Aitken Sally N, Libby William J
Department of Forestry and Resource Management, University of California, Berkeley, California, 94720.
Evolution. 1994 Aug;48(4):1009-1019. doi: 10.1111/j.1558-5646.1994.tb05289.x.
Patterns of allozyme variation within and between two of the subspecies of Pinus contorta were examined for the evolutionary relationship between them. In coastal northern California, these subspecies are parapatric. Pinus contorta ssp. contorta occurs on grassy coastal bluffs on the lowest and youngest of a sequence of five marine terraces; P. contorta ssp. bolanderi is endemic to a pygmy forest ecosystem that occurs on the increasingly older and harsher soils of the third, fourth, and fifth terraces. The soils of the upper three terraces are characterized by extreme podzolization, low pH, low nutrient availability, summer drought (with periodic fires), and winter surface flooding above the hardpan. Dune and cliff soils support a tall redwood and Douglas-fir forest between the terraces. Analyses of seeds collected from 11 pygmy-forest and 6 coastal populations showed ssp. bolanderi to have significantly less allozyme variation than spp. contorta. The two subspecies did not show the phylogenetic dichotomy in allozyme allelic constitutions expected for subspecific classification. Within ssp. bolanderi, the pattern of genetic distances correlated better with edaphic differences among sites than with geographic distance. It appears that ssp. bolanderi is a recently evolved derivative of ssp. contorta, and that the low degree of allozyme differentiation among the bolanderi populations may be due to colonization of the sites by small numbers of individuals, or to hitchhiking of allozyme loci linked to loci undergoing strong selection imposed by the severe edaphic conditions typical of bolanderi sites.
为了研究扭叶松两个亚种之间的进化关系，对其种内和种间的等位酶变异模式进行了研究。在加利福尼亚州北部沿海地区，这两个亚种是邻域分布的。扭叶松指名亚种生长在五个海蚀阶地中最低且最年轻的阶地上的草地海岸悬崖上；博兰德扭叶松亚种则特有于一个侏儒森林生态系统，该生态系统出现在第三、第四和第五阶地越来越古老且条件恶劣的土壤上。上部三个阶地的土壤具有极端灰化、低pH值、低养分有效性、夏季干旱（伴有周期性火灾）以及硬磐之上冬季地表积水的特征。沙丘和悬崖土壤在阶地之间支撑着高大的红杉和花旗松森林。对从11个侏儒森林种群和6个沿海种群收集的种子进行分析表明，博兰德扭叶松亚种的等位酶变异明显少于扭叶松指名亚种。这两个亚种在等位酶等位基因构成上并未表现出亚种分类所预期的系统发育二分法。在博兰德扭叶松亚种内，遗传距离模式与不同地点的土壤差异的相关性比与地理距离的相关性更好。看起来博兰德扭叶松亚种是扭叶松指名亚种最近进化而来的衍生物，并且博兰德扭叶松种群中等位酶分化程度较低可能是由于少数个体对这些地点的殖民，或者是与受到博兰德扭叶松典型恶劣土壤条件强烈选择的位点连锁的等位酶位点的搭便车效应。
