Evol Ecol (2012) 26:1011–1023 DOI 10.1007/s10682-011-9532-4 ORIGINAL PAPER

Climate and the evolution of serpentine endemism in California Brian L. Anacker • Susan P. Harrison

Received: 1 July 2011 / Accepted: 13 October 2011 / Published online: 12 November 2011 Ó Springer Science+Business Media B.V. 2011

Abstract We asked whether evolutionary transitions to serpentine endemism are associated with transitions to more favorable environments. Theory and observation suggest that benign (e.g., high rainfall and less extreme temperatures) climates should favor the evolution of habitat specialism, both because such climates may facilitate persistence of small populations with novel adaptations, and because competition with non-specialists may be stronger in benign climates. Non-climatic factors, such as habitat availability, should also be associated with transitions to habitat specialism. We examined phylogenetic transitions to serpentine endemism in 23 Californian plant taxa. We contrasted transitions from serpentine-intolerant ancestors, where speciation entails novel adaptations to serpentine, with transitions from serpentine-tolerant ancestors, where the formation of a new serpentine-endemic taxon may result from an altered competitive environment. We found that transitions to endemism were strongly associated with transitions to regions with more benign climates, but only in the case of endemics arising from intolerant ancestors. In contrast, transitions to endemism from both types of ancestor were associated with transitions to regions with greater habitat availability. These results are consistent with the expectation that benign climates promote the persistence of small populations with novel adaptations both before and after speciation. Keywords

Serpentine  Endemism  Climate  California flora

Introduction Biodiversity hotspots are found in areas with warm and wet climates, reflecting high richness in climatically benign (high rainfall and less extreme temperatures) regions and Electronic supplementary material The online version of this article (doi:10.1007/s10682-011-9532-4) contains supplementary material, which is available to authorized users. B. L. Anacker (&)  S. P. Harrison Department of Environmental Science and Policy, University of California Davis, One Shields Avenue, Davis, CA 95616, USA e-mail: [email protected]

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low species richness in climatically harsh regions (Myers et al. 2000; Hawkins et al. 2003; Currie et al. 2004; Field et al. 2009). An important mechanism for the relationship of diversity and climate at large scales may include higher levels of habitat specialization in warm, wet regions (Gentry 1986; Futuyma and Moreno 1988; Gaston and Blackburn 2000). For plants, specialists may be favored in benign climates such as the tropics if high competition restricts species to a subset of habitats or if the persistence of small populations with novel adaptations is permitted by benign climates. Conversely, strong seasonality and extreme climates in poleward regions may select for habitat generalists (Jansson and Dynesius 2002; Jetz et al. 2004). Non-climatic factors, such as habitat availability and configuration, should also directly influence the level of habitat specialization and endemism, due to the increased probability of the origin and persistence of specialization where habitats are extensive (Anacker 2011), reducing the primacy of climate in explaining diversity. However, habitat factors may also interact with climate (Fagan et al. 2005; Harrison et al. 2008); for example, habitat specialists may go extinct in climatically harsh areas in which their habitat is rare, because of their limited capacity to migrate among habitat patches to track a shifting climate (Jansson and Dynesius 2002; Ackerly 2003). Here, we ask if favorable environments (those with benign climates and extensive habitat availability) promote habitat specialization, focusing on endemism to infertile serpentine soils. Serpentine soils occur as island-like rocky outcrops (e.g., Fig. 1a) with low nutrient levels, excessive Mg/Ca ratios, and low water availability (Kruckeberg 1984; Safford et al. 2005; Harrison et al. 2006). Worldwide, there are at least 3,000 taxa endemic to serpentine (Anacker 2011), and these taxa are characterized by small geographic ranges and population sizes (Harrison et al. 2008). Low-fertility soils are classically considered to be refuges from competition because slow-growing, stress-tolerating soil endemics are unable to compete with faster-growing species; in the absence of competition, some endemics have been shown to grow equally well or better on fertile soils (Tansley 1917; Kruckeberg 1954; Sharitz and McCormick 1973). Serpentine endemism is more common in tropical and Mediterranean biomes than elsewhere, and within California there are more endemics in the rainy north than the more arid south (Brooks 1987; Harrison et al. 2008; Anacker 2011). Endemics tend to inhabit environments with higher rainfall and less extreme seasonal temperatures than their nonendemic congeners (Harrison et al. 2008). It also appears that some taxa are restricted to serpentine in wet and productive parts of their ranges but are soil generalists in colder or more arid regions (Brooks 1987; Safford et al. 2005). Climate has long been thought to play a key role in the evolutionary origin and persistence of serpentine endemics in California (Stebbins and Major 1965; Raven and Axelrod 1978), yet a phylogenetic examination of climate divergences and the evolution of serpentine endemism has never been conducted. There are two possible transitions that lead to endemism: Transitions from serpentine tolerators to endemism (T ? E) or transitions from serpentine non-tolerators to endemism (Nt ? E). Serpentine tolerators, or ‘‘bodenvags,’’ are species that commonly occur on both serpentine and nonserpentine soils; nontolerators, or serpentine ‘‘avoiders,’’ seldom or never occur on serpentine. Endemics may be operationally defined as species that nearly always ([85%) occur on serpentine (Kruckeberg 1984; Safford et al. 2005). Because serpentine soils occur as fine-grained patches within a coarser mosaic of climates, it is common for nontolerators to occur in the same regions and same climates as tolerators and endemics. If benign climates promote the dominance of fast-growing, highly competitive taxa on fertile soils, and if this causes the restriction of more slowly growing, stress-tolerant taxa to poor soils such as serpentine, we expect large climate divergences for tolerator-to-endemic

