an estimate of t he relative strengths of gene flow and genetic drift, was large, indic ating that genetic exchange among P. glaucus populations is relatively unrestricted. The contrasting patterns of spatial differentiation for host-use traits and lack of differentiation for electrophoretically d etectable variation implies that differential selection among populati ons will be counterbalanced by gene flow, thereby maintaining genetic variation for host-use traits.