The Geography of Phytochemical Races

3.2 North American Conifers 155
agreement with average values for 20 other conifer species (Hamrick et al., 1981):
78.6% polymorphic loci per population as opposed to 67%; 2.35 alleles per locus
compared to 2.29, and 0.327 for mean individual heterozygosities, compared to
0.207, the mean conifer value. Hiebert and Hamrick (1983) suggested that the latter
value was higher for the bristlecone populations primarily because of an even
distribution of allele frequencies at the polymorphic loci.
There are two possible explanations for the present distribution of P. longaeva
in the eastern Great Basin. In the fi rst, long-distance dispersal of a small number
of seeds via birds could have established the species’ presence in the area. In this
case, one would predict a relatively low level of intrapopulational variation, since
each population would have originated from a very limited sampling of the parental
genome and a relatively high level of interpopulational variation because the populations
would not, in all probability, have arisen from related parents. In the second
scenario, the present distribution of bristlecone pines refl ects fragments of a once
continuous distribution individual populations, of which became separated from
others through a series of climate changes, resulting in extinction of lower-elevation
stands associated with Pleistocene glacial advance and retreat. In this situation, one
would expect to see higher levels of intrapopulational and lower levels of interpopulational
variation. Since most of the observed genetic variation occurs within
populations, the second explanation for the present distribution seems the more
likely. This view is supported by paleo-ecological observations, including radiocarbon
dating of bristlecone pine needles, 10,000–40,000 years old, found in pack rat
middens at much lower elevations (1900 m) than those at which the trees exist at
the present time. It was also noted by the authors that, since glacial periods tend to
be longer than interglacials, it is likely that bristlecone pines spent a considerable
amount of time in large, continuous populations. The low level of genetic variation
observed by Hiebert and Hamrick (1983) suggests that bristlecone pine populations
in this area have not lived in isolation from one another for a long time.
Pinus torreyana C. Parry ex Carrière, the Torrey pine, occurs in southern
California and exhibits an interesting natural distribution consisting of two disjunct
populations, one on Santa Rosa Island, which lies about 50 km south of Santa
Barbara (Santa Barbara County), and one on the mainland immediately south of the
town of Del Mar on the northern edge of San Diego (San Diego County). They are
separated by about 280 km. The relationships and taxonomy of Torrey pine were
discussed in detail by Haller (1966, 1986), who described the two populations as
two subspecies, P. torreyana subsp. torreyana of the mainland, and P. torreyana
subsp. insularis Haller from the island. Features upon which the distinction was
made included crown shape, needle color, cone features, seed width, and seed color.
A further distinction between the two forms came from a study of the composition
of their turpentines (Zavarin et al., 1967). Table 3.2 summarizes their results. Two
qualitative differences can be noted; n-decylaldehyde (ten-carbon hydrocarbon aldehyde)
and cineole [41] were present in small amounts in the mainland material, but
were not seen in material from the island. Concentration differences were observed
with all components but most were not statistically signifi cant. Two quantitative differences
that were judged signifi cant by statistical tests involved limonene [24] and