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Integrating Ecological and Evolutionary Theory of Biological Invasions 87
introduced range is by definition a novel habitat. Thus, introduced taxa of
hybrid origin may be able to invade areas that are unavailable to the parental
species. Second, we propose that hybridization might be expected to be more
frequent in the novel range, because the introduction process unites distinct
genotypes of a species or distinct species whose distributions do not overlap
in the native range.
In addition to leading to bottlenecks in population size, and potentially to
increased hybridization, introductions may be viewed as founder events.The
genetic consequences of founder events have long been a proposed mechanism
for rapid evolution, and even speciation (e.g., Mayr 1954; Barton and
Charlesworth 1984; Gavrilets and Hastings 1996; Regan et al. 2003). Founder
events may lead to “genetic revolutions” through breaking up what have been
called co-adapted gene complexes (Mayr 1954), and can increase additive
genetic variation in phenotypic traits (Goodnight 1987; Bryant and Meffert
1995). Such genetic revolutions poise the population for rapid response to a
new selective regime. Thus, founder events may be implicated in strong invasion,
as they lead to changes that do not occur in the native populations of an
invasive species. Lee (2002) proposed that the genetic architecture and the
amount of available additive genetic variance contribute to whether an introduced
species will become invasive, or not. While this is likely quite true, and
an important area of research that is much neglected, genetic architecture
alone may not be adequate to explain strong invasion. Rather, founder effects,
or other changes associated with introductions such as release from enemies
(e.g., EICA), must also be invoked for genetic architecture to contribute to
strong invasion.
6.3 Proposed Refinements to Hypotheses, Predictions
and Tests
There likely is room for improvements to all of the above hypotheses. Here, we
offer several suggestions for refining the enemy release and evolution of
increased competitive ability hypotheses, then move on to discuss synergies
among these hypotheses.
6.3.1 Refining the Enemy Release Hypothesis
Given the multiple scenarios for interactions between enemies, introduced
species and native species, it is important to evaluate critically our expectations
for the outcomes of such interactions. This should include differences in
predictions for generalist and specialist enemies (Müller-Schärer et al. 2004;
Joshi and Vrieling 2005). An analytical framework combining comparisons of