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FIGURE 1. As genetic distance between mating colonists increases, so too should heterosis in their progeny—up to a point—then, progeny fitness declines as outbreeding depression becomes important.

would expect that invasiveness would occur after multiple introductions of a species, because multiple introductions would be necessary for providing genotypes from disparate sources. In fact, species that are intentionally introduced would have an advantage in this regard. Second, we would expect that invasiveness would occur after a lag time, during which hybridization and selection would act to create and increase invasive genotypes. As noted in our introduction, both of these phenomena have occurred so frequently that they have attracted the attention of students of invasive species. In fact, invasive species often originate from multiple foci, each with an independent origin (for example, see Cook et al., 1998; Moody and Mack, 1988). If these foci spread and coalesce, there is an opportunity for hybridization among these independent lineages.

Finally, we might expect that if the evolution of invasiveness followed a bout of hybridization between well differentiated populations, then the resulting populations should likely be more genetically diverse than were their progenitors. This suggestion may seem surprising because of the commonly held view that invasives should be relatively genetically depauperate as a result of the bottlenecks associated with their colonization dynamics (Barrett and Husband, 1990). On the other hand, hybridization between well differentiated populations resulting from introductions from different sources ought to leave relatively high levels of within-population polymorphism as a “signature. ”

We have found two such examples. Echium plantagineum is a noxious



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