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Fig. 1 Maps of serpentine outcrops (a), annual precipitation (b), annual temperature (c), and potential evapotranspiration (d) in California

transitions (Fig. 2a). This scenario is consistent with the classical idea of paleoendemism (Kruckeberg 1954, 1984, 1991; Raven and Axelrod 1978). Paleoendemic or ‘‘relictual’’ species in this context are those that were once widespread but were outcompeted in nonserpentine habitats as the climate and competitive environment changed. The loss of non-serpentine populations, termed ‘biotype depletion’ by the above authors, resulted in speciation as the surviving serpentine populations became increasingly geographically separated from their nearest relatives on other soils. Alternatively, if benign climates promote the survival of small founding populations with novel adaptations, climate divergences should be large for nontolerator-to-endemic transitions (Fig. 2b). This scenario is most consistent with neoendemism. Neoendemic or ‘‘insular’’ species are those that arose from serpentine-intolerant ancestors, where presumably a small population was founded on serpentine by individuals with adaptations for tolerance. Strong selection against migrants and hybrids across the soil boundary can then result in relatively rapid ecological speciation (Stebbins 1942; Stebbins and Major 1965; Macnair and Gardner 1998; Kay et al. 2011).

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Fig. 2 Illustration of alternative pathways for the formation of serpentine endemics with respect to climate. As the spatial gradient in climate changed between t1 and t2 (a), the once widespread tolerator species was outcompeted in nonserpentine habitats in wetter regions leading to the loss of non-serpentine populations. The surviving serpentine populations became increasingly geographically separated from their nearest relatives leading to speciation and the reconstruction of a tolerator-to-endemic transition. Alternatively, if benign climates (high rainfall and less extreme temperatures) promote the survival of small founding populations with novel adaptations (b), populations can establish and persist on serpentine, but only in in wetter regions, resulting in ecological speciation and the reconstruction of a nontolerator-to-endemic transition

Regardless of the ancestral condition, we expect that endemics are more likely to arise in areas with more serpentine soil, because habitat availability should favor the origin and persistence of habitat specialization. There is support for both tolerator-to-endemic transitions and nontolerator-to-endemic transitions, at least in the California flora (Anacker et al. 2011), but an important question remains: Which of the above two pathways generates the observed association between favorable climates and endemism at large scales? We compare climate divergences associated with nontolerator-to-endemic transitions and tolerator-to-endemic transitions using phylogenetically independent contrasts of environmental variables and serpentine affinity for the California flora (Purvis and Rambaut 1995; Evans et al. 2005). We also test the relative ages of the two types of transition. This work represents the first test of environmental divergences associated with

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evolutionary transitions towards serpentine endemism, and is among the first to incorporate multiple genera and phylogenetic uncertainty into a contrast analysis.

Materials and methods Study system We use serpentine affinity data and phylogenies for 23 genera as our study group (see Anacker et al. 2011 for details on this dataset; genera listed here in Supplemental Material S1). Our sample contained 485 taxa (47 endemics (E), 282 nontolerators (Nt), and 156 tolerators (T)]), which is representative of the proportion of serpentine endemic taxa per genus found among the 103 genera in the California flora (see Supplemental Material S1 for serpentine affinity by genus). The genera are from 17 families; multiple genera were sampled in only two families (Asteraceae [6] and Apiaceae [2]). Even in these two families, no two sampled genera are sister genera. The genera used in our sample were selected to ensure that we had close to exhaustive sampling with respect to California taxa in each genus (mean of 99%). Within the genera, endemism was shown to be the derived state, to be infrequent in most genera, to be associated with a decrease in diversification rate, and reversals from endemic states were extremely rare (Anacker et al. 2011). Classifying species The serpentine affinity classifications were derived from a published database (Safford et al. 2005), which is based on a compilation of observational information on plant affinity for serpentine soils, and were confirmed or revised based on the opinion of clade experts. Endemics are taxa that have [85% of their known occurrences on serpentine. Tolerators are taxa that are found to exist widely on both serpentine and nonserpentine soils. Nontolerators are taxa that have never been observed on serpentine. We acknowledge that some nontolerators occur in regions of the state where serpentine is not available, and thus their serpentine affinity is relatively uncertain (listed in Table S3 of Anacker et al. 2011). This applied to just 13% of the nontolerator taxa in this paper. As mentioned above, it is classically considered that tolerator lineages give rise to serpentine endemics through the ecologically-driven extinction of populations on nonserpentine soils (e.g., Raven and Axelrod 1978). Nontolerators, on the other hand, are thought to colonize serpentine and rapidly evolve the novel trait of tolerance, with reproductive isolation then arising from strong selection against hybrids and leading to a new endemic with a nearby nontolerant sister lineage (Kay et al. 2011). The two scenarios could be mistaken for one another in a case where a species classified as a ‘‘tolerator’’ actually consisted of mostly nontolerant populations plus one (or more) newly arisen tolerant population(s) on their way to evolving endemism; however, we think this is an improbable situation, given the rarity of speciation events and the unlikelihood that the latter population(s) would be detected. Phylogenetic trees We aligned sequences from GenBank, including ITS, ETS, trnL, rp116, and ndhF regions, using BioEdit ver. 7.0 with ClustalW (Hall 1999) and made manual adjustments as

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necessary. Markov chain Monte Carlo (MCMC) phylogenetic analyses were run using the alignments in Mr. Bayes [ver. 3.1.2] (Ronquist and Huelsenbeck 2003) to obtain a posterior distribution of phylogenetic trees for each genus. We used the General Time Reversal nucleotide substitution model for each genus, with a proportion of invariable sites and a gamma distribution of rates across sites. Four independent runs of Mr. Bayes were conducted for each genus. Each run consisted of one cold chain and three heated chains that were sampled every 50,000 generations for a total of 10 million generations. For each run, the initial one million ‘‘burn-in’’ trees were removed. We saved trees after the log-likelihood score converged, assessed by comparing the standard deviation of split frequencies between paired runs using Tracer ver. 1.4. The posterior distribution of trees for all four runs was then combined to make 720 trees per genus. Branch lengths were transformed using nonparametric rate smoothing in TreeEdit to create ultrametric trees (Rambaut and Charleston 2001). Phylogenetically independent contrasts, described below, were conducted on this sample of the posterior distribution. Topologies estimated using Bayesian analysis of molecular data were generally similar to previous reports, but were often improvements because additional taxa were included and nodal supports were higher (see Anacker et al. 2011 for complete details on the resulting topologies, including Fig. S3). However, we acknowledge that we do not have a perfect understanding of species-level relationships for the genera in our sample and that further phylogenetic analysis, including more genes, may improve phylogenetic resolution in some clades and affect branch length estimates. Environmental data Mean climate values for each taxon were determined by intersecting species ranges (Viers et al. 2006) with a California state climate model (Daly et al. 1994). The spatial average of the 30 year mean for each of nine climate variables was calculated for each taxon: annual mean temperature, temperature seasonality (standard deviation * 100), summer maximum temperature, winter minimum temperature, annual precipitation, winter precipitation (sum of wettest month), summer precipitation (sum of driest month), the precipitation seasonality (CV), and potential evapotranspiration (PET). To obtain a multivariate description of climate, we ran a principal components analysis on the climate variables, subsetted to prevent any pairwise correlations above r = 0.75 (Pearson’s correlation coefficient) (Supplemental Material S2). We used the first three PCs because they cumulatively accounted for 90.8% of the total variance. PC1 primarily represented PET and annual temperature; PC2 had a high positive loading of temperature seasonality and PC3 had a large negative loading for annual precipitation (Supplemental Material S2). Two additional indices of aridity were derived using PET and annual precipitation: aridity1 = PET divided by annual precipitation, corresponding to aridity caused by drought; aridity2 = PET minus annual precipitation, corresponding to aridity associated with high temperatures, also known as ‘‘climatic water deficit.’’ As a measure of habitat availability, we calculate the percentage of the species range that is serpentine by intersecting the range maps with a geologic map of California (Jennings and Strand 1977). In California, the amount of serpentine in an ecological subunit is positively correlated with both the average serpentine patch size (r = 0.89) and the number of serpentine patches (r = 0.79). Also, while the total amount of serpentine in a region is a predictor of endemic species richness, measures of spatial configuration (e.g. numbers, shape, isolation of outcrops) are not (Harrison et al. 2006). Thus, we use just area as our metric of habitat availability. The distributions of annual precipitation, winter precipitation, summer

