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Abstract Hybridization offers insight into speciation and the forces that maintain barriers to reproduction, and hybrid zones provide excellent opportunities to test how environment shapes barriers to reproduction and hybrid fitness. A hybrid zone between the killifish, Fundulus heteroclitus and Fundulus grandis, had been identified in northeastern Florida, although the spatial structure and parameters that affect the distribution of the two species remain unknown. The present study aimed to determine the fine-scale spatial genetic patterns of the hybrid zone to test the hypothesis that species ranges are influenced by changes in dominant vegetation and to determine how differences in reproductive barriers between the two species influence the observed patterns. The area of overlap between the two species spanned ~37 km and showed a mosaic pattern of hybridization, suggesting the spatial structure of the hybrid zone is largely influenced by the environment. Environmental association analysis, however, suggested that while dominant vegetation had a significant influence on the spatial structure of the hybrid zone, a combination of environmental factors was driving the observed patterns. Hybridization tended to be rare at sites where F. heteroclitus was the more abundant species, suggesting that differences in preference for conspecifics can lead to differences in rates of introgression into parental taxa and likely result in a range-shift as opposed to adaptation in the face of climate change. 

Abstract Host shifts to new plant species can drive speciation for plant-feeding insects, but how commonly do host shifts also drive diversification for the parasites of those same insects? Oak gall wasps induce galls on oak trees and shifts to novel tree hosts and new tree organs have been implicated as drivers of oak gall wasp speciation. Gall wasps are themselves attacked by many insect parasites, which must find their hosts on the correct tree species and organ, but also must navigate the morphologically variable galls with which they interact. Thus, we ask whether host shifts to new trees, organs, or gall morphologies correlate with gall parasite diversification. We delimit species and infer phylogenies for two genera of gall kleptoparasites, Synergus and Ceroptres, reared from a variety of North American oak galls. We find that most species were reared from galls induced by just one gall wasp species, and no parasite species was reared from galls of more than four species. Most kleptoparasite divergence events correlate with shifts to non-ancestral galls. These shifts often involved changes in tree habitat, gall location, and gall morphology. Host shifts are thus implicated in driving diversification for both oak gall wasps and their kleptoparasitic associates.