** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** WEEDS ARE AGRICULTURE'S MOST COSTLY PEST CATEGORY AND A MAJOR DETERRENT TO THE ADOPTION OF MORE ENVIRONMENTALLY SUSTAINABLE FARMING STRATEGIES. WEEDS INVADING CROP FIELDS FACE A RANGE OF NEW CONDITIONS THEY MUST ADAPT TO BEFORE THEY CAN BECOME THE PLANTS WE KNOW TO BE SO DIFFICULT TO ERADICATE. WHILE THE EVOLUTION OF HERBICIDE RESISTANCE IN PROBLEMATIC WEEDS HAS BEEN WELL-STUDIED, CHANGES IN OTHER TRAITS THAT ALLOW WEEDS TO INITIALLY ESTABLISH IN AGRICULTURAL ENVIRONMENTS ARE NOT WELL UNDERSTOOD. SUCH ADAPTIVE TRAIT CHANGES HAVE BEEN PROPOSED TO OCCUR THROUGH TWO MECHANISMS: EVOLUTION, WHEREBY FAVORABLE GENES SPREAD THROUGH THE POPULATION; OR PLASTICITY, WHEREBY ORGANISMS CHANGE THEIR TRAITS IN DIFFERENT ENVIRONMENTS WITHOUT ANY NEED FOR GENETIC CHANGE. IN THIS STUDY, I WILL ADDRESS THE RELATIVE ROLES OF EVOLUTION AND PLASTICITY IN DRIVING ADAPTATION TO CROP FIELDS USING THREE SPECIES OF COMMON ANNUAL WEEDS - CANADIAN HORSEWEED, COMMON RAGWEED, AND COMMON LAMBSQUARTERS - SOURCEDFROM REGIONALLY PAIRED AGRICULTURAL AND NON-AGRICULTURAL ENVIRONMENTS. FIRST, I WILL USE DNA DATA TO DETERMINE WHETHER WEEDY PLANTS FOUND WITHIN CROP FIELDS DIFFER GENETICALLY FROM THOSE FOUND IN NEARBY NON-AGRICULTURAL AREAS. I WILL THEN FIND DIRECT EVIDENCE FOR EVOLUTION AND/OR PLASTICITY OF KEY WEEDY PLANT TRAITS BY GROWING AGRICULTURAL AND NON-AGRICULTURAL POPULATIONS IN BOTH AN AGRICULTURAL AND NON-AGRICULTURAL ENVIRONMENT. FINALLY, I WILL USE GENE EXPRESSION DATA TO FIND CANDIDATE MOLECULAR MECHANISMS FOR THE TRAITS I IDENTIFIED AS BEING KEY DRIVERS OF ADAPTATION IN MY WEEDY SPECIES. THE RESULTS OF THIS STUDY WILL DEEPEN OUR UNDERSTANDING OF THE MECHANISMS THAT ENABLE WEEDS TO ADAPT TO FARMERS' FIELDS. THIS IS A KEY STEP IN DEVELOPING EVOLUTIONARILY-INFORMED MANAGEMENT STRATEGIES THAT WILL PROMOTE MORE AGRICULTURAL SYSTEMS THAT ARE LESS RELIANT ON FINANCIALLY AND ENVIRONMENTALLY COSTLY HERBICIDES. MORE BROADLY, THIS STUDY WILL ENCOURAGE FUTURE RESEARCH ON THESE CRITICAL U.S. WEED MANAGEMENT TARGETS. IN ADDITION TO ADVANCING SCIENTIFIC KNOWLEDGE, THIS PROJECT WILL PREPARE THE PROJECT DIRECTOR FOR A SUCCESSFUL CAREER RESEARCHING AGRICULTURAL WEEDS, WHILE PROVIDING CRITICAL RESEARCH EXPERIENCES FOR HIGH SCHOOL AND UNDERGRADUATE STUDENTS.