The level of gene flow between diverging lineages ultimately determines the outcome of a speciation event. If secondary contact occurs before this process is complete, reproductive isolation barriers must exist or evolve to prevent hybridization. The selective pressures facilitating and maintaining genetic divergence do not always involve an observable phenotypic response, thus cryptic species form. The inability to distinguish between sibling species can be a particularly serious problem in groups responsible for pathogen transmission. Culicoides biting midges occur almost world-wide and vector many disease-causing pathogens that affect wildlife and livestock. In North America, the C. variipennis species complex contains three currently recognized species, only one of which is a vector, and limited molecular and morphological differences have hindered vector surveillance. Here, genomic methods were used to investigate speciation and genetic structure within this complex. Single nucleotide polymorphism (SNP) data were generated using ddRAD sequencing for 206 individuals originating from 17 locations throughout the United States and Canada. Clustering analyses consistently suggest the occurrence of five putative species with significant differentiation occurring in both sympatric and allopatric populations. Evidence of hybridization was detected in three different species pairings, indicating a lack of pre-zygotic reproductive isolation within the complex. Mitochondrial genes were used to trace the hybrid parentage of these individuals, which illuminated discordance with the SNP data. In this study, we highlight the potential role of geographic, ecological, and behavioral isolation in speciation and in maintaining species boundaries, despite hybridization and long range dispersal.

As native ranges are often geographically structured, invasive species originating from a single source population only carry a fraction of the genetic diversity present in their native range. This invasion pathway is thus often associated with a drastic loss of genetic diversity resulting from a founder event. However, the fraction of diversity brought to the invasive range may vary under different invasion histories, increasing with the size of the propagule, the number of re-introduction events, and/or the total genetic diversity represented by the various source populations in a multiple-introduction scenario. In this study, we generated a SNP dataset for the invasive termite Reticulitermes flavipes from 23 native populations in the eastern United States and six introduced populations throughout the world. Using population genetic analyses and approximate Bayesian computation (ABC), we investigated its worldwide invasion history. We found a complex invasion pathway with multiple events out of the native range and bridgehead introductions from the introduced population in France. Our data suggest that extensive long-distance jump dispersal appears common in both the native and introduced ranges of this species, likely through human transportation. Overall, our results show that similar to multiple introduction events into the invasive range, admixture in the native range prior to invasion can potentially favor invasion success by increasing the genetic diversity that is later transferred to the introduced range.