Daniel Axelrod’s Madro-Tertiary laurels would prove prickly, however. Theoretical fruition always bears the seeds of doubt. G. Ledyard Stebbins’s later thinking on desert origins seems an example of this. In his 1952 paper, “Aridity as a Stimulus to Plant Evolution,” he had written that genetic change and natural selection would speed up in both semiarid and arid regions. This had supported Axelrod’s idea of regional desert as a fast-evolving, recent phenomenon. In a book on flowering plants published in 1974, however, Stebbins had changed his mind about desert as an evolutionary frontier.
He had maintained in his 1952 paper that slowly evolving populations “nearly all existed in environments which have remained relatively constant and continuously favorable,” like “the great forest belts.” But the idea that desert is an unfavorable habitat is an anthropomorphic one. Desert is unfavorable to humans, but not to organisms that are adapted to it. By this logic, it is not the “favorable” habitat that causes slow evolution, but the “constant” one. Finding a much higher number of species per genus in the shifting patchwork of West Coast semiarid grassland and woodland than in true desert, he surmised that evolution accelerates mainly in the former, whereas desert organisms evolve more slowly, as in other “extreme habitats” such as ancient forest. In his 1974 book, Stebbins had come to regard desert not as an evolutionary frontier but as an evolutionary museum where species accumulate like specimens preserved in the dry air.
Stebbins did not present his “museum desert” scenario in opposition to Axelrod’s recent desert paradigm. He illustrated it with “museum specimens” that may be fairly recent. As examples of well-adapted, stable desert bushes whose ancestors probably had evolved quickly in “intermediate” habitats, he cited “two of the principal groups of shrubby species found in arid regions of the Northern Hemisphere . . . the genus Artemisia, the sagebrushes, and the various shrubby genera of the Chenopodiaceae, such as Atriplex [shadscale], Eurotia [mule fat], Grayia [hopsage], and Sarcobatus [greasewood].” Noting that these shrubs have many herb relatives like mugwort and goosefoot, he postulated that they had herb ancestors in the early Tertiary:
At that time, the prevailing climate throughout the Northern Hemisphere was mesic, with ample rainfall throughout the year, so that continuous forests existed across most of North America and Eurasia. Nevertheless, there were probably “ecological islands” of more xeric climate, resulting from the rain shadows formed by isolated mountain ranges, or from other unusual combinations of ecological conditions. As the overall climate became more arid during the Tertiary Period, these xeric islands increased in size, and new ecological islands probably appeared.
The forest trees and mesic shrubs that formed the “sea” of forest surrounding these islands were already too specialized and rigid in their requirements to be able to give rise to xeric shrubs optimally adapted to the conditions on them. . . . Given this restriction, and the ease of seed transport and establishment that both now and formerly has been characteristic of families such as the Compositae and the Chenopodiaceae, one can imagine without much difficulty that the evolution of xeric shrubs from semixeric herbs belonging to these families could take place more quickly and easily than could the evolution of xeromorphic adaptations in trees and shrubs belonging to genera such as Acer [maple], Alnus [alder], Betula [birch] . . . and other inhabitants of the surrounding forests.
Stebbins’s “museum specimen” examples may be atypically recent for desert bushes, however. Sagebrush (Artemisia tridentata) is not a “desert relict” like those he had cited in his 1965 article with Jack Major. It is not an isolated species with few relatives, but a widespread one with many relatives. The possibility that sagebrush evolved rapidly as climate dried since the Miocene epoch doesn’t address the question of how desert relicts like ocotillo originated.
Stebbins’s 1974 book seemed tacitly to acknowledge this. It reiterated his and Major’s idea that relicts originated from large “radiating complexes” that had occupied “intermediate semiarid or open regions” but had “become extinct there, having succumbed to competition from recently evolved more successful and dominant groups.” Surviving only in deserts, “where the rate of speciation is rather low,” the isolated relicts “persisted long after their ancestors have become extinct.” They didn’t evolve quickly from Miocene epoch Arcto-Tertiary woodland herbs as Stebbins thought sagebrush might have. Their abundance and diversity in North American desert imply a much longer past—a much older desert.
