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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, Colorado Plateau Geosystems)

TABLE 2THE GEOLOGIC TIME SCALE (IN MILLIONS OF YEARS)

The Sierras were first uplifted beginning about 80 million years ago and subsequently eroded into an undulating plain that rose gradually 50 million years ago to Tibet-like heights in east-central Nevada. The present Sierra Nevada includes remnants of this old surface as well as younger granitic rocks that intruded as subduction occurred in the Coast Ranges. A second period of more rapid uplift of the Sierra Nevada began around 3 million years ago. The Klamaths either represent the northern end of the Sierra offset westward by a fault 130 million years ago or are the remains of oceanic terranes lying west of the northern continuation of the Sierra Nevada. During much of the Eocene, 56 to 34 million years ago, the Klamath region was an island with a stable land surface that eroded in a tropical climate, and much of this land surface still exists. The uplift of the Coast Ranges began in Southern California around 30 million years ago and migrated northward with the north end of the San Andreas fault and the Mendocino Triple Junction, where it currently continues. The Coast Ranges began as an offshore submerged subduction complex but were fully joined to the continent by 5 million years ago. The east-west Transverse Ranges arose at around that time because of a bend in the San Andreas fault that resulted in compression during the northward movement of the Pacific plate along the fault.

The Central Valley is one of the largest and flattest valleys in the world. It is believed to have been created when a large slab of oceanic plate was thrust over the North American continental margin during the collison of North America with a west-dipping subduction zone. A new east-dipping subduction zone formed west of this slab in the modern Coast Ranges, leaving a broad gap in between. It lay beneath ocean until about 5 to 15 million years ago depending on location. Between about 5 and 2 million years ago, mountain uplift created a marine embayment in the Central Valley encircled by mountains and draining to the ocean from the southern valley near Monterey Bay. Continued mountain uplift later blocked this seaway, and by about 600,000 years ago the Central Valley was a freshwater basin draining through the region of present-day San Francisco Bay (Harden 2004).

Deserts in southeasternmost California contain ancient continental rocks that attest to their having been part of ancient North America rather than accreting onto the edge, as did most of the west coast. The deserts also harbor more extensive fossils of ancient terrestrial life than the rest of the state, including Eocene terrestrial vertebrates. However, their history as deserts is quite recent (see below).

The California Channel Islands are seafloor ridges, transported northward, rotated, and uplifted by movement on the San Andreas fault beginning 28 million years ago. Although the four smaller islands were inundated at various times in the Pleistocene, beginning 1.8 million years ago, parts of the four largest islands have been continuously exposed for at least 600,000 years, and it is even possible that they had ancient (Miocene) connections to the continent. During the lower sea levels of the Pleistocene, the four northern islands were united into the superisland of Santa Rosa lying only 6 kilometers from the mainland. Santa Cruz, the closest island today, is now 30 kilometers from shore.

CLIMATIC HISTORY

The global climate has generally cooled and dried over the past 50 million years, and plate tectonics have played an important role by altering oceanic circulation and atmospheric composition. Major climate-changing events have included the opening and widening of Antarctic ocean passageways, the uplift of the Himalayas and consequent decline in atmospheric CO₂ due to chemical weathering, and the arrival from the west of Panama and closure of the Central American seaway. Superimposed on these longer-term trends are oscillations on the order of tens to a few hundreds of thousands of years, caused by variation in the eccentricity, obliquity, and precession of the earth’s orbit around the sun (Milankovitch cycles). The interaction of these forces is complex. In particular, it appears that the tectonically driven changes have increased the sensitivity of the climate system to orbital forcing, leading to increasingly rapid and extreme climatic fluctuations toward the present day (Zachos et al. 2001).

