The Geography of Phytochemical Races

156 3 After the Ice
Table 3.2 Comparison of concentrations of selected components from Santa Rosa Island and
mainland (Del Mar) populations of Pinus torreyana turpentine (from Zavarin et al., 1967)
Compound Santa Rosa Del Mar
Limonene 73.4 (2.1) a 84.2 (2.2)
β-Phellandrene 8.7 (1.2) 0.1 (0.15)
Cineole nd 0.9 (0.5)
n-Decylaldehyde nd 0.4 (0.2)
a Percentage of total terpenes (mean deviation).
ß-phellandrene [130]. The ß-phellandrene difference amounts to a factor of nearly
90, which represents a signifi cantly different fl ow of carbon through that particular
part of the terpene biosynthetic pathway.
Land bridges, rafting islands, and long-distance dispersal have all been suggested
as means by which the present distribution became established. Geological
evidence suggests that the island separated from the mainland during the Miocene,
and that it has experienced rotation both northward and westward (Axelrod,
1980). Based on the amount and rate of genetic divergence between the mainland
and island populations, however, Ledig and Conkle (1983) tentatively concluded
that Torrey pine reached the island from the mainland more recently by “sweepstakes”
dispersal. Coupled with their genetic information, they cited the absence
of a Pleistocene land bridge (Junger and Johnson, 1980) and a disharmonic fauna
(Wenner and Johnson, 1980) as evidence for the “sweepstakes” hypothesis. Haller
(1986), however, pointed out that the fl ora of the larger Californian islands is neither
depauperate nor disharmonic relative to areas of comparable size and habitat variation
on the mainland. He also noted that P. torreyana is not alone as an endemic
on Santa Rosa; Quercus tomentella Engelm., Prunus lyonii (Eastw.) Sargent, and
Lyonothamnus fl oribundus A. Gray are also relic species that were well known on
the mainland during the Tertiary, and that they have not been known on the mainland
for more than 3 million years (see Haller for citations). Other possibilities
include a formerly much more extensive distribution of Torrey pine, including other
islands, and that both current populations are relics. After summarizing the various
sources of evidence, Haller (1986) concluded that the island population can be
explained perfectly well by means of either past land connections or by “normal”
dispersal (Axelrod, 1952). More recently, Waters and Schaal (1991) added valuable
information from their comparison of chloroplast DNA from plants collected at
the two sites. The two populations were monomorphic at over 150 restriction sites
(26 enzymes) and the chloroplast genomes were identical in length. Those authors
suggested that P. torreyana has been monomorphic since before the populations
became isolated. It is perhaps unnecessary to note that situations such as the Torrey
pine, which appear simple, are only deceptively so. Their resolution requires all of
the tools available to the biogeographer.
These examples of terpenoid analyses represent only a sampling of the information
available on pines. Several other studies involving North American and
Mexican pines are listed in Table 3.3.