Isolation by environment (IBE) is a population genomic pattern that arises when ecological barriers reduce gene flow between populations. Although current evidence suggests IBE is common in nature, few studies have evaluated the underlying mechanisms that generate IBE patterns. In this study, we evaluate five proposed mechanisms of IBE (natural selection against immigrants, sexual selection against immigrants, selection against hybrids, biased dispersal, environment-based phenological differences) that may give rise to host-associated differentiation within a sympatric population of the redheaded pine sawfly, Neodiprion lecontei, a species for which IBE has previously been detected. We first characterize the three pine species used by N. lecontei at the site, finding morphological and chemical differences among the hosts that could generate divergent selection on sawfly host-use traits. Next, using morphometrics and ddRAD sequencing, we detect modest phenotypic and genetic differentiation among sawflies originating from different pines that is consistent with recent, in situ divergence. Finally, via a series of laboratory assays – including assessments of larval performance on different hosts, adult mate and host preferences, hybrid fitness, and adult eclosion timing – we find evidence that multiple mechanisms contribute to IBE in N. lecontei. Overall, our results suggest IBE can emerge quickly, possibly due to multiple mechanisms acting in concert to reduce migration between different environments.

Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: DEB-1257739Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: DEB-CAREER-1750946Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: IOS-2101059Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: IOS-1656625Funding provided by: National Institute of Food and AgricultureCrossref Funder Registry ID: http://dx.doi.org/10.13039/100005825Award Number: 2015-67011-22803Funding provided by: BIO5 Institute, University of ArizonaCrossref Funder Registry ID: http://dx.doi.org/10.13039/100019002Award Number: