According to the “hybrid bridge hypothesis”, hybrid plants that are morphologically, genetically, and spatially intermediate between parent species can make it easier for pathogens to acquire new hosts (either by direct transfer to hybrids or indirect transfer to new hosts using hybrids as a “stepping stone”) (Floate and Whitham 1993).

The addition of novel, including invasive, organisms in new ranges creates opportunities for evolution and makes prediction of impacts more difficult. Adaptations arising in plants when they move to new environments can produce invasive descendants. The tropical alga Caulerpa taxifolia evolved tolerance of low temperatures through its cultivation in an outdoor tank at the Stuttgart Zoo (Meinesz 1999). Evolution in native animals in response to the entry of invaders can make it difficult to restore the web of interactions characteristic of the native community. For example, the native checkerspot butterfly Euphydrias editha evolved a change in host preference from a native to an introduced plant in response to a decline of its native host and an increase of a novel, invasive host, Plantago lanceolata (Singer et al. 1993). By the time the native host is restored to its original abundance, if ever, the insect population might have lost the capacity to use it.

Introduced biological controls can become less effective because of evolved changes in the virulence of control organisms or changes in the resistance of target organisms. Myxoma virus was released in Australia to control the introduced European rabbit Oryctolagus cuniculus. Within a few years, the virulence of the virus had declined, and the resistance of the rabbits had increased sharply (Dwyer et al. 1990). In other cases, nonindigenous biological control organisms appear to become more effective by adapting to novel hosts. An ichneumonid parasitic wasp, Bathyplectes curculionis, imported to the United States to control the alfalfa weevil, Hypera postica, was originally ineffective against the Egyptian alfalfa weevil, Hypera brunneipennis. Initial dissections showed that 35-40% of the wasp’s eggs were destroyed by the immune response of the larval weevil, whereas samples taken 15 years later showed only 5% egg loss (Salt and van den Bosch 1967).

Evolutionary adjustments of control and target organisms in biological control programs are not well known, but in documented cases in other contexts invading organisms have adapted to their new environments and organisms have adapted to new invaders (Mooney and Cleland 2001). Huey et al. (2000) demonstrated that introduction of a new fruit fly into the West Coast of North America resulted in the evolution, in only 20 years, of an apparently adaptive cline related to wing size throughout the fly’s vast new latitudinal range extending from southern California to British Columbia. The cline that developed in North American female flies was similar to that found in the European native populations. The developmental basis of the cline of wing size was different between the European flies and the invader in North America, although the functional result was the same and provided additional evidence of the adaptive advantage of this set of

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