Hybridization helps colonizers become conquerors.

Species colonizing new environments face an uphill battle to become established. Because colonizing populations are often small, opportunities for sexual reproduction may be limited by availability of mates or viable pollen—an example of an Allee effect (1, 2). The presence of amore abundant related species with similar climatic, habitat, and nutritional needs can further reduce the odds of colonizer establishment (3, 4). Despite these hurdles, there are numerous examples of colonizing species increasing in number and range to the detriment of the recipient ecological and human communities. The field of invasion biology studies such species to further our understanding of the fundamental mechanisms that allow colonizing species to prosper (5), as well as the more applied goals of predicting, preventing, and controlling invasions (6). Mesgaran et al.’s study (7) contributes to these goals by illustrating how colonizing species can exploit the presence of congeners to overcome the problems associated with low numbers through hybridization and the subsequent reemergence of colonizer genotypes. Hybridization between colonizing and resident species has been documented across diverse taxa including birds, fishes, and, particularly, plants (8). The consequences of such hybridization are varied: Both parental species may coexist through formation of a stable hybrid zone (9), a rare colonizer may be lost through genetic swamping by the more abundant resident (10), or both parental species may decline or disappear to be replaced by genetically diverse hybrid swarms (11) or a small number of hybrid genotypes with superior fitness to either parental species (12). Mathematical models, such as those developed by Mesgaran et al. (7), serve as a useful tool for understanding the ecological and genetic conditions under which these varied outcomes might arise (13). Mesgaran et al. (7) built a model describing the population dynamics and genetics of a colonizing plant, an established sister species, and hybrid genotypes that interact through pollinator-mediated gene flow and density-dependent fecundity. In contrast to prior theoretical work (e.g., ref. 14), the model assumes no ecological fitness differences between genotypes. Instead, the authors focus on how prezygotic (pollinator preference) and postzygotic (relative compatibility) forces determine genotype interactions. They explore how hybridization influences colonizer establishment by determining the minimum colonizer population size necessary to avoid population declines (the Allee threshold), and they examine the long-term fate of the colonizing and resident species by comparing how their relative frequencies change through time, with or without hybridization (Fig. 1). Two noteworthy results arise. First, colonizers capable of hybridizing with a resident congener need fewer founding individuals to establish than when no hybridization occurs, provided hybrids are more likely to successfully backcross with the colonizer than with the resident. Pollinator preference for the colonizer and colonizer-like genotypes further lowers the Allee Fig. 1. Schematic showing the fate of a small number of colonizers (yellow circles) interacting with a related resident species (red circles). In the absence of hybridization (Upper), the rare colonizer is swamped by incompatible pollen from the resident, resulting in low fecundity. Colonizers decline to vanishingly small frequency as they are replaced by resident seedlings. When hybridization occurs (Lower), colonizer genes persist in hybrids (orange circles) and colonizer genotypes reassemble through preferential backcrossing among colonizer-like individuals.