There were other, less tentative seeds of doubt. Axelrod’s paradigm was more ascendant with life scientists than earth scientists, some of whom had long questioned whether West Coast mountains and rain shadows are as recent as generally believed. “Some scientists have tried to deduce past climates and topography from fossil evidence,” wrote Jeffrey P. Schaffer, a geographer at Napa Valley College and author of many books on California geology and wilderness areas. “However, there can be problems with this method because it assumes that past climatic gradients were identical to current ones, that climatic tolerances of plants have not changed with time, that species migrate together, and that plant distributions respond to average environments more than to infrequent, extreme conditions.”
Schaffer implied that the Madro-Tertiary paradigm’s geological critics balanced or outweighed its biological supporters: “Daniel Axelrod in 1957, 1959, 1966, and 1980, and with William Ting in 1960, claimed to have seen changes in the composition of Nevada’s Neogene paleofloras (plants of the Miocene and Pliocene epochs) that imply the creation of a rain shadow, presumably due to Sierran uplift. Mark Christensen, Herbert Meyer, and Jack Wolfe have addressed serious problems with Axelrod’s data, methodology, and paleoaltitude reconstruction assumptions.” And although U.S. biologists largely embraced the Madro-Tertiary paradigm, “earth-old desert” persisted south of the border like a tough if frost-sensitive plant. “Desert landscapes are the most ancient ones in our planet,” wrote two Mexican scientists in 1992. “For instance, imprints of xerophilous plants are known from deposits of the upper Paleozoic (Markov, 1965). The formation of modern deserts began in the upper Cretaceous and continued at different places in the Paleocene.”
The previous year, Jerzy Rzedowski had reiterated his objections to Axelrod’s paradigm. He’d noted that desert plant fossils continued to be much more elusive than other kinds, and that there was “little hope that such fossils have been preserved more than sporadically.”
Thus the age of the native flora of arid regions is particularly hard to estimate. Axelrod (1979) asserts that the “Sonoran Desert” of northwest Mexico and the southwestern U.S. has existed about as it is since the Pleistocene, indicating that its vegetation has evolved from enclaves of dry climate that have come into existence since the early Miocene or perhaps since the late Eocene. Nevertheless, there are no substantial reasons to assume that arid climate is such a recent phenomenon in the latitudes of Mexico, and its highly diversified xerophilic flora is more suggestive of a prolonged period of evolution, perhaps initiated in the Cretaceous itself. In support of this idea are discoveries of fossil remains of Prosopis [mesquite], of Vauquelinia [Arizona rosewood], and of Agave in the Eocene of the western United States, as well as of Fouquieria [ocotillo], Pachycormus [elephant tree], and Condalia [abrojo] in the Miocene of the same region [Axelrod 1979].
In addition, various authors (Axelrod, 1950, Raven, 1963, Wells and Hunziker, 1976) insist that Larrea [creosote bush] arrived in Mexico from South America in the Quaternary [ice age] or perhaps a little earlier. The southern origin of this important element of the xerophile vegetation of North America is probable, but there is no convincing evidence of its time of arrival. There exist, at the same time, clear indications that plant interchange between zones of North and South America, at least, began in older times.
Rzedowski again pointed to Mexico’s wealth of endemic plants as suggestive of ancient ecosystems:
This phenomenon is particularly spectacular in arid and semiarid zones, where endemism often pertains not only to high level taxonomic groups but also to biological forms. . . . Thus, for example, the cactus family, although originating in South America, has attained here its maximum diversity, abundance, and importance, containing around 900 species, of which more than 95 percent are restricted to Megamexico 1 [Rzedowski’s term for northern Mexico and adjacent, ecologically similar parts of the United States].
The family Fouquieriaceae [the ocotillo family], equally endemic to Megamexico 1 and in all probability originating here, is distinguished by the biological forms of its representatives, which are strange even among the xerophytes. No less suggestive are the variations offered by the species of Agave, a genus not limited to Mexico but which has its greatest taxonomic and morphologic diversity, and probably originated, here. A similar situation applies with Yucca, equally with Dasylirion [a yuccalike genus], Nolina [another yuccalike genus], Krameria [one of the relict families consisting of one genus], and various other genera.
Rzedowski’s 1991 remarks did not contain much new information, although they still posed a respectable challenge to Axelrod’s paradigm. But a few years earlier, oddly enough, the Alta California desert’s most spectacular endemic plant had surprised U.S. scientists by failing to support some of his paradigm’s assumptions about it.