of rarity-weighted richness for Heritage Network–listed species (Figure 7) are the greater San Francisco Bay Area, part of coastal and interior Southern California, Death Valley, Hawaii, the Florida Panhandle, and the southern Appalachians. The first two reflect large numbers of plants threatened by both natural rarity and urbanization. Plants comprise 113 of 135 imperiled species in the San Francisco Bay Area and 72 of 81 in the Southern California hotspot (Stein et al. 2000). The Death Valley hotspot comprises fish, snails, insects, and plants confined to one or a few desert springs. Appalachia’s imperiled species are mainly freshwater mussels, fish, and amphibians; Hawaii’s include many birds; and Florida’s include many reptiles and amphibians, although plants are also important in all three (Stein et al. 2000).
Within California, concentrations of rarity-weighted richness for nearly all groups of species (including mammals, birds, reptiles, and invertebrates) occur in the San Francisco and the Los Angeles–San Diego urban coastal regions (CDFW 2003). High levels of sampling effort and high degrees of endangerment may contribute to this pattern. For plants (Figure 8a), there are additional and presumably more natural hotspots in the Klamath-Siskiyous, Modoc Plateau, southeastern Sierra, Central Coast, and Mojave Desert. For amphibians (Figure 8b), the southern Sierra Nevada and Transverse Ranges stand out; and for fish (Figure 8c), the Modoc Plateau and Death Valley are especially rich in small-ranged species.
FIGURE 8a. Hotspots of rarity-weighted species richness in California. (a) For plants, particularly rich areas occur in the greater San Francisco Bay Area and western San Diego County, reflecting both ecological reality and human activity.
FIGURE 8b. For fish, important hotspots of restricted species include the Modoc region and the Mojave Desert.
FIGURE 8c. For amphibians, the southern Sierra Nevada is a hotspot of rare species diversity. (Source: California Natural Diversity Database; CDFW 2003. Copyright California Department of Fish and Wildlife.)
ORIGINS OF ENDEMIC SPECIES
Paleoendemism and Neoendemism
As pointed out by Stebbins and Major (1965), species may be restricted to a narrow geographic area for two general reasons: either they were once widespread and are now confined to a subset of their former ranges (paleoendemism), or they evolved recently and have had inadequate time to spread (neoendemism). (Some authors consider a third category, “holoendemics,” or species restricted by their habitat requirements.) Although humans have had enormous impacts on biological diversity, there are relatively few “anthropoendemics” whose small ranges are attributable to humans (Kruckeberg and Rabinowitz 1985).
Paleoendemics or relictual taxa may have fossil records far beyond their contemporary ranges, like ironwood (Lyonothamnus), found in western Nevada fossil beds but now confined to the Channel Islands. They often occur in discontinuous populations, like the coastal redwood (Sequoia sempervirens) and giant sequoia (Sequoiadendron giganteum), thought to be remnants of once-broader distributions. Usually their habitats are more benign than the surroundings, with higher-than-average summer rainfall and/or mild summer temperatures. Paleoendemics are also classically diagnosed by lacking close relatives nearby, since they represent old and shrinking lineages. The closest relatives of many Californian paleoendemics are found in eastern North America or eastern Asia (Stebbins and Major 1965; Raven and Axelrod 1978). By contrast, neoendemics belong to lineages undergoing recent speciation, so they may occur in complexes of closely related and adjacent species and may be poorly differentiated in morphology, genetics, and/or reproductive compatibility. Examples are discussed in Chapters 3 and 4.
The concepts of paleoendemism and neoendemism have also been applied to edaphic (soil) endemism. Paleoendemism is illustrated by species such as leather oak (Quercus durata), which is confined to serpentine soils but scattered across California and is believed to have once occurred widely on other soils but to have become restricted to serpentine through changes in the climate and/or competitive environment. Neoendemism is exemplified by species such as Layia discoidea, which appears to have arisen recently from a nonserpentine ancestor and to have been restricted to serpentine ever since it evolved (Baldwin 2005).
The distinction is not absolute, of course. Although Lyonothamnus is paleoendemic as a genus, its extant representative, L. floribundus, with its two distinctive subspecies, probably evolved recently on the Channel Islands (Erwin and Schorn 2000). The Streptanthus glandulosus complex appears to consist of a widespread species that became fragmented (similar to a paleoendemic), but some of its members have speciated recently as a result of their restriction to serpentine soil (Mayer et al. 1998). Although the prefixes paleo- and neo- imply differences in age, it should not be assumed that paleoendemic lineages are older unless (as is seldom true) this is actually tested.
It is usually assumed that neoendemism accounts for most of the wealth of endemism in California. It is certainly true that most of the studies of plant endemism in the state have focused on the evolutionary processes giving rise to new species in the region. As a background to the following chapters, the rest of this section briefly reviews modes of speciation by which new endemics evolve. The fundamental challenge is to understand how a single lineage can give rise to two (or more) descendant lineages that remain on separate evolutionary pathways rather than lose their integrity through gene flow. Geographic barriers, natural selection, hybridization, chromosomal rearrangements, and genetic architecture play roles that have been studied and debated for decades.
Geographic Speciation
One basic distinction is between geographic and ecological speciation, where the former implies a strong role for external barriers to gene flow as opposed to natural selection. In the classic model of gradual allopatric divergence, an ancestral lineage becomes subdivided by a new mountain range, water body, or similar obstacle. Sequential fragmentation may result in complexes of species with parallel patterns of genetic distance, morphological variation, and variable interfertility. Relatives that co-occur tend to have diverged longer ago than relatives that do not, suggesting that isolation promoted their initial divergence (Baldwin 2006). This is a classic and uncontroversial mode of speciation. It has been studied using the methods of biogeography, where the distributions of closely related taxa are interpreted in light of geologic and climatic events, and more recently using phylogeography, where the genetic patterns within lineages are similarly correlated to historical earth surface events. Biogeographic and phylogeographic evidence suggest that nearly all North American mountain chains are “suture zones,” that is, places where plant and animal lineages have diverged and sometimes come into secondary contact (Swenson and Howard 2007).
Ecological Speciation
Natural selection in response to new ecological opportunities is central to ecological speciation. The most spectacular examples are the adaptive radiations that sometimes occur on newly formed islands or in islandlike habitats (e.g., Price 2008). Speciation into new niches also takes place in habitats already full of species, but formidable obstacles make its success unlikely (Levin 2004). Ecological speciation begins with a small lineage colonizing a habitat in which it is ill adapted. It is more likely to go extinct than to continue evolving because of its small population size, its low genetic variability, and the reduction in its potential population growth implied by the existence of strong selective pressures. Also standing in the way of successful adaptation are gene flow from populations in the ancestral habitat and correlations between adaptive and nonadaptive genetic traits. Even if the fledgling species becomes well adapted to its new environment, it risks being genetically swamped by its progenitor species until intrinsic reproductive isolation evolves, which
