The Geography of Phytochemical Races

100 2 Examples Within Continents
Arizona yielded a series of pulchellins [209], with various arrangements of substituents
on the hydroxyl groups (Herz and Roy, 1969; Yoshioka et al., 1970). These
chemical data provide some support for taxonomic distinctions that have been made
within G. puclhella over the years. Thus, Turner and Whalen (1975) recognized the
Gulf Coast populations as a distinct taxon, G. pulchella var. picta (Sweet) A. Gray.
The population that occurs in Val Verde County (referred to as the Rio Grande
Collection), and is characterized by the presence of spathulin, falls within the area
occupied by G. pulchella var. pulchella. (But very close to an area in which var.
australis B. Turner and Whalen occurs.) Stoutamire (1977) found that the western
populations, those from New Mexico and Arizona, were easily distinguished
from their eastern counterparts by means of chromosome behavior and pollen stainability.
He considered that a distinct boundary exists between the New Mexico
and Texas populations. The fi nding of a series of eudesmanolides, the pulchellins,
uniquely in the western populations lends support to this view (Stoutamire, 1977).
The westernmost populations of var. pulchella have been recognized at one time or
another as a distinct species, G. neomexicana (Biddulph, 1944) or as a subspecies
of G. pulchella (Stoutamire, 1954). A recent examination of allozyme variation in
G. pulchella (Heywood and Levin, 1984) revealed little variation among the varieties.
Neither morphological nor chromosomal races were revealed by clustering of
population genetic distances. Genetic distances were signifi cantly correlated with
geographical distances “. . . suggesting that gene fl ow may be an important deterrent
to differentiation.”
2.7.10 Dudleya (Crassulaceae)
Dudleya consists of about 40 species that occur in California, Arizona, and Baja
California. A comparatively high degree of morphological variation along with a
tendency to hybridize in nature combines to make taxonomy diffi cult in the genus.
In an effort to fi nd additional characters by which species and species groups might
be distinguished, Manheim et al. (1979) examined some of the components of
the epicuticular wax of 19 species of Dudleya subgen. Dudleya (eight from Baja
California; 11 from California). Thin layer chromatography revealed two different
profi les, one representative of more northerly and/or montane sites, the other
centered in Baja California. The northern/montane species have taraxerone [210]
as the major triterpenoid, while plants from the southern sites are characterized
by ß-amyrin acetate [211] (see Fig. 2.63). Those workers reported a correlation
between triterpene type and corolla tube length where species with taraxerone have
shorter tubes (petals united for less than one-third of their length), while those with
ß-amyrin have corolla tubes fused for more than one-third of their length. They
concluded that the correlation between two seemingly unrelated features likely has
phylogenetic signifi cance.
By way of contrast, a study of the leaf cuticle alkanes of Sedum lanceolatum
Torrey (Crassulaceae) from 44 populations showed no correlation with elevation.