The evidence for character displacement as a widespread response to competition is now building. This progress is largely the result of the establishment of rigorous criteria for demonstrating character displacement in the animal literature. There are, however, relatively few well-supported examples of character displacement in plants. This review explores the potential for character displacement in plants by addressing the following questions: (1) Why aren't examples of character displacement in plants more common? (2) What are the requirements for character displacement to occur and how do plant populations meet those requirements? (3) What are the criteria for testing the pattern and process of character displacement and what methods can and have been used to address these criteria in the plant literature? (4) What are some additional approaches for studying character displacement in plants? While more research is needed, the few plant systems in which character displacement hypotheses have been rigorously tested suggest that character displacement may play a role in shaping plant communities. Plants are especially amenable to character displacement studies because of the experimental ease with which they can be used in common gardens, selection analyses, and breeding designs. A deeper investigation of character displacement in plants is critical for a more complete understanding of the ecological and evolutionary processes that permit the coexistence of plant species.

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Stain-resistant carpets and nonstick pots were once the epitome of "better living through chemistry," their space-age properties conferred by molecules known as perfluoroalkyl and polyfluoroalkyl substances (PFAS). But in the early 2000s, researchers began to discover that PFAS were somehow reaching the farthest corners of the planet-from polar bears in Alaska (1) to pilot whales in the Faroe Islands of the North Atlantic (2). These molecules contain chains of carbon peppered with fluorine atoms, which together form one of the strongest known chemical bonds. That helps these chemicals excel at repelling grease and water but also makes them astonishingly resistant to degradation in the environment (3). Amid a flurry of new studies, scientists are still figuring out what risks these ubiquitous "forever chemicals" pose to public health (see "PFAS Politics"). Epidemiologists and toxicologists point to myriad possible consequences, including thyroid disease, liver damage, and kidney and testicular cancers (4). Impacts on the immune system are a particular concern.Animal models and human studies have provided strong evidence that PFAS alter the immune system, diminishing the ability to fight disease or respond to a vaccine. These studies have heightened urgency as nations across the globe grapple with the coronavirus disease 2019 (COVID-19) pandemic and engage in a vaccination campaign of historic proportions. Researchers are intent on better understanding how PFAS affect coronavirus and other infectious diseases-as well as the vaccinations meant to stymie them.But many questions remain: Scientists don't know the toxicity levels of most PFAS or how mixtures of PFAS may interact to affect immune health. Even for the most commonly studied PFAS, little is known about the mechanics of how these substances interact with the immune system. A Troubling FindingPeople are exposed to forever chemicals through contaminated water, food, and air, as well as countless products including cosmetics and upholstery. In 2015, the Centers for Disease Control and Prevention (CDC) National Health and Nutrition Examination Survey reported that PFAS were found in the blood of nearly all Americans sampled (5). US companies no longer manufacture the two best-known PFAS, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS). But these legacy PFAS persist in the environment, even as thousands of others remain in production.About a decade ago, researchers started to detect signs of immune system after-effects in humans. In 2008, environmental epidemiologist Philippe Grandjean of the University of Southern Denmark in Odense read a study on PFOS that worried him. The work, by Margie Peden-Adams, then of the Medical University of South Carolina in Charleston, and colleagues, suggested that PFOS in mice, at levels similar to those found in humans, could suppress the immune system (6).Grandjean, who is also an adjunct professor of environmental health at the Harvard T.H. Chan School Animal models and human studies suggest that forever chem...

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Invasive plants have the potential to alter phenotypic selection on floral traits in native plant populations. If native plants can evolve in response to this altered selection pressure, the evolution of floral traits may play an important role in permitting long-term coexistence of native and invasive plants.