Hawkins Bradford A, Diniz-Filho José Alexandre Felizola, Jaramillo Carlos A, Soeller Stephen A
Department of Ecology and Evolutionary Biology, University of California, Irvine, California 92697 USA.
Am Nat. 2007 Aug;170 Suppl 2:S16-27. doi: 10.1086/519009.
We tested the proposition that there are more species in the tropics because basal clades adapted to warm paleoclimates have been lost in regions now experiencing cool climates. Molecular phylogenies were used to classify species as "basal" and "derived" based on their family, and their richness patterns were contrasted. Path models also evaluated environmental predictors of richness patterns. As predicted, basal clades are more diverse in the lowland tropics, whereas derived clades are more diverse in the extratropics and high-altitude tropics. Seventy-four percent of the variation in bird richness was explained by environmental variables, but models differed for basal and derived groups. The overall gradient is described by the spatial pattern of basal clades, although there are differences in the Old and New Worlds. We conclude that in ecological time, the global richness gradient reflects birds' responses to climatic gradients, partially operating via plants. Over evolutionary time, the gradient primarily reflects the extirpation of species in older clades from parts of the world that have become cooler in the present. A strong secondary effect arises from dispersal of clades from centers of origin and subsequent radiations. Overall, the diversity gradient is well explained by niche conservatism and the "time-for-speciation" hypothesis.
热带地区物种更多是因为适应温暖古气候的基部类群在现在气候凉爽的地区已经消失。分子系统发育学被用于根据物种所属科将其分类为“基部”和“衍生”类群，并对比它们的丰富度模式。路径模型也评估了丰富度模式的环境预测因子。正如预测的那样，基部类群在低地热带地区更为多样，而衍生类群在温带和高海拔热带地区更为多样。鸟类丰富度74%的变异可由环境变量解释，但基部类群和衍生类群的模型有所不同。尽管新旧世界存在差异，但总体梯度由基部类群的空间模式描述。我们得出结论，在生态时间尺度上，全球丰富度梯度反映了鸟类对气候梯度的响应，部分是通过植物起作用。在进化时间尺度上，该梯度主要反映了古老类群中的物种从目前已经变冷的世界部分地区灭绝。一个强大的次要效应源于类群从起源中心的扩散及随后的辐射。总体而言，多样性梯度能很好地由生态位保守性和“物种形成时间”假说解释。
