the new range can potentially evolve. This scenario seems likely in the history of the plant invaders Echium plantagineum (Patterson’s curse) in Australia and Lythrum salicaria (purple loosestrife) in North America; in both cases, there is evidence that multiple introductions from Europe resulted in considerable genetic diversity in introduced populations (Thomson et al. 1987, Burdon and Brown 1986). Such diversity is likely to favor rapid evolution of local races, despite recent efforts at biological control.

The idea that the mixing of distinct genetic lineages through multiple introductions can give rise to novel phenotypes is supported by evidence from hybridization studies in several flowering plant groups. For example, morphological and genetic evidence demonstrates that the recently derived allotetraploid ruderals Tragopogon mirus and T. miscellus originated in the U.S. Pacific Northwest through hybridization between allopatric species from Europe that were introduced in historical times (reviewed in Novak et al. 1991). Similarly, molecular evidence indicates that the common cattail Typha glauca, an important component of wetlands surrounding the Great Lakes, is a stabilized F1 hybrid between the native T. latifolia and introduced European T. angustifolia (Kuehn et al. 1999). Not all hybridization events result in the origin of stable F1 hybrids. Recent molecular studies of putative hybrids between the native Californian cord grass (Spartina foliosa) and S. alternifolia, an introduced species from the East Coast of North America, demonstrated extensive introgression between the two parental species in the San Francisco Bay (Ayres et al. 1999). Abbot (1992) discusses other examples of new plant invaders that arose from hybridization. Rhymer and Simberloff (1996) review cases in which such hybridization and introgression of plants and animals pose ecological and genetic threats to the survival of native species.

The capacity of nonindigenous species to evolve after their arrival in a new range complicates our ability to predict their postarrival behavior. Novel genotypes emerge through strong selection, hybridization, and the sharing of genes among members of the same species drawn from different parts of the native range. Selection in the new range sorts among these new genetic products, and the result can be organisms more adapted than their ancestors to the new range. As cited above, there are cases in the United States of new species, created through allotetraploidy, whose existence, to say nothing about their role in the new range, could not have been predicted. Those results have implications for current quarantine practices. With few exceptions, nonindigenous species already found in the United States are not barred from further entry. That practice has the unintended result of potentially allowing the introduction of a species’ genotypes that were previously unrepresented in the United States. As has already occurred repeatedly, the continuous potential exists for the eventual assembly of new, invasive genotypes from among an array of genotypes each of which by itself is innocuous or at least only has the ability to become naturalized, not invasive. The

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