Ten cases of divergence in the seedling ecology of Dudleya (Crassulaceae)

Dudleya is a genus of succulents consisting of 49 terminal taxa. Many are restricted to narrow geographic ranges with closely related forms living in differing climates. Previously, we found an intriguing set of correlations among nine more or less sympatric Dudleya: species with a smaller mature body size had a lower tolerance for an arid inland climate compared to larger bodied species. Thus, we were motivated to test for rules caused by convergent evolution. We sampled 20 populations from locations across much of the range of the genus. The 20 populations were placed into 10 pairs of close relatives. For each pair, one form was judged to be more mesophilic and the other to be more xerophilic, based on climate-of-origin. We measured germination rate, survival through the summer drought in a coastal garden, and survival through the summer in an inland garden. We hypothesized (among other things) that the xerophilic taxa would have larger mature body sizes and greater rates of survival than mesophilic relatives; however, this and other expected patterns were not repeated across the 10 pairs. Members of pairs have diverged both in various morphological traits and in seedling ecology, but evolution has seemingly not converged on rules. For nearly all taxa, habitat dependence was clear, i.e. plants survived significantly better through the summer at the coastal garden than at the inland garden. Quite possibly the correlations we previously found were caused by divergence between particular lineages coupled with phylogenetic conservatism. Considering the 10 pairs, Dudleya divergences appear to each evolve individualistically.

仙女杯属（Dudleya）是多肉植物的一个属，包含49个末端分类群（terminal taxa）。该属多数类群的地理分布范围狭窄，近缘类群栖息于迥异的气候环境中。此前，我们在9个大致同域分布（sympatric）的仙女杯属类群中发现了一组有趣的关联：相较于成熟个体体型更大的物种，成熟体型更小的物种对干旱内陆气候的耐受能力更低。因此，我们旨在验证由趋同演化（convergent evolution）所塑造的演化规律。

我们从该属绝大部分分布范围内的区域采集了20个居群（population），并将其划分为10组近缘类群对。针对每组类群对，依据其起源气候，将其中一个类群归为喜湿型（mesophilic），另一个归为耐旱型（xerophilic）。我们测定了三类实验指标：种子发芽率、滨海人工栽培园的夏季干旱存活率，以及内陆人工栽培园的夏季存活率。

我们提出了多项假说，其中一项核心假说为：相较于喜湿型近缘类群，耐旱型类群将具备更大的成熟体型与更高的存活率。然而，这一预期模式及其他预设规律并未在10组类群对中得到验证。各组类群对的成员在多项形态性状（morphological traits）及幼苗生态特征上均产生了分化，但演化过程似乎并未形成普适性的规律。

几乎所有类群均表现出显著的生境依赖性，即植株在滨海人工栽培园的夏季存活率显著高于内陆人工栽培园。我们此前观测到的关联，极有可能是特定谱系的分化与系统发育保守性（phylogenetic conservatism）共同作用的结果。综合10组类群对的结果来看，仙女杯属的类群分化似乎均以独立的模式进行演化。