<p>Incorporation of evolutionary theory into experimental investigations of natural populations was important to the development of plant ecology. We review the prominent role grasses (Poaceae) have played in this historical development. Beginning in the 1920s, the ecotype concept prompted researchers to investigate genetically based differentiation in agronomically important forage grasses using common gardens, first in relation to climatological and biotic factors (grazing and competition), and later in relation to latitude and elevation at collection sites. Adaptive variation was documented for populations in relation to edaphic factors such as soil nutrients and moisture, salinity, serpentine conditions, and heavy metals. Molecular analyses using allozyme and DNA markers show selection favors specific genetic loci in grass populations from different habitats or under changing climatic conditions. Consistent with theoretical expectations for breeding-system effects on genetic structure, highly outcrossing grass species show lower molecular variation among populations and greater variation within populations. In contrast, highly selfing grasses show more genetic variation partitioned among populations and correspndingly reduced variation within populations. Progress has been made in identifying quantitative trait loci important to genetic differentiation in phenotypic traits. Biotic factors such as herbivory, competition and microbial symbionts can act as agents of natural selection and adaptation in grass species. Despite inconsistencies in usage, “ecotype” persists in the literature, but the term has value as evidenced by recent adaptation studies in the Poaceae. Applications of the ecotype concept include reclamation and phytoremediation of sites polluted by metalliferous wastes, restoration of habitats, evolutionary responses to a changing climate, and adaptation of weedy and invasive species.</p>