The earth’s transition “from greenhouse to icehouse” has been reconstructed mainly from the carbon and oxygen isotopic composition of foraminiferan shells recovered from Antarctic deep-sea drilling (Figure 10). Warming trends from the mid-Paleocene (59 Ma) to early Eocene (52–50 Ma) produced the Eocene climatic optimum, a time when much of the earth experienced climates resembling today’s wet tropics, except for caps of temperate climate at the poles. This was followed by a long period of cooling, leading to the formation of Antarctic ice sheets by the early Oligocene (34 Ma). Moderate warming from the late Oligocene (26 Ma) to the middle Miocene (15 Ma) reduced the extent of oceanic ice, although temperatures never regained their Eocene levels. Cooling resumed from the middle Miocene to the middle Pliocene (6 Ma). The Northern Hemisphere Glaciation, beginning 3.2 million years ago, marked the beginning of extreme oscillations, with more than 20 relatively long glacial periods interrupted by shorter interglacials (Figure 10; Zachos et al. 2001). The present interglacial period began at the Pleistocene-Holocene boundary 12,000 years ago and is expected to end with another ice age unless disrupted by anthropogenic additions of greenhouse gases.

Today’s five mediterranean climate regions with their characteristic winter rainfall and summer drought are found on west coasts between roughly 30° and 42°; latitude (Figures 2, 3). In these locations, the jet stream brings winter storm systems from maritime rather than interior sources, leading to cool, rainy winters instead of cold, snowy ones. The high-pressure systems that create the world’s major deserts (from 23° to 30° latitude) shift poleward in summer with the earth’s tilt, creating the desertlike summer drought. Upwellings of cold deep-ocean water along the coast, which produce a marine layer of cool air capped by warmer inland air, are also a key ingredient of both the summer drought and the relatively gentle winter weather. Timing and duration of the summer droughts vary considerably among the five world regions, with California’s being among the longest and driest (Dallman 1998).

FIGURE 10. Global relative temperatures from the Paleocene to the present, based on oxygen isotope measurements from deep-sea sediments and ice cores. (Based on data from J. Zachos; see also Zachos et al. 2008)

Because of its relevance to plant and animal evolution, the history of the mediterranean climate is of great interest. In today’s mediterranean zones, tropical-like climates began to give way to more seasonal ones as many as 40 million years ago. Precipitation began to peak in the winter by the middle Miocene. However, the fully mediterranean climate with its near-complete summer drought emerged only after the onset of the Pleistocene brought the development of cold offshore currents. While glaciation in the Northern Hemisphere was well under way 2.7 million years ago, the modern ocean current system was not in place until about 1.5 million years ago (Ravelo and Wara 2004). The uplift of the state’s major mountain ranges in the past 5 million years contributed to increasingly steep internal climate gradients. Based on plant fossil evidence, Raven and Axelrod (1978) argued that some summer rainfall persisted in California until one million years ago, but this has yet to be corroborated with geophysical evidence.

Another important question is how severely the climate fluctuated during glacial-interglacial cycles. The conventional wisdom, largely from plant-based evidence (see later sections of this chapter), is that the climate was colder and rainier but remained mediterranean during glacial periods. However, isotopes indicate that during the coldest parts of the last several glacial periods, expanded oceanic ice sheets blocked the cold oceanic current system, producing warmer and rainier conditions in California resembling a prolonged El Niño. Fossil pollen indicates that while this advance warming speeded the recovery of the interglacial vegetation, it caused declines in abundance of the fog-dependent coastal redwoods (Herbert et al. 2001).

FLORISTIC HISTORY

Origin of the Flora According to Raven and Axelrod

Building on their decades of research in plant evolutionary biology and paleobotany, respectively, Raven and Axelrod (1978) described the biotic history of the California Floristic Province beginning with the Eocene, when many modern plant families diversified and most of the world had a warm, wet, essentially tropical climate. The Sierra was a low coastal mountain range, and the Klamaths were offshore islands. The Coast Ranges had not yet emerged. Many parts of present temperate North America and Eurasia were covered in a tropical rainforest flora containing families and species that are now largely extinct north of the humid subtropics, including laurels (Lauraceae) and palms (Palmae), among others.

