In the course of this review we have discussed the mechanisms by which invasive species evolve in response to their new biotic and abiotic environments, and how invasive species have altered the evolutionary trajectory of native species with which they interact. While it is not surprising that an invasive species would evolve in their new habitat in response to a new set of selective pressures, it is surprising that there are a number of clear examples of evolutionary shifts in native species in response to the presence of invaders, given the small number of generations involved in interactions, and the short period for which such interactions have been studied by ecologists and evolutionary biologists. Much of the evidence we have reviewed has been observed in islands, reinforcing what we know of islands as evolutionary hotspots. While few generalizations can be made across taxa and across environments, we can venture a few tentative conclusions. First, invasive predators may have the most dramatic effects, as the extinctions they cause represent an irreversible removal of evolutionary potential. Second, few examples of extinction have been associated with competitive interactions. This indicates either that extinction by competition is a slower process than extinction by predation, such that the end product of the process is not likely to be observed on the time scale of most scientific studies, or that communities are not as "full" as most ecological theories presume. Third, interactions between invasive and native biota demonstrate how global changes that alter community structure can have persistent and unexpected consequences.
The biota of the Earth is undergoing a dramatic transformation. The spatial patterning, structure, and functioning of most of the ecosystems of the world have been altered by the activities of humankind. There is every indication that these trends will intensify as the size of the human population continues to grow, even in systems that have been set aside for protection, because of the global changes that have been set in motion that are affecting the atmosphere and the climate. Although some aspects of global change, such as climate change, may be reversed by societal actions, this will not be possible for biotic exchange. The mixing of formerly separated biota, and the extinctions these introductions may cause, are essentially irreversible. Since the beginnings of the Age of Exploration, humans have purposefully and inadvertently moved biological material across barriers that, for recent evolutionary time, have separated the unique biotic realms of the continental land masses. We are now developing a whole new cosmopolitan assemblage of organisms across the surface of the Earth with large consequences not only for the functioning of ecosystems but also for the future evolutionary trajectory of life.
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This paper was presented at the National Academy of Sciences colloquium, "The Future of Evolution," held March 16-20, 2000, at the Arnold and Mabel Beckman Center in Irvine, CA.