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C.R. Largiadèr
marily depends on the relative fitness of invaders and their immigration rate.
However, the model assumed a constant population size, and that the two
parental species produce (at their respective proportions) a sufficient number
offspring to account for the loss of hybrid individuals in each generation.
Thus, the accelerating effect in replacing the native species of hybridization
without introgression may strongly depend on the fecundity of the native
species, which may be further reduced indirectly by competition with hybrids
for limited habitat resources.
So far, empirical evidence for such cases has been quite rare in animals. I
also have not found a single well-documented case in plants, though there are
several examples of crosses of plant species yielding sterile hybrid progeny
(Ellstrand 1992). However, it may well be that many such cases have remained
unnoticed, since computer models predict that such replacements of local
populations may occur very rapidly within a few generations (Huxel 1999;
Wolf et al. 2001). Furthermore, if sterile hybrid individuals have reduced vigor
(e.g., die at very early life stages), then their presence may be overlooked. A
case where the latter could apply concerns the formerly widespread and now
threatened European mink (Mustela lutreola), whose populations have
declined almost everywhere throughout its range. Several hypotheses have
been put forward to explain the disappearance of this species. One important
cause seems competition with the American mink (M. vison), which was
introduced for fur farming or accidentally released in many parts of Europe in
the 1920s–1930s. Based on crossing experiments with the two species, which
showed that hybrid embryos are resorbed, it was suggested that, because the
breeding season of M. vison starts earlier and because M. vison males are
stronger than the European mink males, the M. vison males would mate with
the native M. lutreola females, thereby preventing M. lutreola reproduction
(Maran and Henttonen 1995).
Current genetic evidence for hybridization confined primarily to the F1-
generation involves freshwater fish species. In a first example, genetic data
collected over a period of 8 years from a stream in western Montana (northwestern
North America) indicated a rapid displacement of native bull trout
(Salvelinus confluentus) by introduced brook trout (S. fontinalis), with very
low introgression (Leary et al. 1993). Only two of 75 hybrids detected throughout
western Montana were not F1-individuals, and a comparison of the
genetic data of mtDNA and ten diagnostic nuclear markers showed that both
sexes of each species interbreed with the corresponding sex of the other.
A second, more recent example concerns two freshwater minnow species
(Pseudorasbora pumila and P. parva) in Japan (Konishi and Takata 2004). P.
parva native to western Japan has been accidentally introduced during the
transplantations of other cyprinid fish species into eastern Japan. Over the
last 30 years, P. parva has largely replaced P. pumila native to eastern Japan. In
the contact zone of the two species, only F1-hybrids showing exclusively
mtDNA haplotypes of P. pumila have been detected, even after following the