Among plants, basic life history features may be altered by natural selection. Many Eurasian plants introduced to North America have shown ecotypic adaptation to latitudinal differences in climate.Wild carrot (Dau-cus carota), for example, showed genetically based variation in age of reproduction along a latitudinal gradient from North Carolina to Ontario (Lacey 1988). This pattern is estimated to have evolved over about 150 generations. In Chile, burr medic (Medicago polymorpha) is naturalized over a 1,000-km range extending from the arid north to the humid south of the country (Del Pozo et al. 2002). In common garden tests, plants from different localities varied over a range of 46 days in time of flowering, as well as in winter growth vigor and other characteristics. Curly dock (Rumex crispus) (Hume and Cavers 1982), barnyard grass (Echinochloa crus-galli) (Roy et al. 2000), and common cocklebur (Xanthium strumarium) (Tranel and Wassom 2001) likewise showed ecotypic adaptation over their alien ranges. Recently, populations of salt cedar (Tamarix ramosissima) in Arizona and Montana were found to differ genetically in seedling investment in root biomass, with greater investment in northern populations (Sexton et al. 2002).
Detailed studies of common St.John's wort (Hypericum perforatum) over its latitudinal range in North America suggested that this species has shown rapid adaptation to habitat conditions (Maron et al. 2004). Genetic analyses indicated that this species was introduced to North America on multiple occasions and shows high genetic variability. North American populations showed distinct patterns of latitudinal adaptation, just as they do in Europe. Populations in particular North American locations, however, were not always derived from European populations occupying similar climatic regimes. Thus, it appears that natural selection has played a major role in adapting this species to latitudinal habitat differences in about 150 yr or less, corresponding to about 12-15 plant generations.
Shifts in mating systems are often seen in alien species. Genetic shifts toward self-fertilization, apomixis, or vegetative reproduction are often seen in alien plants (Brown and Marshall 1981). Presumably, the selective value of such shifts is increased reproduction in favorable environments in which, at least initially, the opportunity for outcrossing is very limited. On the other hand, shifts toward increased outcrossing are seen in some species (Brown and Marshall 1981). Where an introduction unit is genetically diverse, this may reflect strong, new selection pressures favoring recombination genotypes produced by outcrossing.
A clear example of a shift from sexual to asexual reproduction is provided by bulbous bluegrass (Poa bulbosa).This grass is widespread in western Europe, where it reproduces primarily by sexually produced seed (Novak and Welfley 1997). Bulbous bluegrass has become established widely in North America, but here its primary reproductive mode is by asexually produced bulblets. Many of the individual grass florets develop directly into these bulblets, and others develop into flowers that normally do not set seed. Some sexual reproduction may occur, however, since this species maintains a high level of genetic variation in North America.
In some cases, a shift from sexual to asexual reproduction may only occur in small, isolated populations of alien plants.Yellow starthistle (Centaurea solsticialis), as noted earlier, is an aggressive forb invader of rangelands in much of western North America. Most populations have remained strongly outcrossing. A small population in San Diego County, California, however, shows a high level of inbreeding (Sun and Ritland 1998). A strong genetic bottleneck during the establishment of this population may have eliminated alleles, leading to inbreeding depression and a reduction in the frequency of genotypes among which outcrossing could occur.
Examples of a shift toward outcrossing are less frequent. An increased frequency of outcrossing has been seen in weedy grasses in areas to which they have been introduced (Brown and Marshall 1981). Slender wild oat (Avena barbata) and soft brome (Bromis mollis) exhibit less than 1% outcrossing in European populations. Slender wild oat outcrosses at 2-7% in California and soft brome at 10% in Australia.
Other basic life history patterns, such as a shift from annual to perennial habit, may also occur as a result of strong selection in the alien environment. In annual bluegrass (Poa annua), for example, perennial ecotypes tend to develop in areas of intensively mowed turf (McElroy et al. 2002). This species, native to Europe, is a problem grass in lawns and golf courses.
