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C.R. Largiadèr
than anadromous ones to introgression from genetically distinct lineages,
including some lineages that had been isolated for more than a million years.
By contrast, within major genetic lineages, anadromous populations appear to
be more susceptible to introgression. In general, there is a great variability
observed in the extent of introgression in different populations, ranging from
hardly detectable introgression of exotic genes following several decades of
intensive introductions (Largiadèr and Scholl 1996), to nearly complete displacement
of the native gene pools (Largiadèr and Scholl 1995).
Regarding the literature on anthropogenic introgressive hybridization in
mammals, evidence of gene flow between domesticated varieties and wild
conspecifics or closely related species seems to have particularly increased in
the last few years. This involves, for example, the European wildcat (Felis silvestris
silvestris) and domestic cats (F. s. catus; Beaumont et al. 2001; Lecis et
al. 2006), domestic ferrets (Mustela furo) and the European polecat (M. putorius;
Davison et al. 1999), wolves (Canis lupus) and domestic dogs (C. l. familiaris;
Randi and Lucchini 2002; Vilà et al. 2003), coyotes (C. latrans) and dogs
(Adams et al. 2003), bison (Bison bison) and domestic cattle (Bos taurus; Halbert
et al. 2005), and other bovine species (Nijman et al. 2003).
Although there is no systematic survey dealing specifically with maninduced
inter- and intraspecific introgressive hybridization in plant species to
assess the extent of these phenomena as a threat to native biological diversity,
plants still represent probably the best-studied group in this context.Vilà et al.
(2000) have recently compiled and reviewed a large number of human-mediated
plant hybridizations, which encompass a wide range of taxonomic
groups and the full range of potential outcomes, as described above for animal
species. Also, the gene flow between domesticated and wild plant species
has received increased attention since the advent of transgenics. A recent
review reports that 12 of the world’s 13 most important crop species hybridize
with wild relatives (Ellstrand et al. 2002), and substantial evidence has now
been compiled that at least 48 cultivated plant taxa hybridize with one or
more wild relatives somewhere in the world (Ellstrand 2003a, 2003b). Two further
reviews focused on the extinction of rare species through hybridization
with numerically superior invading species (Ellstrand and Elam 1993; Levin
et al. 1996).Yet, there are also cases of small invading populations threatening
larger populations of native species. For example, smooth cord grass
(Spartina alterniflora), which was introduced into the salt marshes of San
Francisco Bay in the mid-1970s, shows a much higher male fitness than does
the native California cord grass (S. foliosa). This fitness difference seems to
reverse the direction of gene flow, which would normally be expected to occur
from the more abundant to the rarer taxon, and thus may ultimately lead to
the extinction of the more widespread, abundant native species (S. foliosa)
through progressive introgression of genes of the rare invader (S. alterniflora)
into the native gene pool (Anttila et al. 1998).