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precipitation, PET, habitat availability, and PC2 were log normal; we log-transformed these variables prior to calculating means. To test if extending the extent of our study beyond California affects our results, we determined species mean values based on global occurrence records and global climate grids. Results were qualitatively similar to those presented below, and are provided in the supplemental information (see Supplemental Material S3). Contrast analysis To calculate phylogenetically independent contrasts of changes in serpentine affinity (ternary) and environmental variables (continuous), we used the Brunch algorithm of ‘‘comparative analysis by independent contrasts’’ (CAIC) as implemented in the R package ‘‘CAIC’’ (Purvis and Rambaut 1995). Contrasts were made by subtracting the non-endemic (nontolerators or tolerator) value from the endemic value, thus meaning that positive contrasts reflect that the change to endemism was associated with an increase in the variable under consideration. As is typical of a contrast analysis, we test the statistical distribution of contrasts against the null expectation of zero. However, to account for phylogenetic uncertainty, we calculate contrasts for many samples of the posterior distribution of each genus, separated by transition type (Nt ? E, T ? E), and examine the resulting posterior distributions of the test statistics for deviation from zero. For each environmental variable, we randomly sampled one tree from the posterior distribution of each of the 23 genera, calculated independent contrasts for each of the 23 trees in the sample, and combined the resulting contrast values into a data vector. We repeated this procedure 1,000 times, sampling trees with replacement. Depending on the random draw of trees, the number of contrasts ranged from 42 to 44 per vector. We then split each data vector of 42 to 44 values into two data vectors, separating the Nt ? E contrasts from the T ? E contrasts, each containing roughly the same number of contrasts. For example, for a particular set of 1,000 vectors, the mean number of contrasts (±one standard deviation) per vector was 42.7 (±1.0), where 21.5 (±1.5) were Nt ? E contrasts and 21.2 (±1.5) were T ? E contrasts. We conducted a one-tailed t test of the *21 values in each data vector against zero, storing the t-statistic. For the two posterior distributions of 1,000 t-statistics (one for Nt ? E and the second for T ? E), we assessed for significant departure from zero in the predicted direction by assessing if a 95% confidence interval is entirely within the realm of significance given a critical value for a one-tailed t test with degrees of freedom of 20 (the mean number of contrast values used to calculate the t-statistics, minus one; t = ± 1.72). Importantly, this did not require calculating a new t-statistic for the two posterior distributions of 1,000 tstatistics, avoiding pseudo-replication. For example, if each draw of 23 trees produced the same contrasts, but the t-value was very small and non significant for any single draw, and we then generated 1,000 t-values and calculated a new t-statistic for the distribution, we would get a highly significant departure from zero. Rather, we tested the distribution of the 1,000 t-values from the original draws for significant departure from zero by simply counting how many of the 1,000 t-values are greater than, or less than, depending on the variable, the critical value. In the example above, zero of 1,000 t-values would be above or below the critical value and significance would be 1.0. To assess if nontolerator-to-endemic transitions were younger than tolerator-to-endemic transitions, for each of the 1,000 random draws of trees, we determined the length of the branch between the node in consideration and the tree tip, creating one branch length per contrast, for a total of 42–44 branch lengths per random draw. For each vector of branch

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lengths, we conducted a one-tailed t test of 42–44 branch lengths against the contrast type (Nt ? E and T ? E), storing the t-statistic. Significance was assessed as above, except degrees of freedom of 42 was used (the mean number of contrast values used to calculate the t-statistics, minus one; t = 1.68).

Results Environment and species distributions A comparison of the overall and genus-level means of the environmental variables by serpentine affinity (Table 1, Supplemental Material S4) shows that endemics clearly occupied wetter regions with more serpentine habitat than non-endemics, but this Table 1 Results of the phylogenetic contrast analyses Environmental variables

