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ers than are native genera supports the view that successful invasives are preadapted and do not evolve invasiveness in situ. Certain specific cases of invasives fit this model well. For example, the fact that invasiveness can sometimes be reversed by a biological control agent [(e.g., prickly pear in Australia (Dodd, 1959) and Klamath weed in the American Pacific Northwest (Huffaker and Kennett, 1959)] suggests that invasiveness can appear simply once an organism is released from its primary biological enemies. Also, it has been observed that “a strong predictor of invasiveness . . . is whether the organism has been invasive . . . elsewhere” (Ewel et al., 1999, p. 627). Although such correlates may be statistically strong, they are typically weak in predicting invasions, leading one reviewer of the field to assert, “serendipity is often an important element in successful invasions” (Gray, 1986, p. 655) and another to lament, “It could be that invasions . . . are intrinsically unpredictable ” (Williamson, 1999, p. 10).

But for some successful invasive species, it may well be that a series of events after colonization is more important than intrinsic “colonizing ability.” In fact, two enigmatic phenomena associated with successful invasives suggest that many species are not preadapted to become successful invasives and that the right circumstances must transpire for invasiveness to occur (and perhaps evolve). The first is the observation that there is often a considerable lag phase between the establishment of local populations and their aggressive spread (Ewel et al., 1999; Mack, 1985). For example, Kowarik (1995) reviewed 184 invasive woody species with known dates of first cultivation in Brandenburg, Germany. The mean delay in invasion was 131 years for shrubs and 170 years for trees. Delays on the order of decades may occur for herbaceous invasives as well (Pyek and Prach, 1993). If these species were simply preadapted, then we would expect evidence of invasiveness relatively quickly. Second, multiple introductions often are correlated with the eventual success of non-native species establishment and invasiveness (Barrett and Husband, 1990). For example, North America's most successful invasive birds, the European Starling and the House Sparrow, both became invasive only after repeated introduction (Ehrlich et al., 1988). Collectively considered, these observations suggest genetic change and adaptive response play a role in the ultimate establishment of some invasive species.

We contend that hybridization may result in critical evolutionary changes that create an opportunity for increased invasiveness. As Anderson and Stebbins (1954) pointed out, “hybridization between populations having very different genetic systems of adaptation may lead to . . . new adaptive systems, adapted to new ecological niches ” (Anderson and Stebbins, 1954, p. 378). Stebbins further examined what he came to call “the catalytic effects of such hybridization ” (Stebbins, 1974) in subsequent articles (Stebbins 1959, 1969). Although Anderson and Stebbins did not

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