Relatively recent increases in molecular and geographic data for many taxa in different areas of the world have provided scientists with tools to evaluate biodiversity using evolutionary or phylogeny‐based indices (reviewed by Laffan, 2018). These measures provide quantitative estimates of the portion of the tree of life contained in a taxon or community, aiming to answer the question of what percentage of the phylogeny would be lost if that taxon or community is not conserved (Faith, 1992; Purvis et. al., 2000). One of the most widely used phylogenetic indices is phylogenetic diversity (PD), which measures the evolutionary history captured by a set of species (or any biodiversity unit) on the tree of life (Faith, 1992). A higher PD value can represent either a set of taxa that represent longer branches than expected and/or are overdispersed in the phylogeny (Fig. 1A, B). Use of PD was proposed more than two decades ago (Faith, 1992) as a method for finding sets of taxa that could be highlighted as priorities for conservation and has recently been proposed as a biodiversity metric by several international conservation organizations (see IPBES, 2019). For example, the International Union for the Conservation of Nature (IUCN) has recently established a Phylogenetic Diversity Task Force (https://www.iucn.org/commissions/ssc‐groups/disciplinary‐groups/phylogenetic‐diversity‐task‐force), a global expert group, aimed at incorporating PD into practical conservation strategies.

Several studies of geographic patterns of PD have been conducted for the floras of different ecoregions (for a review, see Scherson et al., 2018), generally showing that relatively more humid regions (temperate and tropical) tend to concentrate more PD than arid zones. Coincident with the concentration of PD, plant diversity in hot deserts is lower than the diversity in more humid regions when measured in terms of species richness (Barthlott et al., 2005), and humid regions tend to have lower extinction rates leading to higher species accumulation over time. Greater resource availability contributes to the increased species richness in humid compared to arid zones (Worm and Tittensor, 2018).

Tropical regions in the world constitute most of the biodiversity hotspots (Myers et al., 2000), and only 1.82% of hotspots correspond to arid zones (Olson et al., 2001), leaving deserts underprioritized in national and international biodiversity conservation strategies. However, hot deserts are areas of high endemism (meaning that their taxa are unique and geographically restricted) and vulnerability. In addition, plant species in these areas are adapted to dry conditions, a relevant attribute when considering their evolutionary, ecological, and economic potential in the face of global change (Ward, 2016).

相对而言，世界不同地区许多生物分类的分子和地理数据相对较新增加，为科学家提供了使用基于进化或基于系统进化论的指数评估生物多样性的工具（Laffan综述，2018年）。这些措施提供了对分类单元或群落中生命树的部分的定量估计，旨在回答以下问题：如果该分类单元或群落不保守，将损失多少系统发育百分比（Faith，1992 ; Purvis等。等，2000）。最广泛使用的系统发育指标之一是系统发育多样性（PD），它衡量生命树上一组物种（或任何生物多样性单位）捕获的进化历史（Faith，1992年））。较高的PD值可以代表一组比预期更长的分支和/或在系统发育中过度分散的分类单元（图1A，B）。二十多年前（Faith，1992）提出使用PD作为寻找分类单元集的一种方法，可以将其作为保护的重点，最近已被多个国际保护组织提出作为生物多样性指标（见IPBES，2019年））。例如，国际自然保护联盟（IUCN）最近成立了系统发育多样性工作组（https://www.iucn.org/commissions/ssc-groups/disciplinary-groups/phylogenetic-diversity-task-force ），这是一个全球专家小组，旨在将PD纳入实际的保护策略。

已经针对不同生态区域的植物区系进行了PD地理分布的若干研究（综述见Scherson等人，2018），总体上显示相对较干旱的地区（温带和热带）倾向于比干旱地区集中更多的PD 。与局部放电浓度一致的是，从物种丰富度的角度衡量，炎热沙漠中的植物多样性低于较湿润地区的多样性（Barthlott等，2005），并且湿润地区的灭绝率较低，导致物种较高随着时间的推移积累。与干旱地区相比，更多的资源可利用性增加了湿地物种的丰富度（Worm and Tittensor，2018）。

世界上的热带地区构成了生物多样性热点中的大部分（Myers等，2000），只有1.82％的热点对应于干旱区（Olson等，2001），在国家和国际生物多样性保护战略中沙漠没有得到应有的重视。但是，炎热的沙漠是高度流行的地区（这意味着它们的分类群是独特的，并且在地理上受到限制）和脆弱性。此外，这些地区的植物物种适应干旱条件，这是在面对全球变化时考虑其进化，生态和经济潜力时的相关属性（Ward，2016）。