Deep History and Biogeography of La Frontera - Design with the Desert: Conservation and Sustainable Development - page 122

the Axelrod model of landscape evolution is the most useful, although it is susceptible to
revisions in the timing of the uplift of the Sierra Madre Occidental.
6.3.2 Evolution of the Deserts
Although the modern North American climatic regimes and biotic provinces were
established in the Miocene Revolution, the deserts were not yet in existence. Based on a
series of fossil floras in California, Axelrod 14 inferred that the Sonoran Desert formed as
the result of a drying trend in the middle Miocene (15-8 mya), displacing thornscrub to
more southerly latitudes. Some of the species in the new desertscrub communities such as
brea ( Cercidium praecox ), guayacán ( Guaiacum coulteri ), tree ocotillo ( Fouquieria macdougalii ),
organpipe cactus ( Stenocereus thurberi ), and senita ( Lophocereus schottii ) were probably
segregated out of thornscrub. Bradley 25 demonstrated that the behavioral and physiological
adaptations of reptiles that allow them to thrive in hot, dry environments are not unique
to deserts and evolved in more mesic habitats. Other plants such as foothills paloverde
( Cercidium microphyllum ), ironwood ( Olneya tesota ), and saguaro may have evolved with the
Sonoran Desert. However, saguaro is not closely related to the columnar cacti of tropical
deciduous forest (etcho, Pachycereus pecten-aboriginum or saguira, Stenocereus montanus ) or
thornscrub (organpipe or senita). It was derived from Neobuxbaumea , a genus of columnar
cacti restricted to central Mexico south of the Sierra Madre Occidental.
Another important chapter in the history of the Sonoran Desert pertains to Baja California,
which was attached to the Mexican mainland. As the Gulf of California formed, a strip of
land stocked with tropical plants and animals drifted in splendid isolation northwestward
to meet California and form the Baja California Peninsula. 14,26,27 Natural selection shaped
them into many unique endemics including boojum tree or cirio ( Fouquieria columnaris ).
As with the mainland Sonoran Desert species, the biogeographical affinities of many Baja
California plant and animals are with central Mexico south of the Sierra Madre Occidental.
The nearest relatives of many Baja California reptiles are found today on the Pacific coast
of south-central Mexico in the Balsas Basin or Sierra Madre Sur. 27 The geologic history
of Baja California was reconstructed differently in each of the three papers cited earlier.
The initial rifting was completed by 10-12 mya with the formation of the proto-Gulf of
California. However, Grismer 27 argues that Baja California formed later as the modern
Gulf of California formed in the latest Miocene and that most of the evolution occurred
after 5.5 mya. At least some of the marine sediments in the Imperial Formation of southern
California appear to be older than this, suggesting greater age of the Gulf of California. 28
Thus, the Sonoran Desert was in existence by the late Miocene (5-8 mya). Despite the
absence of fossils, the regional geologic events that reshaped western North America likely
affected the Chihuahuan Desert as well. Aridity intensified on the Mexican Plateau due to
increasing rainshadow effects of the Sierra Madre Occidental to the west, the Sierra Madre
Oriental to the east, and the Rocky Mountains to the north. Cold had a greater role in the
evolution of the Chihuahuan Desert biota than in the Sonoran Desert as the Continental
Divide was uplifted, and there were no barriers to the icy “blue northers “(incursions of
Arctic air). Apparently, the deserts are perhaps the youngest biotic communities of North
America. The geologic events that shaped the landscape and altered regional climates were
mostly in the early-middle Miocene, suggesting that the speciation of many of the prominent
desertscrub plants occurred earlier in tropical deciduous forest or thornscrub. Subsequently,
changing climates and immigration rather than evolution were the major impacts on the
desert biota—dramatically shifting species ranges and community compositions.