Most transplant experiments across species geographic range boundaries indicate that adaptation to stressful environments outside the range is often constrained. However, the mechanisms of these constraints remain poorly understood. We used extended generation crosses from diverged high and low elevation populations. In experiments across low elevation range boundaries, there was selection on the parental lines for abiotic stress tolerance and resistance to herbivores. However, in support of a defense-tolerance tradeoff, extended generation crosses showed non-independent segregation of these traits in the lab across a drought-stress gradient and in the field across the low elevation range boundary. Genotypic variation in a marker from a region of the genome containing a candidate gene (MYC2) was associated with change in the genetic tradeoff. Thus, using crosses and forward genetics, we found experimental genetic and molecular evidence for a pleiotropic tradeoff that could constrain the evolution of range expansion.