Adaptive radiation and its relationship to ecological and non-ecological speciation in a group of walnut infesting Tephritid flies

Adaptive radiation is an evolutionary pattern in which a population differentiates into an array of species that inhabit different environments. Descendent taxa differ in morphological and physiological traits. There are alternative hypotheses for how ecological divergence occurs and generates adaptive radiation. Under the ecological hypothesis, divergent selection pressures cause related populations to adapt to varying conditions, initiating speciation and contributing to their reproductive isolation (RI). Non-ecological speciation may also generate adaptive radiation. Species first form in allopatry and then come back into secondary contact. Competition for limited resources then secondarily causes character displacement, allowing these species to coexistence in sympatry. A issue concerning adaptive radiation in walnut-infesting Rhagoletis (Diptera: Tephritidae) fruit flies is the chronology in which host-related ecological adaptation evolves relative to non-host related ecological RI. From studies of diapause, life history timing and genomic DNA sequencing, we present evidence that walnut flies first evolved differences in their adult eclosion times in allopatry on different Juglans host plants along with other non-host related pre- and post-mating barriers. These differences contribute to the ability of walnut flies to co-exist in three secondary contact zones on shared Juglans hosts, supporting the ecological hypothesis for their adaptive radiation. We found no evidence for character displacement in eclosion time, a trait causing allochronic isolation in univoltine Rhagoletis and allowing walnut flies to temporally partition walnut fruit resources where they co-occur. We discuss how the interplay of ecological and non-ecological processes may synergistically influence the radiation of walnut flies and other organismal groups.