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consider the case of invasive species, they did acknowledge that human activities could be a powerful agent for bringing together cross-compatible species that had been previously isolated by ecology or geography.

Indeed, Abbott (1992) observed that interspecific hybridization involving non-native plant species has often served as a stimulus for the evolution of entirely new, and sometimes invasive, species. Specifically, he noted that hybridization involving a non-native species and another (either native or non-native) has led to a number of new sexually reproducing plant species. The 10 examples he gives are either stabilized introgressants or allopolyploids. Some of these species have remained localized, but most have spread successfully far beyond their sites of origin. The latter group of his examples, plus many more we have accumulated, are listed in Table 1.

Abbott, Anderson, and Stebbins focused on interspecific hybridization. But their ideas should work equally well for hybridization among previously isolated populations of the same species. Therefore, we proceed below with a broad perspective.

We extend the ideas of Stebbins, Anderson, and Abbott to specifically address hybridization as a stimulus for the evolution of invasiveness. We restrict our examples to plants, but the model we develop may apply to other organisms as well. Below, we first provide many examples in which hybridization seems to have served as a stimulus for the evolution of a new invasive line. Second, we explain why plants with a history of hybridization may have a fitness advantage relative to those without such a history. Third, we discuss some scenarios that might lead to such hybridization. Finally, we examine how our model for interspecific hybridization could work equally well for hybridization between previously isolated populations of the same species.

MATERIALS AND METHODS

We sought at least 25 well documented examples of the evolution of invasiveness in plants after a spontaneous hybridization event. We did not intend our review to be exhaustive, but instead concentrated on finding the most convincing examples.

We used four criteria for choosing our examples:

  1. More evidence than intermediate morphology must be available to support the hybrid origin of the invasive lineage. Intermediate morphology does not necessarily support the hypothesis of hybridity (Rieseberg and Ellstrand, 1993). Species-specific genetically based traits such as chromosomes, isozymes, and/or DNA-based markers provide more reliable evidence for hybrid parentage. The hypothesis also can receive support



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