Biology Reference
In-Depth Information
19.
Dispersing Aliens and Speciation
“Under the biological species concept, taxa that are fully reproductively iso-
lated are called separate species, and identifying the conditions under which
reproductive isolation will (or will not) evolve to completion is therefore cru-
cial to understanding speciation.”
—V
IA
(1999)
Three herbaceous composites of the genus
Tr agopogon
have been widely
introduced and naturalized in North America from Europe: salsify (
T. por-
rifolius
), yellow salsify (
T. dubius
), and yellow goat's beard (
T. pratensis
). In
eastern Washington State, these species became established, usually in
waste areas in towns, between 1916 and 1930. Marion Owenby, a botanist
at Washington State University (then known as State College of Washing-
ton), was aware of these plants and their hybrids, which comprised all
three pair combinations of the species.These hybrids were common, but
sterile. In 1949, however, he detected colonies of two forms that were
similar to hybrids but were larger, coarser, and produced fertile seeds
(Owenby 1950). One form, found in two colonies in Pullman and
Palouse,Washington, was similar to the hybrid of
dubius
and
porrifolius
.The
second, also found in two colonies in Moscow, Idaho, was similar to the
hybrid of
dubius
and
pratensis
. On examination, these two forms turned
out to be tetraploid in chromosome number (n = 12) rather than diploid
(n = 6) as in the naturalized species and their hybrids. Since these
tetraploid colonies were flourishing and not backcrossing with other
tragopogons, Owenby concluded that they were new, reproductively iso-
lated species, arising by the doubling of chromosome number in some of
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