However, the more northerly and mountainous interior regions of this Eocene world contained what earlier authors had called the Arcto-Tertiary Geoflora, a rich mixture of trees, shrubs, and herbs whose descendants are now found in the temperate forests of East Asia and eastern North America. This flora has sometimes been described as resembling a modern redwood forest, although with many more species of both angiosperms and conifers. It included the ancestors of today’s Californian coastal forests, which were found north of 44° latitude, as well as the ancestors of the drier montane Sierran forests, which were then found farther south. Both of these elements shifted coastward as cooling and drying accelerated in the Oligocene to Miocene, and many of the warm tropical forest elements became extinct. As revealed by fossil assemblages, these forests were enriched by the co-occurrence of species that are now segregated by elevation and habitat. Five million years ago, the mediterranean climate was clearly developing, but fossil floras indicate a less seasonal climate; the wide occurrence of Abies and Picea suggests cooler summers, while the presence of now-extinct Per-sea, Castanea, and Ulmus suggest wetter summers. Late Pliocene decreases in summer rainfall increasingly restricted conifers to high elevations, and cooling temperatures restricted broad-leaved evergreens to low elevations. Pleistocene vegetation was essentially modern, except that it was shifted downward in elevation and latitude compared to the present; conifers were more widespread due to cooler summers, and hardwoods survived in mild coastal climates. During the early Holocene warm period 8,000 to 4,000 years ago (called the “Xerothermic” by early authors), the remnants of the Arcto-Tertiary flora retreated toward their present coastal, riparian, and higher-elevation refuges.

Together with other authors, Raven and Axelrod (1978) considered California one of the most important areas for survival of the Arcto-Tertiary flora, second in the United States only to the considerably richer forests of Appalachia. The refuge existed because the climate remained consistently equable during and since the Tertiary, without widespread glaciation or extreme aridity. Within California the most significant Arcto-Tertiary refuge is thought to be the Klamath-Siskiyou region, where the greater total and summer rainfall, milder winters, and cooler summers amount to a climate resembling that of the Miocene and Pliocene. Elsewhere, Arcto-Tertiary forests became restricted to small patches during the mid-Holocene warm period, and monodominant stands of species such as Pinus jeffreyi, Pinus ponderosa, and Pseudotsuga menziesii developed in recent millennia through the progressive loss of other species.

Today’s Arcto-Tertiary flora, as defined by Raven and Axelrod (1978), comprises just over half the species in the California Floristic Province and is the source of most of its paleoendemics. Some Arcto-Tertiary taxa that meet the standards for very strict paleoendemism, such as having their closest relatives outside of western North America, are Quercus sadleriana, Picea breweri, Berberis nervosa, and Pinus albicaulis. Paleoendemics in a slightly broader sense, such as plants having few close relatives in western North America, include Chrysolepis chrysophylla, Taxus brevifolia, Torreya californica, Calycanthus occidentalis, Dirca occidentalis, Lithocarpus densiflorus, and Acer circinatum. Neoendemics in California are not normally thought of as being of Arcto-Tertiary origin. However, Raven and Axelrod (1978: 16) listed 50 Arcto-Tertiary genera that have undergone significant speciation in California (e.g., Allium, Aster, Bromus, Calochortus, Delphinium, Iris, Lomatium, Lupinus, Silene, Viola). Together, these genera comprise 645 taxa, of which 253 are strictly endemic to California (see Chapter 3 for further analysis).

