Biology Reference
In-Depth Information
5.
Hybridization and
Evolution of Exotics
“It is interesting to note that two Old World [
Rhamnus
] species
that have been brought together to a habitat on a different continent,
away from their place of origin, have hybridized naturally and probably
produced several generations of fertile hybrids. . . . The aggressive nature
of these new hybrids should be of concern to local stewards and naturalists.”
—G
IL
-
AD AND
R
EZNICEK
(1997)
In 1879, a new salt marsh cordgrass appeared along the coast of Hamp-
shire, England.To some, it seemed to be a luxuriant form of the native salt
marsh cordgrass,
Spartina maritima
. Others deemed it a new species and
named it
S. townsendii
. The new form spread slowly along the coast, but
in the late 1880s, a change occurred that initiated a rapid acceleration of
its spread. As it turned out, that change was a chromosome doubling that
created a plant that was eventually named
S. anglica
(Gray et al. 1991). Fur-
ther study revealed that
S. townsendii
is actually the sterile hybrid of
S. mar-
itima
and
S. alterniflora
,a North American salt marsh cordgrass that had
been introduced to the coast of England.These two cordgrasses are close
relatives with similar chromosome numbers (Baumel et al. 2002a). Chro-
mosome doubling restored the fertility of
S. anglica
and simultaneously
isolated it reproductively from its parental forms.
Thus, a new species arose through hybridization and chromosome
doubling. Furthermore,
S. anglica
proved to be highly competitive and
invasive, eventually colonizing estuary areas all around the British Isles, the
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