It falls outside the scope of the formal taxonomic categories.它不属于正规植物分类学的分类阶元的范畴。
For instance, all birds and reptiles are believed to have descended from a single common ancestor, so this taxonomic grouping is called monophyletic.例如，所有的鸟类和爬行动物被认为是从一个单一的共同祖先传下来的，因此这个分类学的分组被称为单系类群。
