The Geography of Phytochemical Races

150 3 After the Ice
Zavarin et al. (1991) assumed that the same genetic mechanism was operational
in P. albicaulis and assigned the intervals of 0–8%, 8–56%, and 56–100% 3-carene
to the genotypes cc, Cc, and CC, respectively. Next, they compared the individual
genotype ratios with the theoretical ratio using the Hardy–Weinberg law and found
the values to be in agreement. Next, they plotted the occurrence of allele C against
latitude, which revealed that the highest values characterized the Californian populations.
Populations north of 42°N had frequencies of allele C in the vicinity of 70%,
with the value dropping to roughly 45% in populations north of ca. 45°N latitude.
Zavarin et al. (1991) also discussed the relationship between total monoterpene
variability (∑V) and latitude as it applied to the evolutionary history of P. albicaulis.
The least variable populations of P. albicaulis occurred among the Californian populations,
with increasing values of ∑V characterizing populations north of 42°N
latitude. The greatest variation in monoterpenes was found within the northernmost
populations (>48°N). The signifi cance of these values can be appreciated as they
apply to alternative explanations for the present-day distribution of the species. One
suggestion is that P. albicaulis evolved from an ancestor of subsect. Cembrae, all
members of which are Asian, either before the separation of North America and
Eurasia at the end of the Mesozoic, or after the ancestor had crossed the Bering
Land Bridge and become established in North America. This view is based upon
the characteristics that the four Asian species have in common, but are otherwise
rare in the genus (wingless seeds and closed cones at maturity). The alternative
view is that P. albicaulis evolved from an ancestor in subsect. Strobi, and that the
peculiar cone and seed features developed independently from the Asian taxa. The
level of chemical variation is not consistent with a recent arrival, but rather supports
the view that the species has been in western North America long enough for it to
accumulate considerable variation. Adding weight to this scenario is the fact that
the differences between coastal and Rocky Mountain forms of P. albicaulis are very
similar to differences observed in other conifers, that is, Pinus fl exilis, Pseudotsuga
menziesii, Abies concolor, A. lasiocarpa, and the bristlecone pines, all of which
have been explained by the disappearance of suitable habitats through the formation
of the arid Great Basin.
An explanation was also put forward by Zavarin et al. (1991) to account for
the low level of variation in the southern populations, which is contrary to what
might have been expected, had this area served as a refuge during periods of glacial
advance. It was suggested that the refuge existed a good deal to the north, possibly
in the Klamath area (northwestern California and southwestern Oregon), to which
reference has been made above for other taxa, and that populations lying to the
south are of more recent origin, and thus have had much less time to diversify.
Ponderosa pine, Pinus ponderosa Laws., is among the most widespread of
western conifers, with a range that extends from British Columbia along the coast
to southern California and along the Rocky Mountains as far south as Mexico.
It occurs at mid-elevations in drier habitats and often covers considerable acreage.
Three varieties have been recognized (Critchfi eld and Little, 1966), the typical
western var. ponderosa, the northeastern var. scopulorum Engelm., and the southern
var. arizonica (Engelm.) Shaw. Not unexpectedly, not all workers agree with this