weed of Australia. The average population there was found to be more diverse than were those genetically analyzed in its native range in Europe (Burdon and Brown, 1986). This species has been introduced more than once to Australia, both intentionally and unintentionally (Piggin and Sheppard, 1995). Similarly, North American populations of the introduced weed cheatgrass, Bromus tectorum, were found to have increased within-population genetic variation as compared with populations from its source range in Europe and northern Africa (Novak and Mack, 1993). Again, there is ample evidence of multiple introductions (Novak et al., 1993).
Discussions of the population biology of invasives have focused largely on their ecology and on the evolutionary consequences of the invasive process. The evolution of invasiveness as an adaptive trait has been largely neglected. We have extended—and, indeed, hybridized—the ideas of Stebbins, Anderson, and Abbott concerning the evolutionary significance of hybridization to offer one model for the evolution of invasiveness. That is, hybridization can, through one or more mechanisms, catalyze the evolution of invasiveness. Human dispersal and human disturbance both act to accelerate the process and increase the opportunities for hybrid lineages to take hold. The process is not unique to plants. In fact, evidence recently has emerged that “a new, aggressive Phytophthora pathogen of alder trees in Europe” seems to have arisen through interspecific hybridization (Brashier et al., 1999, p. 5878). Likewise, hybridization between different honeybee subspecies has given rise to the infamous Africanized bees of the New World (Camazine and Morse, 1988).
Certain caveats are in order. We recognize that only a fraction of hybridization events will lead to the evolution of invasiveness. We do not claim that all invasive species have evolved invasiveness. As we note in our introduction, sometimes certain ecological explanations appear to be the most parsimonious, such as encountering an unfilled niche, competitive superiority, or ecological release. Nor do we claim that hybridization is the sole evolutionary pathway to invasiveness. Other evolutionary pathways to invasiveness already have received some attention. For example, weeds have evolved to mimic unrelated crops and have become successful invaders of agroecosystems (Barrett, 1983). Also, Jain and Martins (1979) observed that a single gene mutation apparently is responsible for the appearance of invasiveness of rose clover in California.
At the moment, evolution of invasiveness remains an underappreciated area of research on a topic of great applied and basic importance. We have shown that one way to get a handle on studying such evolution is to use examples that have a genetic signature for reconstructing past