The Geography of Phytochemical Races

174 4 Intercontinental Disjunctions
Fig. 4.1 Compounds
299–302, fl avonoids from
Datisca
considered the ancestral type from phylogenetic analyses, was widely distributed
throughout the range of the species in pre-Pleistocene times. Climatic cooling
would have resulted in retreat of the species south of the Transverse Ranges but
with localized populations surviving north of the Transverse Ranges. The existence
of relictual populations in Monterey and Santa Cruz Counties would be consistent
with the existence of other relictual taxa in suitable habitats in that area (Raven and
Axelrod, 1978). A northern population that survived the climatic changes could
have become fi xed for unique chloroplast DNA mutations and given rise to the
present populations by range extension. The existence of the DG-N’ population in
Mendocino County attests to the potential for differentiation within the northern
form. The authors also pointed out that the differentiation of these two forms may
have occurred several million years ago and in fact have nothing to do with Pleistocene
climate extremes.
A comparison of the cpDNA data for D. cannabina and D. glomerata revealed a
sequence divergence of 0.87% ± 0.17%, which Liston et al. (1992) translated into a divergence
time of 8.7 million years (± 1.7 million years) placing the divergence in the
late Miocene. Sampling of D. cannabina was not suffi cient to allow further speculation
on the relationship between the two species. The authors noted in their concluding
remarks that this pair of species represents another system where genetic differentiation
has occurred with comparatively little accompanying morphological change.
4.1.2 Armeria maritima (Plumbaginaceae)
This example involves Armeria maritima (Mill.) Willd., which is distributed over
central and coastal Europe, Great Britain, and parts of North America. Attempts
to accommodate the amount of variation that characterizes this taxon are refl ected
in the existence of several infraspecifi c taxa, among which are subsp. maritima
native to the coasts of Europe from northern Spain to Norway, subsp. calaminaria
(Petri) Rothm. that appears principally in metalliferous surroundings, such as mine
tailings; subsp. sibirica (Tuscz ex Boiss.) Nyman that occurs in the Arctic and in
subarctic areas; and subsp. californica (Boiss.) Posild. Areas of occurrence of these
subspecies are not necessarily contiguous as indicated by specimens of subsp.