As the climate became drier in the mid- to late Eocene and the Arcto-Tertiary forests shifted coastward, a more drought-adapted flora began to expand northward into western North America. Fossil floras containing sclerophylls (species with hard, thick, drought-adapted leaves), such as species of Quercus, Arbutus, Pinus, and various laurels (Lauraceae), have been found in deposits as old as 50 million years. Axelrod named this broad assemblage the Madro-Tertiary Geoflora, based on a resemblance to the flora of today’s Sierra Madre of northern Mexico. Axelrod speculated that this flora might have its ancient roots in Mediterranean Europe and/or in localized dry habitats such as rocky southfacing slopes in low-latitude North America. Some members, such as Arbutus, Cupressus, Erodium, Lavatera, Quercus durata, and Q. berberidifolia, were noted to have close relatives in the Mediterranean Basin; Raven and Axelrod termed these taxa “Madrean-Tethyan” after the ancient Tethys Sea that separated Laurasia from Gondwana. By the Miocene, the rich subtropical semiarid Madro-Tertiary (or Madrean) flora dominated interior Southern California. Besides the above taxa, it included Palmae, Lyonothamnus, Ceanothus, Arctostaphylos, Heteromeles, and Rhus. Continued drying in the Pliocene caused Madrean chaparral to expand farther into the Sierra foothills and coastal California and to lose some of its more tropical elements such as Acacia sensu lato. Raven and Axelrod visualized the Madrean flora meeting the Arcto-Tertiary flora at relatively abrupt boundaries along climatic gradients. These boundaries fluctuated during Pleistocene climatic cycles and largely arrived at their present configuration during the early Holocene warm period.

Raven and Axelrod (1978) believed about one-third of species in the California Floristic Province were of Madrean origin. The majority of the neoendemics belong to Madrean genera that radiated extensively in the province and are almost completely endemic at the genus level (e.g., Clarkia, Hesperolinon, Lasthenia, Mimulus, Phacelia). The neoendemic genera are believed to have undergone most of their diversification in the late Pliocene and the Pleistocene as the mountains rose and the climate became fully mediterranean. In the view of Raven and Axelrod (1978) and other classic authors (e.g., Stebbins and Major 1965), much of the speciation was stimulated by the climate-driven advances and retreats of Madrean vegetation across rugged landscapes, which created many opportunities for fragmentation, divergence, reproductive isolation, and/or subsequent hybridization. Preadaptation to summer drought and fire helped to determine which genera thrived and speciated in the new climate. Most annual herbs, and most shrubs that obligately recruit by seed after fire, are Madrean. There are also Madrean paleoendemics in the mountains of Southern California, representing wetter elements of the Madro-Tertiary flora that survived under occasional summer rainfall.

Raven and Axelrod also identified a second group of drought-adapted species that they termed the “warm temperate desert” element of the flora, which they thought moved into the California Floristic Province from the south during the mid-Holocene warm period and colonized the interior Coast Ranges and southern Central Valley. This group comprises 604 species, or 13.5 percent of the flora of the province. Raven and Axelrod (1978) estimated there were 44 endemics in the Central Valley, most of which they considered young (dating to the mid-Holocene warm period) and of desert origin.

Desert floras are relatively poor in endemic species because of the recency of the desert climate, according to Raven and Axelrod (1978). The Mojave and Great Basin Deserts were largely covered by pinyonjuniper woodlands during the Pleistocene. The Sonoran Desert was subtropical woodland. Having undergone less cooling and having retained moderate summer rainfall, the Sonoran Desert contains more relictual subtropical taxa. Raven and Axelrod (1978) enumerated a total of 102 genera and 935 species in the Californian deserts, including 9 species endemic to the Great Basin, 44 to the Inyo region that includes the White Mountains and Death Valley, 22 to the Mojave, and 8 to the Sonoran (Colorado) Desert.

Raven and Axelrod (1978) cited the geography of Californian endemism in support of their conclusions, relying largely on an analysis by Stebbins and Major (1965). Especially high concentrations of neoendemics were found in “intermediately” warm and dry mediterranean-type vegetation, particularly in coastal Southern California, the Sierran foothills, and the central Coast Ranges. Paleoendemics were found to be most prevalent in the Klamath-Siskiyou, the northern Coast Ranges, the Channel Islands (as also described in Raven 1965), and northern Baja California. Scarcity of both neo- and paleoendemics was noted in the climatically youthful Central Valley, deserts, and high Sierra.

