Achillea millefolium, yarrow (photo copyright S. Shamoff). 
between subspecies as wide-ranging geographical races, and va¬ 
rieties as localized, ecologically differentiated populations. In ex¬ 
isting floras of Oregon, it is usual to find subspecies and varieties 
used almost interchangeably; that is, the same population group¬ 
ings that one author classifies as varieties are, by a different au¬ 
thor, treated as subspecies. No simple cure to this problem exists 
at present; it is something we have learned to live with despite 
the apparent inconsistency. Therefore, in composing this Check¬ 
list, our practice was to consider each genus individually and to 
use whichever rank was accepted by the monographer or special¬ 
ist whose treatment we chose to follow. In the genus Erigeron, 
for example, where we mostly used the publications of Arthur 
Cronquist to characterize the Oregon taxa, only the rank of vari¬ 
ety is accepted in the Checklist. However, in the genus Antennaria 
we utilize subspecies only, as was done in the detailed mono¬ 
graphic studies by R. J. Bayer and G. L. Stebbins. In the genera 
Aster and Artemisia, both subspecies and varieties are mentioned, 
but the categories are conceptually nearly identical; we simply 
wanted to avoid having to create new combinations for the sake 
of uniformity. 
In the Asteraceae, as in many other families of vascular plants, 
species often are not rigid, unvarying entities but rather are ge¬ 
netically and evolutionarily flexible and open to change. Their 
flexibility arises mainly from hybridization; that is, genetic ex¬ 
change between morphologically or ecologically differentiated 
populations. At one extreme, hybrid plants may be found which 
Chaenactisdouglasii, pincushion (photo copyright S. Shamoff). 
are intermediate between otherwise well defined species. In the 
Checklist, we comment upon some examples of this phenom¬ 
enon and assign names indicative of the plant's hybrid origin. 
Grindelia integrifolia X G. nana var. nana is one such case, with 
plants of this parentage occurring at numerous sites in the 
Willamette Valley. Some interspecific hybrids are designated by 
a species epithet preceded by an "X" as in Wyethia Xcusickii, the 
hybrid between W helianthoides and W. amplexicaulis. 
A much more common form of hybridization is the intergra¬ 
dation one sees between subspecies of a single species. In fact, 
the existence of morphologically (hence, genetically) intermedi¬ 
ate populations connecting one extreme form with another is 
often the justification we have for placing the extremes in a single 
species but named as different subspecies or varieties. In the 
Checklist, we sometimes remark that intergradation between races 
of a species is so extensive that it is impractical to describe sub¬ 
species or varieties; that is, the variation tends to be continuous 
rather than bimodal. Excellent examples of this pattern are seen 
in the common yarrow, Achillea millefolium, and in Scolder's 
hawkweed, Hieracium scouleri. 
A third kind of hybrid evolution to be mentioned is the 
development of polyploid complexes, of which the Asteraceae 
contain many notable examples. The evolution of species in such 
groups proceeds by the combining of whole chromosome sets 
derived from well differentiated ancestral species, followed by 
the stabilization of derived hybrid species through chromosome 
Kalmiopsis Volume 7, 2001 
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