Alien plants, both perennial and annual, have shown a variety of patterns of adaptation to physical habitat conditions of their new environment. Rose clover (Trifolium hirtum), for example, was introduced to California from the Mediterranean region of Eurasia in the late 1940s as a forage plant (Jain and Martins 1979). From pasture plantings, it has spread, in some places, to roadsides and other disturbed habitats in central and northern California. Comparison of the morphology and population dynamics of pasture and roadside populations showed that several differences existed between these populations. In roadside populations, the flower calyces of plants tended to be hairier than those of plants in pasture populations and to remain attached to the fruits after they fell from the plant. This was correlated with higher seed germination, but lower seedling survival, in the following season. In greenhouse experiments under identical conditions, roadside plants also tended to flower earlier and to produce about 22% more flowering heads.Thus, over a period of only about 20 yr, evolutionary differentiation of pasture and roadside populations of rose clover had occurred. Subterranean clover (Trifolium subterraneum) in Australia, introduced both deliberately and inadvertently, similarly shows a number of strains distinct from planted varieties (Cocks and Phillips 1979). Some of these may have originated by interbreeding and natural selection in Australia.
In California, slender wild oat (Avena barbata), an alien introduced from Europe, has evolved distinct races. One race is adapted to the semiarid grasslands and oak woodlands of the Central Valley and lower foothills and the other to the valleys of the coastal mountains and the higher foothills of the Sierra Nevada Mountains (Clegg and Allard 1972). Populations in the grassland and oak woodland habitat are characterized by black, hairy lemmas and a series of five enzyme loci that are monomorphic. Coastal mountain and higher Sierran foothill populations show paler, less hairy lemmas and high levels of polymorphism for enzyme loci.
Many other annual forbs and grasses show genetically based patterns of adaptation to local habitat conditions, especially in characteristics related to reproduction. In Europe, common groundsel (Senecio vulgaris) shows differentiation of ruderal and cropland populations, with cropland plants having larger leaves and more flowering heads (Leiss and Mueller-Scharer 2001). Charlock mustard (Sinapis arvensis) and corn poppy (Papaver rhoeas) exhibit population differences in seed dormancy (Garbutt and Whitcombe 1986; Lane and Lawrence 1995). Canada thistle (Cirsium arvense), a noxious weed in many temperate regions, also shows ecotypes differing in phenology and disease resistance (Donald 1994). Among annual grasses in North America, red brome (Bromus rubens) populations differ in flowering phenology and seed mass in different habitats (Wu and Jain 1978) and cheatgrass (Bromus tectorum) varies in flowering phenology, seed mass, and the response of seed germination to temperature (Rice and Mack 1991; Meyer and Allen 1999).
Forage plants, both forbs and grasses, have been transported worldwide. These plants exhibit a complex pattern of genetic variation due to both artificial and natural selection. Many artificially selected cultivars and locally evolved ecotypes of white clover (Trifolium repens), for example, occur throughout the world (e.g., Gustine et al. 2002).
Rapid evolution of ecotypes adapted to new conditions also occurs in aquatic plants. In 1984, a strain of a tropical marine green alga, Caulerpa taxifolia, is believed to have been released accidentally into the Mediterranean Sea from the Oceanographic Museum of Monaco (Meinesz 1999).This strain apparently arose through selection in European aquaria and is characterized by vigorous clonal growth, resistance to cold water, and lack of sexual reproduction.This strain has not only spread over thousands of hectares of shallow coastal waters along the northern coast of the Mediterranean Sea, but has appeared in coastal waters of southern California and coastal waters far south of its native range in eastern Australia. Caulerpa contains a series of toxins that provide defense against grazing animals and organisms that might attach to its surfaces (Ribera Siguan 1996).Thus, evolutionary change under human selection may create alien forms that are highly invasive in new environments.
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