Evolutionary Adjustments of Invaders and of the Invaded. We turn to contemporary studies to give us some indication of the evolutionary impact of invasive species. Recent studies have shown that invaders can rapidly adapt to the new environments in which they find themselves. Huey et al. (18) demonstrated how an 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 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. Interestingly, the developmental basis for the cline of wing size was different in Europe than for the invader in North America, although the functional result was the same, providing additional evidence for the adaptive advantage of this set of traits.
Drosphilia subobscura were introduced into North America in 1982; shortly thereafter Ayala
et al. (
19) described the invasion as "a grand experiment in evolution." This was certainly an accurate prediction, given the results of Huey
et al. 10 years later, and only 20 years after the beginning of the invasion event.
There are other documented instances of an invading species adapting to its new environment. For example, Johnston and Selander (20) described the evolution of apparently adaptive clines in body size and feather color in English sparrows that were introduced into North America in 1852 and that subsequently established a large geographical range. Further, Cody and Overton (21) described the reduction in distance of dispersability for wind-dispersed seeds of invasive species onto islands in just a few generations in small isolated populations. Similarly, Losos et al. (22) demonstrated that within 10-14 years species of lizards introduced onto a series of island in the Caribbean showed adaptive morphological adjustments.
There are also examples of relatively rapid, nonadaptive, genetic change of invaders as seen in house mice introduced into Madeira; localized differentiation of chromosomal races is the result of genetic drift in isolated valleys (23). Similarly, genetic drift has been responsible for geographic genetic patterns found in the introduced Bufo marinus in Australia (24).
Evolution in Response to an Invader. There are also examples of rapid evolution in native species in response to an introduced species. Carrol and Dingle (25) indicate that populations of the soapberry bug (Jadera hematoloma) have evolved differing beak lengths in response to the introduction of new invasive hosts, within only 50 years time. Singer et al. (26) have shown rapid evolution in the feeding preferences of the Euphydras butterfly for the invading herb, Plantago lanceolata.
Zimmerman (
27) documents an interesting case of evolution in response to an introduced crop species. At least five species of host-specific moths (
Hedylepta) have evolved since the introduction of banana into Hawaii
1,000 years ago. These species were threatened at the time of Zimmerman's study by parasitic wasps and flies introduced for agricultural pest control.
There is a large literature on the evolution of weeds in response to human activities, including agricultural practices. Harlan (28) noted that some weeds have evolved to be crop mimics. Not only are they are similar in their phenological development and morphological appearance to the crops with which they have co-evolved, but also their seeds have evolved a similar appearance so they are not sorted and discarded during harvesting. For example, the lentil mimic (Vicia sativa) has evolved a seed shape and color comparable to the lentil (Lens culinaris). This trait is under control of a single gene. Similarly, Echinochloa crus-galli has evolved mimics to rice, Oryza sativa, which are very difficult to distinguish from the crop.
De Wet and Harlan (29) surmised that many plant weeds might have evolved from natural pioneer species associated with continuous disturbance by humans. Some weeds that have developed in association with agriculture have become crop mimics as described above. Weeds are also derived from hybridization and introgression with crops as happened with Johnson grass (Sorghum halepense) and the cultivated Sorghum bicolor. Weeds have also evolved from abandoned domesticated plants.
Thus there are many cases that have been documented of the evolutionary response to the new environment that an invasive species may encounter as well as cases of the adaptive response of organisms to a new invader.
Hybridization and Introgression. In addition to direct evolutionary responses of organisms involved in invasions there are also very important indirect effects through changes in the genetic structure of invasive species in relation to the new organisms that they encounter. These major effects are related to hybridization and introgression. Rhymer and Simberloff (30) have recently summarized our knowledge in this area. There are many examples extending over many different taxonomic groups, a few of which are noted below. These authors conclude that in the case of invasive species hybridization with native species can cause a loss in fitness in the latter and even a threat of extinction. McMillan and Wilcove (31) have documented that of 3 of 24 species listed as Endangered in the United States and that subsequently went extinct, 3 were the result of hybridization with alien species.
