Biology Reference
In-Depth Information
the hybrid forms.This doubling made possible a normal process of meio-
sis and thus restored fertility to the offspring. He named the
dubius
×
por-
rifolius
form
T. mirus
(“remarkable”) and the
dubius
×
pratensis
form
miscel-
lus
(“mixed”).
In 1990, Novak et al. (1991b) surveyed the distribution of the
Tr ago-
pogon
species in Washington and Idaho and pulled together other infor-
mation that had accumulated since 1949. Both
T. mirus
and
T. miscellus
had
been subsequently discovered in northern Arizona and
T. miscellus
in
Montana and Wyoming. In eastern Washington and northern Idaho, how-
ever, they found that both new tetraploid species had spread considerably
and appeared in new localities within a five-county area.
T. miscellus
,in
fact, was more abundant than
T. porrifolius
and
T. pratensis
.The distribution
of both tetraploid species suggested that they had arisen on multiple occa-
sions. The number and widely scattered localities at which
T. mirus
was
found in particular suggested that the spread of this species was largely the
product of multiple origins, rather than long-distance seed dispersal.
Finally, Novak et al. (1991b) discovered hybrids between
T. mirus
and
T.
miscellus
, raising the possibility that a still different, octoploid species might
arise in the future.
Thus, the introduction of three European species to new environ-
ments in North America has enabled the origin of two new plant species,
with a potential for others.
Setting the Stage for Speciation
In the evolutionary long run, the introduction of species to new geo-
graphical areas, where they are isolated from interbreeding with their
ancestors, will frequently result in these separate ancestral forms becom-
ing differentiated into new species. At some future time, perhaps in thou-
sands or millions of years from now, the speciation pulse now beginning
might compensate for the extinctions that are presently resulting from
alien introductions.The new
Tr agopogon
species are an indication that this
process is beginning and that the bringing together of alien species in a
new environment can facilitate hybridization and the origin of reproduc-
tively isolated species by polyploidy. As we shall see, however, simply
introducing species to new abiotic and biotic environments can lead to
selection that ultimately results in speciation.
For flowering plants, the incidence of speciation by mechanisms
involving polyploidy is estimated to be between 2 and 4% (Otto and
Whitton 2000). Cases of rapid speciation, however, frequently involve
