to the habitat. Hybrid origins and allopolyploidy help overcome these obstacles by creating immediate reproductive barriers between a newly adapted species and its ancestors; however, the new species may be ecologically outcompeted by its relatives unless it inhabits a novel niche (Levin 2004). Other pathways to reproductive isolation include strong selection against hybrids (Kay et al. 2011), selection that incidentally favors differences in reproductive traits (e.g., flowering phenology, floral morphology), and linkage or epistatic interactions between genes involved in adaptation and genes that confer reproductive isolation (Wu et al. 2007).
Progenitor-Derivative Speciation
Another basic distinction is whether speciation results in sister taxa with roughly equal initial population sizes, geographic ranges, and genetic diversity, as in classic gradual allopatric divergence, or whether it involves a small population budding off within the range of a widely distributed species. The latter case, called peripheral isolate formation or progenitor-derivative speciation, leads to a localized neoendemic species that is phylogenetically embedded in its ancestral species. The ancestor then becomes paraphyletic; that is, it does not include all descendants of a single common ancestor. Progenitor-derivative speciation appears common in plants (Grant 1981; Gottlieb 2003; Baldwin 2006). It is related to the classic idea of “catastrophic speciation,” in which a sudden event causes a population decline in a widespread species, allowing a random chromosomal arrangement to become rapidly fixed in a small population that evolves into the derivative species. Chromosomal alterations, novel habitats, breeding system changes, and adaptive morphological differences may contribute to reproductive isolation between progenitor and derivative species. Changes appear to be moderate and to involve a small number of genes, and overall genetic similarity between progenitor and descendant remains high (Gottlieb 2003).
Hybrid and Polyploid Speciation
The origin of new species through hybridization and/or changes in chromosome number is sometimes detected in animals but is central to evolution in plants. In his classic Plant Speciation (1981), Grant argued that patterns of relatedness among plant lineages are so often network-like, due to both modern and ancient hybridization events, that the standard view of evolution as a branching “tree of life” may not apply. Hybridization generates adaptive potential because it increases genome size and allows various duplicate genes to be turned on or off in hybrid progeny. Hybridization between species with different chromosome numbers or structures initially produces progeny of low fertility because of chromosomal incompatibility, but chromosomal doubling or other rearrangements may restore fertility, in addition to causing reproductive isolation between hybrid and parental lineages. The resulting allopolyploid hybrids are thought to be particularly capable of rapid evolution because they have the full chromosomal complement of both parents. Hybridization between parental species with the same chromosome numbers and structure 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