Means by serpentine affinity

Contrast analysis results

E

Number of contrast vectors

Nt

Nt ? E

T

T?E P

Number of contrast vectors

P

Precipitation (cm) Annual

83.08

59.32

77.74

963

0.037*

10

Winter

16.16

10.98

14.81

985

0.015*

118

0.24

0.27

0.26

0

1.0

0

1.0

83.43

77.44

80.95

0

1.0

0

1.0

Summer Seasonality

0.990 0.882

Temperature (°C) Annual

12.69

12.20

12.34

0

1

0

1.0

Winter

0.45

-0.52

-0.15

7

0.993

3

0.997

Summer

29.59

28.90

29.36

0

1.0

0

1.0

Seasonality

50.78

53.62

53.35

23

117.50

125.60

120.74

379

0.621

459

0.541

0.15

0.23

0.16

978

0.022*

271

0.729

27.96

58.33

35.00

902

0.098

23

0.977

0.511

825

0.175

0.977

19

0.981

Aridity PET (cm) Aridity1 Aridity2 (cm) Multivariate climate PC1

1.43

0.47

0.81

489

PC2

-9.67

-9.32

-9.56

0

PC3

2.29

2.30

2.31

2.18

1.86

2.17

1.0

0

1.0

349

0.651

0

1.0

1000

\0.001*

Habitat availability % Serpentine

1000

\0.001*

We randomly sampled one tree from the posterior distribution of each genera and calculated independent contrasts for each tree, conducted a t test on the combined contrasts, and repeated the procedure 1,000 times. We report the number of contrast vectors of t-values (of 1,000) that were significantly different from zero, in the direction of endemics occupying more favorable environments (cooler, wetter, lower aridity, and more habitat). Nt = nontolerator, T = tolerator, E = endemic. *indicates that a 95% confidence interval of the posterior distribution of the test statistic is within the realm of significance. Untransformed means are presented but transformed means were used in the contrast analysis for variables that were log-normal (annual precipitation, winter precipitation, summer precipitation, PET, habitat availability, and PC2)

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difference was especially strong for endemics versus non-tolerators (e.g., for overall means, annual precipitation endemic minus nontolerator = 23.76, endemic minus tolerator = 5.34). Differences in temperature and seasonality variables were less obvious. Given that we conducted independent contrasts below, we did not test the above patterns in overall means for significance, but we report them to help interpret the contrast results and to provide comparison with other studies (Table 1, Supplemental Material S4). The spatial patterns of climate and serpentine in the state are similar at the largest scale, reflecting the absence of serpentine in the arid interior and southern regions (Fig. 1). Despite this, climate variables were not highly correlated with serpentine availability in our dataset (maximum correlation was r = 0.30 for annual precipitation; Supplemental Material S5). Contrast analysis Using the posterior distributions from the phylogenetic analysis, we calculated contrasts of environmental and serpentine affinity data (endemics [E], tolerators [T], and nontolerators [Nt]). We refer to our contrast results in terms of the environmental divergences associated with evolutionary transitions towards endemism. Increased annual precipitation and winter precipitation were significantly associated with nontolerator-to-endemic transitions (Table 1). Likewise, nontolerator-to-endemic transitions were significantly associated with decreased aridity, as measured by aridity1 (PET/P), but not aridity2 (PET–P) nor PET itself (Table 1) (Fig. 3a, b). None of the climate variables were significantly associated with tolerator-to-endemic transitions (Fig. 3c, d). Temperature and multivariate climate variables were not significantly associated with transitions to endemism. Both transition types were significantly associated with increased serpentine availability. Nontolerator-toendemic transitions were not significantly younger than tolerator-to-endemic transitions (mean ± one standard deviation: Nt ? E = 0.39 ± 0.10; T ? E = 0.41 ± 0.09; P = 0.997).

Discussion We found that serpentine-endemic lineages in California occupy geographic regions with higher rainfall than their non-endemic sister lineages, but only in the case of endemics that arose from serpentine-intolerant ancestors. Endemic lineages also occupy regions with greater serpentine availability than their non-endemic sister lineages, regardless of their tolerator or non-tolerator ancestry. We found no association between the origin of serpentine endemism and any temperature-related variables. Transitions to serpentine endemism from a serpentine-intolerant ancestral state are classically considered examples of ecological speciation, in which reproductive isolation is promoted by strong selection against cross-habitat migrants or hybrids (Kay et al. 2011). Such transitions require that a new lineage with the novel trait of serpentine tolerance must arise, colonize serpentine, and survive through environmental adversity long enough to become reproductively isolated; often the resulting species remains rare and geographically localized (Fig. 2b). The best-studied example is Layia discoidea, which exists as a few populations on a single serpentine outcrop in central California, separated by 102–103 m from populations of its much more widely distributed serpentine-intolerant ancestor Layia glandulosa (Baldwin 2005). Benign climates have the clear potential to enhance the

