The Geography of Phytochemical Races

138 3 After the Ice
No geographic structure was revealed by this analysis, with trees from Arch Cape,
Oregon, Whitefi sh, Montana, and sites from northern British Columbia, including
Queen Charlotte Islands, being closely associated. The authors remarked on the
lack of differentiation between coastal and interior populations. A reinvestigation of
red cedar from 55 sites (3–6 trees per site) provided a new data set that was analyzed
by numerical and discriminant-function analyses (von Rudloff et al., 1988). These
analyses confi rmed the low intra- and interpopulational variation seen in the earlier
study, but did reveal small differences between coastal and interior populations. No
correlations between northern and southern populations emerged from the analyses;
likewise, elevation had no effect on terpene composition.
Parallel with the study just described was an examination of genetic variation in red
cedar using enzyme electrophoresis (Yeh, 1988). Useful data from 2300 megagametophytic
samples from 230 trees in eight populations were obtained for 15 enzymes representing
19 loci. Sampling ranged between 48°49′ and 52°37′ N latitude and 116°40′
and 124°10′ W longitude. The number of alleles per locus averaged 1.17 and observed
heterozygosity ranged from 0.024 to 0.055 (average 0.036). The level of variability of
western red cedar approaches the lower limit for primarily outcrossing taxa (Hamrick
et al., 1979) and is in accord with the very low levels of variation reported from the
terpene analyses. Yeh (1988) suggested that the low levels of variation in red cedar, an
outcrossing species that would be expected to be more variable, could be accounted
for by comparatively rapid expansion following a bottleneck. He goes on to suggest
that divergence time since the bottleneck is in the neighborhood of 10,000 years. It is
known from the fossil record that red cedar did not reach the northern end of Vancouver
Island until about 3000 years ago. Estimates of the timing of red cedar migration
on the mainland range from 4900 to 10,000 years ago, which is consistent with the
value obtained from the electrophoretic data. Further insights require information on
effective population size and mutation rate per locus per generation (Yeh, 1988).
3.1.6 Liriodendron tulipifera (Magnoliaceae)
Liriodendron, tulip tree, consists of two species, the North American L. tulipifera L.,
which we will examine here, and the Asian L. chinense (Hensley) Sarg. The relationship
between the two species will be discussed later in this review. The North American
tulip tree occurs naturally throughout a wide range in eastern North America from
28°N to 43°N latitude, predominantly east of the Mississippi River. It is also commonly
planted as a decorative species in many parts of North America (it fares well in
Vancouver, B.C. at 49°13′N). In its natural setting, the tulip tree occurs in two forms,
an upland form that can occur in large populations in the northerly part of its range,
and an ecotype usually found in low, wet habitats along the coastal plains from North
Carolina to Louisiana (Parks and Wendel, 1990, and citations therein; Parks et al.,
1994). The latter report described an electrophoretic study of allozyme variation across
the range of the species, from which three forms emerged based on differences in