necessary for previously isolated populations to come into contact and for hybridization to occur. Our examples demonstrate that invasiveness can evolve. Our model does not represent the only evolutionary pathway to invasiveness, but is clearly an underappreciated mechanism worthy of more consideration in explaining the evolution of invasiveness in plants.
Invasive species have always held a special place for ecologists and evolutionary biologists. Successful invaders that have colonized new regions within historical time provide real-life examples of ecological and evolutionary change. The demographic change from a small number of colonists to a sweeping wave of invaders is a dramatic ecological event. Likewise, those demographic changes—a founder event followed by a massive increase in numbers—may have dramatic evolutionary consequences. Not surprisingly, whole books have been dedicated to the basic science of invasive species (for example, see Elton, 1958; Mooney and Drake, 1986).
Also, the applied biology of invasive species has become increasingly important as intentional and unintentional anthropogenic dispersal moves species from continent to continent at unprecedented rates. Invasive plants and animals are often thought of as agricultural pests, but they also pose a hazard for a variety of human concerns, including health, transportation, and conservation (U.S. Congress, Office of Technology Assessment, 1993). Invasive species not only directly impact human well being, but they also are recognized as agents that alter community structure and ecosystem function (for example, see Horvitz et al., 1998). In the United States alone, the damage wrought by invasive species totals approximately $122 billion per year (Pimentel et al., 2000).
Only a tiny fraction of introduced species become successful invasives (Williamson, 1993). Given that invasives are important for so many reasons, considerable effort has been spent trying to develop generalizations to determine which species are likely to become successful. In particular, ecological, taxonomic, and physiological correlates of invasive success have been sought to predict which introduced species might become successful (for example, see Bazzaz, 1986; Daehler, 1998; Pyek, 1997, 1998; Rejmanek, 1996). Less frequently, possible genetic correlates have been sought (for example, see Gray, 1986). Very little attention has been given to the possibility of the evolution of invasiveness after colonization.
Are invasives “born” (that is, are they released from fitness constraints) or are they “made” (that is, do they evolve invasiveness after colonization)? The fact that certain correlates of invasive success have been identified suggests that invasives are born. Also, Darwin's (1859) observation that non-native genera are more likely to be successful invad-