Sequential speciation and the multiplicative origin of community diversity

Phenotypic and genetic variation in one species can influence the composition of interacting organisms within communities and across ecosystems. As a result, speciation may not be an isolated process, as the origin of one new species could create new niche opportunities for other species to exploit, leading to the genesis of many new taxa in a process termed “sequential speciation.” Here, we test for such a multiplicative effect of sequential speciation in a community of host-specific parasitoids, Diachasma alloeum, Utetes canaliculatus and Diachasmimorpha mellea (Hymenoptera: Braconidae), that attack Rhagoletis pomonella fruit flies (Diptera: Tephritidae). Flies in the R. pomonella species complex radiated by sympatrically shifting and ecologically adapting to new host plants, the most recent example being the apple-infesting host race of R. pomonella formed from hawthorn flies within the last 160 years. Using population genetics, field-based behavioral observations, host odor discrimination assays and analyses of life history timing, we show that the same host-related ecological selection pressures that differentially adapt and reproductively isolate Rhagoletis to their respective host plants (diapause life history timing and host fruit odor discrimination) cascaded through the ecosystem and induce diversification of multiple members in the community of host-specific parasitoids. Thus, divergent selection at lower trophic levels can multiplicatively amplify biodiversity at higher levels on ecological time scales which may sequentially contribute to the rich diversity of life.