Understanding the evolutionary processes that influence fitness is
critical to predicting species' responses to selection. Interactions
among evolutionary processes including gene flow, drift and the strength
of selection can lead to either local adaptation or maladaptation
especially in heterogeneous landscapes. Populations experiencing novel
environments or resources are ideal for understanding the mechanisms
underlying adaptation or maladaptation, specifically in locally
co-evolved interactions. We used the interaction between a native
herbivore that oviposits on a patchily distributed introduced plant that
in turn causes significant mortality to the larvae to test for
signatures of local adaptation in areas where the two co-occurred. We
used whole genome sequencing to explore population structure, patterns
of gene flow and signatures of local adaptation. We found signatures of
local adaptation in response to the introduced plant in the absence of
strong population structure with no genetic differentiation and low
genetic variation. Additionally, we found localized allele frequency
differences within a single population between habitats with and without
the lethal plant, highlighting the effects of strong selection. Our work
highlights the potential for adaptation to occur in a fine-grained
landscape in the presence of gene flow and low genetic variation.