Adaptation of exotic species to the Australian landscape poses a serious threat to the integrity of natural ecosystems and profitability of current farming systems. Ryegrass, an outcrossing species complex (Lolium spp.) and an intractable weed of cropping systems, was used as a candidate to further investigate adaptation within Mediterranean-type environments. Evolutionary differentiation in naturalised populations and their adaptive potential were examined in common gardens by characterising the genetic variability within and between 80 populations collected across 8 cropping regions, and 30 populations from adjacent paddocks differing in key management inputs. Morphological variability within and between populations was not found to be simply the consequence of variable distribution in the number and frequency of taxonomic species which colonised Australia with European settlement. Most plants were intermediate in appearance between L. rigidum Gaud. and L. multiflorum Lam. Ecotypic differentiation between regions was clearly evident even though the vast majority of variation in life history traits was found within sites (except for flowering time, 18%). Overall, as growing season length decreased flowering time and spikelet number per spike also decreased; conversely, florets per spikelet, flag leaf size, and population uniformity increased. Larger leaves and lower maternal investment per seed may confer an adaptive advantage in more arid, cropping intensive habitats where competition to intercept light is intense and moisture availability during seed fill more uncertain. By contrast smaller, more numerous leaves and a wider diversity in life history traits may confer an adaptive advantage in higher rainfall, grazing intensive habitats where the risk of predation (consumption) of plant parts is greater and a wider range of biotic pressures abound. iv However, in spite of the large amount of variability in morphological traits upon which selection could act, no evidence was found to support the claim that ryegrass would change in response to mechanical seed collection at harvest (chaff-cart use) to circumvent such control. The frequency of a deleterious adaptation to acetyl-coenzyme A carboxylase inhibiting herbicides appeared stable in one population after 13 years without herbicide use. Notwithstanding, the frequency of herbicide resistance declined sharply within a 20 m zone adjacent to a susceptible area but this has only limited agronomic benefit. By contrast, the potential to decimate the frequency of resistance by sowing herbicide-susceptible tetraploid ryegrass was identified, and glasshouse results were consistent with the minority cytotype exclusion principle. Possible experimental approaches to field validate this novel concept and key issues to be resolved are discussed.