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.

Maize, genetically modified with the insect toxin genes of Bacillus thuringiensis (Bt), is widely cultivated, yet its impacts on soil organisms are poorly understood. Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with plant roots and may be uniquely sensitive to genetic changes within a plant host. In this field study, the effects of nine different lines of Bt maize and their corresponding non-Bt parental isolines were evaluated on AMF colonization and community diversity in plant roots. Plants were harvested 60 days after sowing, and data were collected on plant growth and per cent AMF colonization of roots. AMF community composition in roots was assessed using 454 pyrosequencing of the 28S rRNA genes, and spatial variation in mycorrhizal communities within replicated experimental field plots was examined. Growth responses, per cent AMF colonization of roots and AMF community diversity in roots did not differ between Bt and non-Bt maize, but root and shoot biomass and per cent colonization by arbuscules varied by maize cultivar. Plot identity had the most significant effect on plant growth, AMF colonization and AMF community composition in roots, indicating spatial heterogeneity in the field. Mycorrhizal fungal communities in maize roots were autocorrelated within approximately 1 m, but at greater distances, AMF community composition of roots differed between plants. Our findings indicate that spatial variation and heterogeneity in the field has a greater effect on the structure of AMF communities than host plant cultivar or modification by Bt toxin genes.