from comparison of artificially synthesized hybrids with the putative spontaneous hybrids and from the relative sterility of the putative hybrids compared with that of the parental species.
The hybridization event preceding the evolution of invasiveness must be spontaneous. Many artificial hybrids have escaped from cultivation to become naturalized invasives (e.g., certain mints, comfrey, poplars, and watercress; cf. Stace, 1975).
The hybrid derivatives must be established as a novel, stabilized lineage and not simply as transient, localized hybrid swarms. In some cases, genetic or reproductive mechanisms may stabilize hybridity (e.g., allopolyploidy, permanent translocation heterozygosity, agamospermy, and clonal spread; cf. Grant, 1981). Some have become new, reproductively isolated, recombinant species. In other cases, introgression may be so extensive that the hybrid lineage swamps out one or both of its parents, becoming a coalescent complex.
The new lineage must exhibit some degree of invasiveness. We define invasive populations as those that are capable of colonizing and persisting in one or more ecosystems in which they were previously absent. The minimal criterion of invasiveness for our hybrid derivative is that it must replace at least one of its parental taxa or invade a habitat in which neither parent is present. We hold to this criterion for those few cases in which one parent is itself invasive.
We did not restrict ourselves to examples of hybridization involving one or more non-natives, because the evolution of invasiveness by hybridization should be independent of the geographical source of the parental material.
We found 28 examples representing 12 families where invasiveness was preceded by hybridization; these examples are detailed in Table 1 and Table 2. We encountered another 2 dozen or so examples of invasive lineages thought to have a hybrid origin (e.g., Lonicera × bella, Oenothera wolfii × Oenothera glazioviana, and Platanus racemosa × Platanus acerifolia). The latter did not sufficiently meet our criteria, mostly because only morphology was offered to support their putative hybrid origin.
In some of our examples, the hybrid-derived lineages have already achieved a taxonomic epithet (detailed in Table 1). In other cases, a new invasive lineage has been identified and studied but not yet named, to our knowledge (detailed in Table 2). In each case, we give the parental species, plant family, habit of the hybrid derivative, its site of origin, and the evidence supporting a history of hybridization for the new lineage. We