An introduced plant species invading a new region either must possess sufficiently high levels of physiological tol erance and plasticity, or it must undergo genetic differentiation to achieve required levels of fitness. These options are not mutually exclusive. Phenotypic plasticity is important for many invasive species from many taxonomic groups and in diverse habitats. On aver age, invasive species have greater phenotypic plasticity than co occurring native species.
Evolution is another potential explanation for invasion success, because it can be rapid enough to be relevant over the timescales at which invasions occur. Invasive plants may evolve by genetic drift and inbreeding in founder popula tions, by intra and interspecific hybridization in the introduced range creating novel genotypes, and by drastic changes in selection regimes imposed by novel environ ments that may cause adaptive evolutionary change.
Hybridization can lead to adaptive evolution in a number of ways, including fixed heterozygosity via polyploidy. Hybridization has been shown an important mechanism of evolution of invasive species and many widespread and successful invaders are recently formed allopolyploid hybrids. Increased performance of hybrid taxa or genotypes has been documented for some genera (e.g., Carpobrotus in California and Fallopia in Central Europe).
The 'evolution of increased competitive ability' (EICA) hypothesis predicts that plants introduced into an environ ment that lacks their usual herbivores will experience selection favoring individuals that allocate less energy to defense and more to growth and reproduction. Many stu dies have found support for this, and some have not, but only a few studies have done a full test of the EICA hypothesis by addressing both growth and defense in the same species. Elegant evidence in support of the EICA hypothesis came from examining herbarium specimens of the alien Pastinaca sativa in North America over 152 years. There were phytochemical shifts toward increased toxicity coincident with the accidental introduction of a major her bivore from the alien plant's native range.
There is reasonable empirical evidence that genetic differentiation through rapid evolutionary change plays an important role in plant invasions. Nevertheless, available evidence suggests that some invaders are 'born' (released from fitness constraints), some are 'made' (they evolve invasiveness after colonization), and that the relative importance of ecological and evo lutionary forces is unique to each plant invasion episode.
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