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Elevated atmospheric CO2 has been shown to rapidly alter plant physiology and ecosystem productivity, but contemporary evolutionary responses to increased CO2 have yet to be demonstrated in the field. At a Mojave Desert FACE (free-air CO2 enrichment) facility, we tested whether an annual grass weed (Bromus madritensis ssp. rubens) has evolved in response to elevated atmospheric CO2. Within 7years, field populations exposed to elevated CO2 evolved lower rates of leaf stomatal conductance; a physiological adaptation known to conserve water in other desert or water-limited ecosystems. Evolution of lower conductance was accompanied by reduced plasticity in upregulating conductance when CO2 was more limiting; this reduction in conductance plasticity suggests that genetic assimilation may be ongoing. Reproductive fitness costs associated with this reduction in phenotypic plasticity were demonstrated under ambient levels of CO2. Our findings suggest that contemporary evolution may facilitate this invasive species’ spread in this desert ecosystem.