The Geography of Phytochemical Races

Chapter 7
Polar Disjunctions
This chapter deals with taxa whose ranges of occurrence are disjunct between the
Northern and Southern Hemispheres. Some of the examples involve taxa with
related members whose distributions fi t into other categories, some of which have
been discussed above. Chrysosplenium, for example, has a distribution that involves
disjunctions between Asia and North America, between eastern and western North
America, and between the Northern Hemisphere and the Southern Hemisphere.
Excellent general information on amphitropical relationships involving taxa
from the Pacifi c Coast of North America and South America appears in a symposium
on the subject published in the 1963 Quarterly Review of Biology (Raven,
1963) with contributions from Lincoln Constance on Apiaceae, Larry Heckard on
Hydrophyllaceae, Kenton Chambers and Robert Ornduff on genera in Asteraceae,
and Peter Raven on an overview of fl oristic relationships between North America
and South America.
7.1 Ambrosia chamissonis (Asteraceae)
Ambrosia chamissonis (Less.) Greene is a shrubby ragweed—often called beach
bur—whose range is characterized by a disjunct distribution between the Pacifi c
Coast of North America, extending from southwestern British Columbia to northern
Baja California (ca. 30°–50° N), and the coast of Chile (ca. 29° and 40° S). The
distribution in California is not uniform, however; there is a gap of several hundred
kilometers south of Monterey where suitable habitats are absent. It was fi rst
reported in Chile in 1892 on Isla de la Mocha near Valdivia (39°46′S, 73°15′W)
(Kohler, 1966; Kohler and Weisser, 1966; as cited by Payne et al., 1973). Following
its introduction, it has now spread several hundred kilometers northward to occur
in what constitutes the Mediterranean zone of South America. Several key reports
have appeared documenting the patterns of morphological and chemical variation
that characterize the taxon, making it one of the better-understood disjunctions
between North America and South America.
The morphological variation within the complex is refl ected in the number of taxonomic
descriptions (ca. 20) that have been published defi ning species, subspecies,
B.A. Bohm, The Geography of Phytochemical Races,
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