Birds
Mallard ducks (
Anas platyrhynchos) that have been introduced into various regions of the world have had large genetic effects. They have hybridized and reduced populations of the New Zealand gray duck (
Anas superciliosa superciliosa), the Hawaiian duck (
Anas wyvilliana), and the Florida mottled duck (
Anas fulvigula fulvigula) (
30).
MammalsSitka deer (Cervus nippon) were introduced into Great Britain from Japan over a hundred years ago. They have hybridized with the native reed deer (Cervus elephaus) although they are different in body size. It appears that the genetic integrity of the native red deer is threatened in some regions (32).
FishThere are a number of cases of hybridization and subsequent introgression in fish, primarily game fish where there are massive introductions of foreign stock. These include trout in western and eastern United States as well as in Europe (33, 34). It has been shown, however, that even small introductions of nonnative species can have large impacts on the genetics of native species through hybridization and introgression, as was found for native pupfish in Texas (33).
PlantsAbbott (35) notes that of 2,834 species listed in the New Flora of the British Isles 1,264 are aliens. There are 70 recognized hybrids between native and alien species and 21 between aliens. About half of these hybrids show some degree of fertility.
There are many examples of the large populations of invading species swamping small populations of native species by hybridization, but in certain cases small populations of an invader can threaten native species that have much larger populations. This is the case with the invading Spartina alterniflora into the San Francisco Bay. It hybridizes with the native Spartina foliosa. The invader has a higher pollen output, and greater male fitness, than the native species and the hybrids and it occupy lower intertidal habitats. In time introgression will threaten the native species (36). Conversely, small populations of rare species can be threatened by hybridization in a number of ways (37), including infertility of the hybrids.
Small populations on islands are particularly vulnerable to extinction by hybridization because they are often less genetically divergent than mainland species and have weak crossing barriers as well as unspecialized pollinators. Levin et al. (37) describe a number of cases of extinction by hybridization on islands, including the endemic shrub Cercocarpus traskaei with the widespread Cercocarpus betuloides and the endangered Lotus scoparius traskiae with the Lotus argophyllus ornithopus. They specifically note that introductions may threaten rare species on islands and give a number of examples from around the world, including threats to the rare Arbutus canariensis and Senecio teneriffae on the Canary Islands, Gossypium tomentosum on the Hawaiian Islands, and Pinguicula vulgaris and Linaria vulgaris in the British Islands. They posit that the threat of extinction of rare species by hybridization is very high and that habitat disruption and invasive species are increasing this threat to the degree that conservation programs should strive to isolate rare species from cross-compatible congeners.
The Origin of New Taxa Through Hybridization and Introgression. While hybridization with invaders can be a threat to species integrity, it can also be a source of new variation and the origin of new species. Spartina alterniflora from the east coast of North America was introduced into Southampton in shipping ballast in the early 19th Century. It subsequently hybridized with the local Spartina maritima, producing a sterile hybrid. The hybrid in turn underwent chromosome doubling to produce the new fertile species, Spartina anglica. Spartina anglica has become very aggressive and occupies large areas of the coastline of the British Isles while at the same time the original invader, Spartina alterniflora, and the native Spartina maritima have maintained limited distributions. The new polyploid evidently has characteristics that enable it to occupy bare tidal flats that were not available to the parents (38). This event was apparently serendipitous and has not been replicated artificially (39).
In addition to the
Spartina anglica there are other cases of alloploids that have originated from hybridization of native and invasive species. These include species of
Tragopogon in North America and
Senecio in Great Britain (
35).
There are also examples of introgressive hybrids between native and weedy species becoming stabilized to form new taxa. The introduced Helianthus annuus hybridized with native Helianthus debilia. The hybrids adapted to the new conditions it encountered to form the subspecies Helianthus annuus texanus. Abbott (35) cites six such cases of origins of new taxa.
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