Data from: Limitations of climate data for inferring species boundaries: insights from speckled rattlesnakes

Phenotypes, DNA, and measures of ecological differences are widely used in species delimitation. Although rarely defined in such studies, ecological divergence is almost always approximated using multivariate climatic data associated with sets of specimens (i.e., the “climatic niche”); the justification for this approach is that species-specific climatic envelopes act as surrogates for physiological tolerances. Using identical statistical procedures, we evaluated the usefulness and validity of the climate-as-proxy assumption by comparing performance of genetic (nDNA SNPs and mitochondrial DNA), phenotypic, and climatic data for objective species delimitation in the speckled rattlesnake (Crotalus mitchellii) complex. Ordination and clustering patterns were largely congruent among intrinsic (heritable) traits (nDNA, mtDNA, phenotype), and discordance is explained by biological processes (e.g., ontogeny, hybridization). In contrast, climatic data did not produce biologically meaningful clusters that were congruent with any intrinsic dataset, but rather corresponded to regional differences in atmospheric circulation and climate, indicating an absence of inherent taxonomic signal in these data. Surrogating climate for physiological tolerances adds artificial weight to evidence of species boundaries, as these data are irrelevant for that purpose. Based on the evidence from congruent clustering of intrinsic datasets, we recommend that three subspecies of C. mitchellii be recognized as species: C. angelensis, C. mitchellii, and C. Pyrrhus.

表型、DNA及生态差异测量指标在物种界定（species delimitation）领域应用广泛。尽管此类研究中鲜有对生态分化作出明确定义，但几乎均通过与标本集关联的多变量气候数据近似表征生态分化，此类数据即所谓“气候生态位（climatic niche）”；该方法的合理性在于，物种特异性的气候包络线可作为生理耐受度的替代指标（surrogates）。本研究采用统一统计流程，通过比较遗传数据（核DNA单核苷酸多态性（nDNA SNPs）与线粒体DNA（mitochondrial DNA））、表型数据及气候数据在斑点响尾蛇（Crotalus mitchellii）复合群的客观物种界定中的表现，评估了“以气候作为生理耐受度替代指标”这一假设的实用性与有效性。内在（可遗传）性状（intrinsic heritable traits）的排序分析（ordination）与聚类分析（clustering）模式整体一致，其间的不一致性可通过个体发育（ontogeny）、杂交（hybridization）等生物学过程解释。与之形成鲜明对比的是，气候数据未能生成与任一内在数据集相契合的具有生物学意义的聚类结果，反而对应于大气环流（atmospheric circulation）与气候的区域差异，表明此类数据中不存在固有的分类学信号。将气候作为生理耐受度的替代指标，会人为抬高物种边界证据的权重，因此类数据与物种界定的核心目标无关。基于内在数据集聚类一致性的证据，我们建议将C. mitchellii的三个亚种（subspecies）认定为独立物种：C. angelensis（天使岛响尾蛇）、C. mitchellii（米切尔响尾蛇）与C. Pyrrhus（皮洛斯响尾蛇）。