Biology Reference
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northernmost locality in southern Peru. The gap between desert areas
in North and South America at this time may have been as short as
2,000 km.
The oldest fossil record of L. tridentata in North America is 18,700 yr
BP, based on material from a fossil packrat midden near the head of the
Gulf of California (Van Devender 1990). This date, together with specu-
lation about Pleistocene glacial climate changes, has led some workers to
speculate that the ancestor of L. tridentata arrived in North America
within the last 1.5 million yr (Hunter et al. 2001).
Fingerprinting analysis of chloroplast and ribosomal DNA from the
five species of Larrea and members of related South American genera,
however, suggested an earlier divergence of L. divaricata and L. tridentata
(Lia et al. 2001). This analysis suggested that these two species diverged
between 8.4 and 4.2 million yr ago, the most likely factor initiating diver-
gence being the dispersal of ancestral L. tridentata into North America.
Whether this dispersal occurred by a single long-distance transport of
seeds or by the stepping-stone occupation of dry habitats in northern
South America and Central America is still uncertain.
In North America, L. tridentata has evolved three races differing in
chromosome number (Hunter et al. 2001). Plants of the Chihuahuan
Desert are diploid, with a chromosome number identical to that of L.
divaricata in Argentina. Plants of the Sonoran Desert are tetraploid, and
those of the Mojave Desert hexaploid.These races follow a climatic gra-
dient of increasing summer heat and aridity from southeast to northwest.
Although the ploidy races are morphologically indistinguishable, it is pre-
sumed that the ploidy levels correspond to some physiological adaptation
to this climatic gradient. Thus, following invasion of North America, L.
tridentata has undergone substantial evolution.
In addition, L. tridentata has established coevolved relationships with
many insects. Some 30 species of insects of 5 orders are associated with
North American creosote bush (Schultz et al. 1977). Many of these are
restricted to creosote bush, suggesting a long period of coevolution. The
biochemical distinctiveness of creosote bush suggests that herbivorous
insects in North America were not likely preadapted to use this plant
immediately after its arrival, and that this fauna has been accumulated
gradually over several million years.
The ability to determine the degree of genetic differentiation among
species and relate differences to evolutionary time by means of molecular
clock relationships gives us an improved ability to understand patterns of
invasion and adaptation. This relationship, coupled with information on
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