In summary, Raven and Axelrod portrayed California’s floristic history as a progressive shift from a largely mesic tropical and warmtemperate flora to a modern flora with a much more arid-adapted character, with new species arising as climatic oscillations across the rugged landscape produced a constant interplay between two distinctive assemblages. The generally equable climate of California enabled the survival of many mesic Arcto-Tertiary relicts. The cycles of cool/moist to warm/dry climates since the Pliocene triggered outbursts of speciation, mostly among the southerly Madro-Tertiary component of the flora, leading to especially high neoendemism within the fully mediterranean climates where the modern vegetation is chaparral.

Raven and Axelrod’s (1978) account is remarkable for its attention to detail. Their sweeping historical analysis is complemented by attention to the numbers of species belonging to each geographic region, life form, and biogeographic origin. Tables in their monograph give the identities of the taxa interpreted as having different biogeographic affinities. For virtually no other large region in the world is there such a comprehensive attempt to link the identities of modern species to their places in a broad account of biogeographic history.

Critiques of the Classic Story

Scientific progress leaves every ambitious accomplishment open to reconsideration. Perhaps the most outdated aspect of the Raven-Axelrod analysis is its reliance on the geoflora concept. Geofloras were envisioned in the early and mid-twentieth century as widespread and long-lasting assemblages that formed in the Tertiary and remained constant in their ecological requirements, changing little through either trait evolution or differential migration and moving as a unit in response to climate change. This is clearly out of step with the modern view that emphasizes the individualistic nature of species range shifts, the recent assembly of modern communities from disparate ancestry, and the significance of adaptation as well as migration in response to climate change (e.g., Davis and Shaw 2001). The existence of an Arcto-Tertiary Geoflora as classically defined was disputed by Wolfe (1978) on the basis of Eocene fossil floras from the Arctic that were predominantly broadleaf evergreen rather than deciduous. However, the notion of a north-temperate deciduous flora at Arctic latitudes in the Tertiary has been borne out in more recent analyses (e.g., Basinger et al. 1994; Brown 1994). More generally, it could be argued that the basic Raven and Axelrod story of the formation of the Californian flora, through range shifts and evolution in contrasting northerly mesic-adapted and southerly drought-adapted assemblages, could still be valid even if the shifts occurred in a less unitary fashion than these authors envisioned (Ackerly 2009).

Modern authors generally substitute more nuanced terms for the old geoflora names. This book follows Ackerly (2009) in using north-temperate” in place of Arcto-Tertiary and “subtropical semiarid” in place of Madro-Tertiary, except when specifically citing Raven and Axelrod.

Terrestrial plant fossils are uncommon in California, and Axelrod’s paleobotanical conclusions involved much interpolation from scarce data. In keeping with the geoflora concept and its principle that species have evolved little, Axelrod employed the assumption that a now-fossilized plant inhabited a climate much like that of its closest living relative. This method has been criticized for ignoring evolution and within-taxon diversity, although it may have some validity if large enough suites of species are used (Basinger et al. 1994). An alternative approach to inferring ancient climates is to use the physiognomic traits of entire fossil assemblages; for example, the proportion of plants with entire (smooth-margined) versus toothed and lobed leaves is strongly correlated with mean annual temperature in climates with year-round rainfall (Wolfe 1978). Another problem was that Axelrod assigned fossil taxa to modern genera using subjective matching of easily visible traits, such as leaf outline and major venation (Edwards 2004); newer approaches to fossil identification emphasize venation patterns and the microscopic examination of epidermal anatomy (Ellis et al. 2009).

More recently, stable isotopes have increasingly allowed paleoclimates to be reconstructed independently of plant fossils. One of the most important findings has been that “the Ice Age” was not a single cold event, as was once believed, nor was the early Holocene warm period an aberrant extreme. Rather, the Pleistocene consisted of many glacial cycles, interspersed by periods that often reached temperatures as warm as the early Holocene (Millar 1996). Complex changes in Californian plant distributions occurred during glacial-interglacial cycles, with both herbs and hardwoods tending to expand as the glacials ended (Edwards 2004).