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Fig. 3 Range maps of sister taxa. Upper row (a, b) shows ranges of taxa representing a nontolerator (Nt) to endemic (E) transition; lower row (c, d) shows ranges of taxa representing a tolerator (T) to endemic (E) transition. Climate divergences were large for nontolerator-to-endemic transitions, but not for toleratorto-endemic transitions

survival of such small neoendemic lineages both before and after speciation, while harsh or fluctuating climates are likely to promote their extinction (Fig. 2b). Our results are consistent with previous theories (Fjeldsa et al. 1997; Jansson and Dynesius 2002; Jetz et al. 2004) and observational evidence (Raven and Axelrod 1978) in supporting such a linkage between climatic favorability and the origin and persistence of habitat specialism. In turn, this mechanism has the potential to explain the observed association between serpentine-endemic richness and climate in California (Harrison et al. 2000; Harrison et al. 2008). Transitions to serpentine endemism from more widespread serpentine-tolerant ancestors are examples of paleoendemism, in which altered environments leave behind relictual populations stranded in refugial habitats, and these populations may form new species that

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are more distinct both geographically and taxonomically from their closest relatives than in the case of neoendemics (Fig. 2a). Here, a well-studied example is the Streptanthus glandulosus complex (Mayer and Soltis 1994; Mayer et al. 1994), which belongs to a genus with ancestrally desert climatic affinities (Axelrod 1977; Raven and Axelrod 1978). Based on the geographic structure of genetic diversity and other evidence, Mayer and Soltis (1994) and Mayer et al. (1994) suggested that the widespread progenitor species of the S. glandulosa complex, underwent range contraction and fragmentation due to climatic cooling, with some of the resulting daughter lineages then becoming serpentine endemics. Our results suggest that such tolerator-to-endemic transitions have been common in the Californian serpentine flora, but no more so in the wetter than the drier regions of the state. This lack of association between Californian serpentine paleoendemism and climate may not be surprising, in retrospect, since the climate has generally become drier and the vegetation structure more open since the Miocene (Raven and Axelrod 1978). In contrast, Arenaria norvegica in Sweden appears to be a classic case in which the reinvasion of forest, in response to post-Pleistocene warming, led to the increasing restriction of this small herb to serpentine outcrops (Brooks 1987). While competition may play a role in the ecological restriction of many taxa to serpentine in California, our results do not support a linkage between competitive intensity and a wetter, more productive climate. We found no differences in the timing of the two types of transition to endemism, and indeed we had no a priori expectation in this regard. However, a more definitive test of the relative ages of neo- and paleoendemic lineages would require multiple gene trees, fossil calibration, and a detailed phylogenetic analysis. The association of endemism with precipitation variables, but not temperature, whether measured as an annual mean or seasonal extremes, reflects the fact that California’s gradient in plant productivity is largely driven by rainfall. The remotely sensed productivity index NDVI (normalized difference vegetation index), is highly correlated with mean annual precipitation (r = 0.88) but uncorrelated with temperatures across the serpentinecontaining regions of the state (Harrison et al. 2006). Our results also demonstrate the role of habitat availability in promoting habitat specialization. Not surprisingly, whether serpentine endemics arose from tolerant or intolerant lineages, they occupied ranges with greater availability of serpentine than their non-endemic relatives. Perhaps less obviously, however, evolving restriction to the patchy serpentine environment does not lead to island-like radiations in the lineages studied here, but rather is associated with decreased rates of subsequent diversification (Anacker et al. 2011). Comparing diversification in serpentine endemic lineages in California versus other regions of the world (e.g., New Caledonia, De Kok 2002) may illuminate whether climate affects the probability of diversification subsequent to the origin of endemism. The general finding that endemism is more common in favorable environments could be obtained in the absence of phylogenetic information (Table 1, Supplemental Material S4), but the use of phylogenies helps to identify the pathway that generates the observed pattern of favorable environments and endemism. In addition, our study is an example of taking comparative analyses beyond single clades, which increases statistical power in the face of rare states and expands the extent of the inferences allowed. Acknowledgments We thank Carl Boettiger, Jonathan Davies, and Joshua Viers for discussion and comments on the manuscript.

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