The Geography of Phytochemical Races

136 3 After the Ice
The two species were sampled from throughout their respective ranges and
subjected to electrophoretic analysis. Twenty-eight populations of M. ferruginea s.l.
and 15 populations of M. pilosa were studied for 13 enzyme systems. Populations of
M. ferruginea subsp. ferruginea along the Pacifi c coast and Cascade Range are not
distinguishable from each other based upon allozymes, and are only slightly different
from populations of subsp. glabella in the Rocky Mountains. This lack of differentiating
allozyme features corresponds well with the high degree of morphological
similarity observed within this taxon (Wells, 1992), but differs from the high degree
of fl avonoid variation described above. What little differentiation there is likely
arose as a result of the isolation of the coastal/Cascades populations from the Rocky
Mountain populations as the Cascades were formed during the Pliocene. A major
result of this orogeny was the development of dry intermontane valleys that lacked
suitable habitats for Menziesia (Daubenmire, 1978). Changes in local climate during
the Pleistocene provided the opportunity for the two forms to reunite in areas,
such as the Columbia River Plateau, which at the present time is too dry to support
the species, except in a very few scattered localities. The allozyme data suggest a
divergence between the coastal/Cascades form and the Rocky Mountain form in
the range of 60,000–80,000 years ago. Morphological evidence suggests an area
of overlap in the vicinity of Mount Hood, Oregon (Hickman and Johnson, 1969;
Wells, 1992).
The most distinctive populations of M. ferruginea were those collected from the
northern limit of the species in Alaska. These populations exhibit the low levels
of intrapopulational genetic variation characteristic of recolonization from propagule
sources that existed in unglaciated refugia in central Alaska (Heusser, 1983).
In contrast, populations from the Alaskan panhandle have alleles characteristic of
populations located farther south along the coast and in the Cascades. Overall, however,
the panhandle populations are less variable than those from the more southerly
sites. It is generally held that the panhandle populations arose by recolonization
from refugia on offshore islands such as the Queen Charlotte Islands (Haida Gwaii)
(ca. 53°N, 132°W).
The genetic identity between the two western taxa is high (I = 0.98) suggesting
a comparatively recent divergence as commented upon above. More surprising
is the high degree of genetic identity between M. ferruginea s.l. and M. pilosa
(I = 0.92). This value points to contact between the two taxa as recently as the Pleistocene,
possibly 400,000–800,000 years ago, much more recent than was previously
thought. It seems reasonable to suggest that the progenitor of the eastern and
western species may have enjoyed a larger, and possibly continuous, distribution
across what are now the northern United States and adjacent Canadian provinces,
and that suitable habitats in the middle of the range were subsequently eliminated
by climatic changes. Intriguingly, there have been reports of Menziesia growing in
the vicinity of Duluth, Minnesota (Gleason, 1952; Rosendahl, 1963). However, the
absence of herbarium records substantiating this claim (Rosendahl, 1963) puts this
suggestion in doubt. Were this claim substantiated, however, suitable habitats for
Menziesia could well have existed in late glacial Picea-Larix forests that occupied
the area (Watts, 1983).