The potential for rapid evolution in invasive species makes them useful for studying adaptive responses of populations to novel environments. However, phenotypic divergence during invasion is not necessarily due to selection, but may be a product of neutral processes resulting from population bottlenecks during colonization and range expansion. We investigated phenotypic adaptation during the establishment and range expansion of the invasive bunchgrass, slender false brome (Brachypodium sylvaticum; Poaceae). Utilizing a novel approach, we made robust comparisons of functional traits using genetic similarity based on unique alleles to determine the genetic probability of contribution from native source regions, and integrated these probabilities into our QST‐FST comparisons for twelve physiological and anatomical traits associated with drought stress in the introduced range. Our results indicate phenotypic divergence greater than neutral expectations in five traits between native and invasive populations, indicating selective divergence occurred during invasive species establishment. Results indicate that the majority of divergence in B. sylvaticum occurred after introduction to the novel environment, but prior to invasive range expansion. This study provides evidence for adaptive genetic differentiation during the establishment of an invasive species, while also describing a robust method for detection of selective processes after species introduction to a novel environment.