Taylor Stacy I, Levy Foster
Department of Biological Sciences, East Tennessee State University, Johnson City, Tennessee 37614 USA.
Present address: Department of Biological Sciences, University of Exeter, Exeter, UK.
New Phytol. 2002 Sep;155(3):437-447. doi: 10.1046/j.1469-8137.2002.00478.x.
• Tests for adaptation to three different soils inhabited by subspecific taxa within Phacelia dubia and for preadaptation to a serpentine soil were conducted to examine the plausibility of an endemic-to-endemic evolutionary pathway. Each taxon performed optimally on its home soil, demonstrating edaphic specialization. None survived on the serpentine. • Hydroponic assays for tolerance to two serpentine factors, elevated magnesium: calcium and elevated nickel, were conducted on population samples and maternal half sib families. Performance was estimated by root length and rosette diameter while leaf dissection served as an indicator of developmental maturity. • Both nickel and magnesium: calcium of typical serpentine inhibited all three taxa. However, the granite outcrop endemic var. georgiana tolerated higher magnesium: calcium than other taxa, its tolerance exceeded that found on its home soil, and there was developmental variation among sibships. • The tolerance uncovered in the endemic var. georgiana suggests that a specialized endemic taxon may encompass variation that could lead to preadaptation to a novel habitat and therefore serve as the raw material for speciation rather than represent an evolutionary dead end.
• 对Dubia花菱草亚种下的三个不同土壤栖息类群进行了适应测试，并对蛇纹石土壤进行了预适应测试，以检验特有物种到特有物种进化途径的合理性。每个类群在其原生土壤上表现最佳，显示出土壤专一性。没有一个能在蛇纹石上存活。
• 对种群样本和母本半同胞家系进行了水培试验，以测试对蛇纹石的两个因素——镁钙比升高和镍含量升高的耐受性。通过根长和莲座直径评估生长情况，同时叶片解剖作为发育成熟度的指标。
• 典型蛇纹石的镍和镁钙比均抑制了所有三个类群。然而，花岗岩露头特有变种乔治亚娜比其他类群能耐受更高的镁钙比，其耐受性超过了在原生土壤上的表现，并且同胞间存在发育差异。
• 特有变种乔治亚娜中发现的耐受性表明，一个特化的特有类群可能包含可导致对新栖息地预适应的变异，因此可作为物种形成的原材料，而不是代表进化的死胡同。
