Genetic or reproductive mechanisms that stabilize hybridity (e.g., allopolyploidy, permanent translocation heterozygosity, agamospermy, and clonal spread) also will fix heterotic genotypes. It may well be that the fitness boost afforded by fixed heterozygosity is all that is necessary to make a hybrid lineage invasive. Given the ubiquity of heterosis in both agricultural and natural systems, we are surprised how rarely fixed heterosis is posited as a role of hybridization in adaptive evolution (but see Grant, 1981). The majority of our examples (especially in Table 1) are capable of fixing heterotic genotypes by agamospermy (e.g., Amelanchier), by allopolyploidy (e.g., Bromus, Cardamine, Sorghum, and Tragopogon), by permanent translocation heterozygosity (Oenothera), and by clonal spread (e.g., Circaea, Fallopia, Glyceria, Mentha, and Stachys).

The case of the invasive S. anglica in the British Isles is perhaps our most notorious example (Gray et al., 1991; Thompson, 1991). This species originated by chromosome doubling of the sterile hybrid between the Old World S. maritima and the New World S. alterniflora. Genetic analysis found fixed heterozygosity at many of this species ' loci, but also showed that S. anglica is almost totally lacking in genetic variation among individuals. Despite its relatively narrow ecological amplitude, it has invaded intertidal flats, replacing more diverse native plant communities, altering succession, and limiting the availability of food to wading birds.

But note that we also were able to use S. anglica as a possible example of invasive success attributable to evolutionary novelty (see Evolutionary novelty above). It is not clear whether invasive success in S. anglica and in our other examples is caused by (i) the fitness benefits conferred by heterosis, (ii) the fixation of an evolutionarily novel genotype by a mode of reproduction that frustrates recombination, or (iii) both. Common garden experiments could test these hypotheses by asking whether hybrids have superior fitness to one or both parental types under specific environmental conditions. We are aware of one such study among our examples, involving Carpobrotus and demonstrating heterosis in the hybrids (Vilà and D'Antonio, 1998).

Populations with a history of isolation and a small population size may accumulate detrimental mutations. In such populations, mildly deleterious alleles become fixed, leading to slow erosion of average fitness (see examples in Mills and Smouse, 1994; Lande, 1995). Hybridization between such populations can afford an opportunity to escape from this mutational load, particularly if recombination permits selection to act to