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[102]. According to Baker's hypothesis, the colonization ability came at the
cost of competitive ability, such that slower colonizing, but competitively
superior specialists would eventually displace the general colonizers. More
recently, the GPG has been used as a framework to explore the relative impor-
tance of phenotypic plasticity and contemporary evolution (see SIA, below) to
the successful spread of NIS into new habitats. Van Doninck et al. [103] found
that a widespread, asexual ostracod species
Darwinula stevensoni
tolerated
salinity and temperature gradients better than the sexual
Vestalenula
molopoensis
, which also has a more restricted range. Parker et al. [104] exam-
ined the relative importance of phenotypic plasticity and adaptation in an inva-
sive weed
Verbascum thapsus
in California. They found significant phenotyp-
ic differences between introduced populations, and phenotypic variance of
quantitative traits was largely partitioned among individuals from the same
family, suggesting that phenotypic differences are largely plastic, rather than
genetic. Genetic evidence suggests that a hybrid invasion by
Phragmites aus-
tralis
in North America is by a single chloroplast haplotype [105]. However,
the GPG hypothesis is not supported in genotypes of the aphid
Myzus persicae
[106]; contrary to predictions of the GPG hypothesis, the mean geometric fit-
ness of obligately parthenogenetic (i.e., asexual) genotypes on different host
plants was not significantly higher than that of cyclically parthenogenetic
genotypes (i.e., those that reproduce both sexually and asexually, respective-
ly). The importance of a GPG is still largely unexplored.
Selection for invasive ability (SIA) hypothesis
The idea that invaders rapidly adapt to novel environments, or that invasive-
ness is somehow selected by human activity is certainly not new. In particu-
lar, Baker was keenly interested in the effects of humans on plant evolution
[107, 108]. These hypotheses have so far received little empirical testing [95,
109]. However, Lee [110] has shown that multiple invasions of freshwater
habitats by the copepod
Eurytemora affinis
have occurred from brackish and
marine populations that have adapted to freshwater. Several other studies
have suggested a change in the mean phenotype of introduced, relative to
native populations [93, 111, 112]. However, it is important to consider the rel-
evance of such a finding to the evolution of 'invasiveness'. For example, an
'evolutionary' change could result from a simple founder effect, whereby
North American populations are established from a relatively small number
of individuals from a relatively restricted area in the native range. This genet-
ic bottleneck is likely to change the mean phenotype of introduced popula-
tions, particularly if populations remain relatively small for several genera-
tions [113]. Therefore, evolutionary change can result merely from genetic
drift, rather than from some sort of selection for invasiveness. Moreover, the
generality of the assertion that invaders are more vigorous in their native
ranges has recently come into question [114]. Regardless, rapid evolutionary
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