Another problematic issue is that plants were designated as Arcto- or Madro-Tertiary by subjective methods that relied on species traits and contemporary distributions as well as fossil evidence. Thus groups such as Brodiaea and its relatives, having many species in the province and few outside it, were designated Madro-Tertiary because “the degree of radiation . . . in the California Floristic Province suggests a relatively great antiquity for that group in Madrean vegetation” (Raven and Axelrod 1978: 50). In other words, they are of Madrean origin because they are diverse in Madrean vegetation. While such inferences could well be correct, corroboration from independent evidence would lead to stronger inferences. Moreover, the concept of biogeographic “origins” has its problematic aspects, since (for example) a genus may be inferred to have arisen in one region or climate but its family in another. For adherents of the geoflora concept, the Eocene is regarded as the period when modern lineages acquired the traits defining their climatic niches, an assumption that has never been tested. Ackerly (2009) notes that Raven and Axelrod seem to equate the geographic region in which greatest diversification occurred with the region of origin, although the two need not be the same. An alternative approach would be to focus on traits, which unlike lineages originated at specific times; phylogenetic methods can be used to ask in what regions and climates a trait arose and how it affected the subsequent spread and diversification of a lineage (Ackerly 2009).

Raven and Axelrod’s classification of species in terms of biogeographic origin has been examined in a number of recent analyses. Ackerly (2003) found that woody species classified as Madro-Tertiary had larger seeds and lower specific leaf area (i.e., thicker leaves) than those classified as Arcto-Tertiary. Harrison and Grace (2007) and Ackerly (2009) found that the geographic distribution of the groups conforms as expected to climatic patterns within the state; Raven and Axelrod’s Arcto-Tertiary species are most numerous in the rainy and mountainous north, Madro-Tertiary species (including those “strongly associated with the California Floristic Province,” many of which are endemics) were most abundant in the Coast Ranges and Sierra Nevada foothills, and desert species tend to be found in the driest parts of the California Floristic Province. Damschen et al. (2010) found that over a six-decade period (1949–2007), as mean temperatures in the Siskiyou region increased by 2°C, the abundance of north-temperate (Arcto-Tertiary) forest herbs declined relative to other species. All these analyses bear out, to some extent, the existence of genuine differences between the lineages subjectively identified by Raven and Axelrod as Arcto-Tertiary and Madro-Tertiary.

Phylogenetic analyses of molecular data are a vast new source of evidence on evolutionary processes that were unavailable to Raven and Axelrod (1978). Chapter 3 considers what is now known about Californian plant endemism in light of this and other new evidence.

• • •

The Californian landscape has come into existence over the past 200 million years, and the modern climate and flora have developed during around 50 million years of global cooling and drying. Rapid geologic and climatic changes within the past 5 million years have left an especially strong imprint on the present-day biota. In the historic account by Raven and Axelrod (1978), the Californian flora arose from the interplay of two main sources: a northerly, temperate assemblage (Arcto-Tertiary) and a subtropical, arid-adapted (Madro-Tertiary) assemblage.

The north-temperate assemblage is thought to have given rise to many paleoendemics with relatives in eastern North America or eastern Asia. They include many broad-leaved deciduous trees and shrubs, conifers, and perennial herbs, and are most prevalent in cool and wet environments. The subtropical seminarid assemblage is considered to have given rise to the majority of neoendemics in the California Floristic Province. These species are often evergreen shrubs, geophytes, or annuals. Their centers of diversity are in mediterranean-type chaparral and coastal scrub habitats. Most neoendemics are thought to have arisen in the past 5 million years or less, since the climate became fully dry in the summer, although recognizable relatives have been found in 20- to 30-million-year-old fossil deposits. However, many aspects of this story have been reexamined in recent decades and many new details added. The next chapter examines both old and new evidence on the origins of Californian plant endemism and asks how well the classic story holds up.