Plant and Animal Endemism in California. Susan Harrison
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produces hybrids with the same chromosome number as the parents. These homoploids face lower initial barriers to fertility than allopolyploid hybrids but are more likely to be genetically swamped by the parental lineage. For reproductive isolation to develop in homoploid hybrids, within-chromosome rearrangements and geographic or ecological isolation may be necessary (Rieseberg 2006). Both allopolyploid and homoploid hybrids may exceed the parental species in the values of quantitative traits, and such “transgressive hybridization” may facilitate the invasion of new niche space (Grant 1981; Rieseberg 2006). Autopolyploids are products of within-species gene duplication rather than hybridization. Like allopolyploids, they enjoy the adaptive benefits of larger genome sizes (Soltis et al. 2004).
Recent molecular studies have confirmed the conclusions of classic authors that many plant lineages are of polyploid origin. Individual polyploid “species” may arise multiple times. Autopolyploidy and genome-wide duplication events are more common relative to allopolyploidy than was once believed. Rapid chromosomal rearrangements, genomic downsizing, and changes in gene expression following polyploid origins are beginning to be studied, as are the relationships of these genomic changes to pollination, reproductive biology, and ecological traits. A growing number of comparative studies illustrate the potential for polyploid hybrids to have new, broader, or more finely partitioned niches than their ancestors. Polyploid lineages may diverge ecologically or reproductively through the loss or silencing of alternative duplicate genes, contributing to their evolutionary dynamism (Soltis et al. 2004).
Hybridization and polyploidy have been credited with important roles in the rapid evolution of Californian flora (Stebbins and Major 1965). A common pattern in the California flora is for close relatives to hybridize intermittently, perhaps in certain zones, yet to remain distinct over their core geographic distributions. This pattern, seen in Arctostaphylos, Chorizanthe, Eriogonum, Monardella, and other taxa, is informally known to botanists as the “California pattern” (Skinner et al. 1995).
TRAITS OF ENDEMIC SPECIES
Rare species are sometimes found to have lower genetic variability, higher rates of selfing, lower reproductive investment, poorer dispersal, higher susceptibility to natural enemies, or less competitive ability than common ones (Kruckeberg and Rabinowitz 1985; Lavergne et al. 2004). These traits are interpreted as factors that may cause rarity, that is, prevent species from achieving higher range sizes or abundances. Other studies find that rare species inhabit less competitive (e.g., rockier) or more benign (e.g., rainier, less seasonal, less fragmented) environments than their common relatives (Lavergne et al. 2004; Harrison et al. 2008); such extrinsic differences may be interpreted as factors that have helped species persist, given that they are rare for other reasons. Finally, some traits such as higher inbreeding and lower genetic variability could be interpreted as either causes or consequences of rarity. Not many strong generalizations have arisen from the literature on the biology of rarity, and the usual conclusion is that rarity is too complex to have a single cause.
To understand high endemism within a geographic region such as California, it would be interesting to ask whether endemics have any consistent attributes, either intrinsic or environmental, that explain their diversity relative to other taxa and other regions. For example, in the Cape flora of South Africa, it has been proposed that plant adaptations to nutrient-poor soils, including fine, oil-rich, flammable foliage and ant-dispersed seeds, produce high rates of speciation by conferring short generation times and low dispersal, thereby leading to exceptional diversity (Cowling et al. 1996). In California, Wells (1969) proposed that the diversification of Arctostaphylos and Ceanothus is linked to the loss of resprouting in these two shrub genera, which regenerate by seeds after fire and thus have shorter generation times than their resprouting relatives. The annual life form is a widespread adaptation (or preadaptation) to California’s summer drought that may similarly facilitate rapid speciation (Raven and Axelrod 1978). There are now formal phylogenetic methods for testing the relationship between such possible “key innovations” and rates of diversification, but they have not yet been applied to California endemism. However, specialization to serpentine in 23 genera of California plants was shown to be the opposite of a key innovation; when lineages become endemic to serpentine, their diversification rates decline (Anacker et al. 2011).
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Endemism is biologically interesting as long as the geographic unit being studied is meaningful in terms of processes that create and maintain diversity. Californian endemism is best studied at the scale of the California Floristic Province, a natural biotic unit defined by a flora with a shared evolutionary history, but this book also considers state-level endemism due to data constraints. Taxonomic scale also influences the study of endemism. This book focuses on full species because some groups of organisms are much more extensively split than others into units below the species level, which by definition have higher levels of endemism.
Species richness is highest in parts of the world where water and solar energy are abundant. The richness of endemic species is also high on islands and in historically stable climates. The mediterranean regions of the world are unusual in being rich in plant endemism, yet not as obviously rich in animal endemism, whereas other global hotspots (nearly all of which are tropical) do not show this disparity.
Endemics arise through the shrinkage of widespread geographic ranges (paleoendemism) and the evolution of new species (neoendemism). Pathways to neoendemism include allopatric divergence, ecological speciation, peripheral isolate formation, and hybridization, all of which are well known in the Californian flora. There is not yet a predictive understanding of the traits that make lineages likely to diversify in particular environments such as California’s.
2
A Brief History of California
Plant and animal diversity in California are clearly linked to the rich complexity of the contemporary landscape, including its rugged topography, (see Figure 1), many climatic zones, varied geologic substrates, and resulting tapestry of vegetation types. This ecological variety has been well described in many places; see, for example, Barbour et al., Terrestrial Vegetation of California (2007), for plant communities and vegetation; and CDFW, Atlas of the Biodiversity of California (2003), for a pictorial overview of plant and animal diversity in relation to the landscape. This chapter presents a brief overview of how California’s ecological landscape evolved (for other accounts, see Edwards 2004; Minnich 2007; Millar 2012). The monograph by P. H. Raven and D. I. Axelrod, The Origins and Relationships of the Californian Flora (1978), is central to this discussion. Raven and Axelrod’s account is exceptional in its sweeping, yet detailed view of the history of a regional flora. Although it has been critiqued on various counts, it has not yet been replaced. This chapter uses the Raven and Axelrod story as an essential starting point. Chapter 3 reconsiders the classic story in light of more recent ideas and evidence.
GEOLOGIC HISTORY
During much of evolutionary history, ocean existed where California is today, and the coastline has gradually grown westward through a series of plate tectonic events (Figure 9; Table 2). Just over 200 million years ago (Ma), the supercontinent Pangaea broke up and North America began colliding at its western edge with smaller oceanic plates known as terranes, causing subduction (movement of one plate beneath another) and accretion (addition of one plate to another). By 140 million years ago, the edge of the continent had reached the present-day location of the Sierra Nevada and its western foothills. Then the same processes shifted farther westward and built the Coast Ranges. Subduction ceased around 28 million years ago when the zone where it was occurring collided with the East Pacific Rise (the midocean trench or spreading center). The subduction zone gave way to a mostly horizontally moving plate boundary fault, namely, the complex of northwest-to-southeast faults known as the San Andreas system, along which the motion is relatively northwest on the west side and southeast on the east side. This change in plate motion ultimately led to the rise of the Coast, Peninsular, and Transverse Ranges.
FIGURE 9. Paleogeography of California in (a) mid-Eocene, 50 Ma; (b) Oligocene, 35 Ma; (c) mid-Miocene, 20 Ma; and (d) late Miocene, 10 Ma. (Source: Ron Blakey,