NARROW HYBRID ZONE BETWEEN 2 SUBSPECIES OF BIG SAGEBRUSH (ARTEMISIA-TRIDENTATA, ASTERACEAE) .2. SELECTION GRADIENTS AND HYBRID FITNESS

The dynamic equilibrium hypothesis proposes that hybrid zones are stab ilized by a balance between dispersal and selection against hybrids. A key prediction of this hypothesis is that hybrids should have lower f itness than either parental taxon, regardless of habitat. Hybrid big s agebrush (Artemisia tridentata ssp. tridentata x ssp. vaseyana) in two narrow hybrid zones do show greatly decreased recruitment. Hybrids in one zone also show increased browsing by grasshoppers, while those in the other zone have increased numbers of aphids. Overall herbivore lo ads, however, are not greater on the hybrids than on the parental subs pecies. Browsing by mule deer is greatest on ssp. vaseyana in both hyb rid zones. Incidence of galls is also greatest on ssp. vaseyana in one hybrid zone. Moreover, browsing by Artemisia weevils is greatest on s ap. tridentata in one hybrid zone. Hybrids produce more flowers than e ither ssp. tridentata or ssp. vaseyana, while seed production rates of hybrids do not differ from those of the parental taxa. Finally, hybri d seeds germinate as well as those of ssp. tridentata and better than those of ssp. vaseyana. Thus, our data do not support the dynamic equi librium hypothesis.