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Invasions of nonindigenous species have caused ecological devastation to natural communities worldwide, yet the biological bases for invasiveness remain poorly understood. Our studies of invasive watermilfoil (Myriophyllum) populations revealed widespread polymorphisms in biparentally inherited nuclear ribosomal DNA sequences, which were not detected in populations of native North American species. Subclones of the polymorphic regions revealed the occurrence of distinct sequences matching those acquired from both nonindigenous and native North American species. Molecular data demonstrate clearly that invasive watermilfoil populations in North America have resulted from hybridization between nonindigenous and native species. These observations suggest that invasiveness in these aggressive aquatic weeds may be linked to heterosis maintained by vegetative propagation.

The extraordinarily wide distributional ranges of aquatic flowering plants have long stimulated phytogeographical discussion. Although aquatic plants occur rarely among the angiosperms, they represent a disproportionately large number of taxa with broad distributions including various intercontinental disjunctions that are manifest even at the species level. Throughout the nineteenth and early twentieth centuries, long-range dispersal by waterfowl was the prevailing explanation for widespread aquatic plant distributions. This explanation gradually fell into disfavor as biologists raised doubts as to the ability of waterfowl to transport propagules across the extensive transoceanic distances between the continents on which an assortment of aquatic taxa now reside. During the twentieth century, the development of biogeographical displacement theory, i.e., "continental drift," steadily began to supplant dispersal as the preferred explanation for discontinuous angiosperm distributions. Our study assesses the dispersal/displacement hypotheses from a temporal standpoint using molecular estimates of divergence time for a diverse sample of phylogenetically related aquatic taxa that exhibit discontinuous intercontinental distributions. With few exceptions, we found divergence times that are far too recent to implicate continental drift as a major determinant of discontinuous distributions in aquatic plants. We suggest that long-distance dispersal by birds should continue to be regarded as a viable explanation for widely disjunct aquatic plant distributions, although such dispersal is likely to have involved a combination of overland as well as transoceanic migratory routes.

“Venture philanthropy” burst loudly onto the scene in the mid- to late 1990s, promoted as a way to revolutionize grantmaking. Today the field has been refined, and its proponents are more modest. The case of venture philanthropy provides insights into the construction and evolution of a “new” organizational field and “new” professional culture, topics that require further scholarly exploration. Qualitative research examining venture philanthropy organizations and their leaders is reported here. Findings suggest that although the dot-com boom was an important prompt, the construction and diffusion of the field depended on opinion leaders who strategically defined, legitimated, and advocated the new model. The fit with existing culture and institutionalization via networks were also important. Implementation difficulties and the business—nonprofit culture clash are among factors forcing evolution of the field. Several avenues for further research on this understudied field, and on other new fields and hybrid professional cultures, are suggested by these findings.