Evolution

The conditions under which a species is introduced to a new region (isolation from parent population, small propagule size [usually], and different physical and biotic environment) should be conducive to rapid evolution. There has been little study of this phenomenon, but some striking examples have emerged. Drosophila subobscura, introduced to the Americas from the Old World around 1980, spread widely and by 2000 had evolved a cline of increasing total wing length with latitude in North America phenotypically similar to that in its native range. The ichneumonid Bathyplectes curculionis, introduced to the western United States for biological control of introduced alfalfa weevils (Hypera spp.), evolved in less than 10 years to become less susceptible to the encapsulation reaction of its host.

A phenomenon widely reported among introduced vertebrates and plants, particularly in North America and Eurasia, is hybridization with native species, sometimes to the point of a sort of genetic extinction of the latter. Although hybridization is known to have played an important role in insect evolution, hybridization between native and recently introduced insects is rarely if ever reported. This absence of data may reflect biological differences or simply less genetic study of insects. There are instances of introduced populations hybridizing with one another, most notably the Italian and African strains of the honey bee. The red imported fire ant hybridizes extensively with the previously introduced black imported fire ant (Solenopsis richteri) in Tennessee.

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