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C.R. Largiadèr
connections between the two river systems have provided the opportunity for
hybridization between long-separated groups, leading to the emergence of a
new, adaptationally distinct sculpin lineage.
16.5 Can we Predict Introgressive Hybridization
and its Outcome?
16.5.1 Genetic Differentiation Between Taxa as an Indicator
In the case of biological invasions, it would be useful for managers to be able
to predict whether an invader will hybridize with a local relative, and if so, to
what extent. A recent review on experimental hybridization (Edmands 2002)
clearly showed that, although pre- and postzygotic isolation are roughly correlated
with divergence time, there is tremendous variation in divergence
time, resulting in variable hybrid vigor, outbreeding depression, or only partial
reproductive compatibility within and among different taxonomic
groups. Thus, the extent of variation observed in this relationship does not
allow one to predict the consequences of a specific encounter of invading and
native taxa based on their genetic divergence.
16.5.2 Habitat Modifications
As can be easily deduced from the examples given in this text, anthropogenic
disturbance of local habitats is a major factor promoting hybridization, and
also affecting the outcome of hybridization. Rhymer and Simberloff (1996)
discuss in detail three forms of habitat modifications.
1. The first form is local habitat modification, leading to a mixing of previously
distinct gene pools. There are indeed many examples, in plants and
in animals, reporting that introgressive hybridization occurs at higher frequency
in disturbed or artificial habitats than in undisturbed natural sites
(e.g., Bleeker and Hurka 2001; Riley et al. 2003).
2. As a second form, they defined regional habitat change promoting geographic
range expansion of one taxon into the range of another, which
would provide an opportunity for hybridization. One notable example is
the genetic introgression between mallards and black ducks described
above.
3. A third type of habitat modification promoting hybridization is the construction
of permanent migration corridors between the ranges of
allopatric taxa. For example, this has presumably led to the emergence of
the new invasive hybrid Cottus taxon described above.