Sheldon P R
Department of Geology, Trinity College, Dublin, Ireland.
Nature. 1987;330(6148):561-3. doi: 10.1038/330561a0.
There are very few high-resolution studies of the fossil record from which to assess the relative frequency of gradualistic and punctuated evolution. Here I report some of the first detailed evidence of phyletic gradualism in benthic macroinvertebrates, based on a study of approximately 15,000 trilobites from central Wales. Over a period of about three million years, as many as eight lineages underwent a net increase in the number of pygidial ribs, a species-diagnostic character. The end members of most lineages have previously been assigned to different species and, in one case, to different genera. In view of intermediate morphologies and temporary trend reversals, however, practical taxonomic subdivision of each lineage proved impossible. The apparent success of earlier Linnean nomenclature (with its implications of discrete species) could easily have been misinterpreted as evidence of punctuation and stasis, and it is probable that detection of many other gradualistic patterns has been hindered by ready application of binominal taxonomy to fossils.
几乎没有高分辨率的化石记录研究可用于评估渐变式进化和间断平衡式进化的相对频率。在此，我基于对来自威尔士中部约15000只三叶虫的研究，报告一些有关底栖大型无脊椎动物系统发育渐变论的首批详细证据。在大约三百万年的时间里，多达八个谱系的尾节肋数量出现净增加，这是一个物种诊断特征。大多数谱系的终端形态先前已被归入不同物种，在一个案例中甚至被归入不同属。然而，鉴于中间形态和暂时的趋势逆转，事实证明对每个谱系进行实际的分类细分是不可能的。早期林奈分类法（其暗示着离散物种）表面上的成功很容易被误解为间断平衡和停滞的证据，而且很可能许多其他渐变模式的发现因将双名分类法轻易应用于化石而受到阻碍。
