The Geography of Phytochemical Races

3.2 North American Conifers 141
from the Texas mainland and Gulf Coast Islands and seven populations of the closely
related A. cumanensis Kunth. from near Vera Cruz, Mexico, where both species grow
sympatrically (Fig. 3.6). In addition to the sesquiterpene fractions, these workers
also examined the volatile terpenes of these plants. Numerical analysis of the combined
chemical data revealed a closer relationship of the island populations with the
population from Vera Cruz than with populations on the Texas mainland. The earlier
suggestion that the island was colonized from achenes originating from the mainland
has to be discarded in view of the strong support for the parental populations lying
farther south, possibly in the vicinity of Vera Cruz as suggested by the newer chemical
data. It is thought that the islands originated within the last 5000 years as a result
of post-Pleistocene sea level changes. Colonization of a newly formed island by
means of a single achene, or a few achenes from a localized source, would explain
the observed morphological homogeneity. The existence of populations on the
mainland that exhibit the dilactone chemistry characteristic of island plants can be
rationalized simply by transport of propagules to the mainland by prevailing winds
during the time of year when A. psilostachya fruits are available.
3.2 North American Conifers
The next series of examples involve conifers whose secondary chemistry has been of
major interest for many years. The resulting literature in this area, particularly with
regard to the application of terpenoids to taxonomic and evolutionary problems, is a
large and rich one. In the paragraphs that follow, only a representative sampling of
the available examples will be described. An important early work is Mirov’s (1967)
book The Genus Pinus, in which he summarized much of the available literature
on the application of turpentine composition to unraveling relationships among the
pines. Von Rudloff’s 1975 review is also a valuable source of information.
The use of volatile chemicals as systematic markers has the obvious advantage of
lending itself to quantifi cation through GLC. In many, if not most, of the cases discussed
below, qualitative differences in monoterpene profi les would not have been
suffi cient to allow distinctions to be made between taxa, or even between individuals
within a population. This is true because most conifers synthesize many of the
same monoterpenes, although often in vastly different relative concentrations. It is
these quantitative differences that have been constructively used in the following
examples. Structures of the terpenes commonly studied are presented in Fig. 3.7.
3.2.1 Abies (Pinaceae)
Abies—the true fi rs—comprise a moderately large genus, 49 species (Mabberley,
1997, p. 1) that include some of the most important commercial species among the
conifers, particularly in North America. Some of the more important North American