concept

Monophyly

The property of a group containing one ancestor and **all** of its descendants, with none left out. It is the only kind of group that corresponds to a real branch on the tree of life, which makes it the goal of any serious classification. The two common failures are leaving descendants out — "reptiles" excluding birds is the classic case — and lumping together things that merely look alike. For a flora the second trap is especially dangerous, because the tree habit, the pitcher trap and the succulent stem have each been invented many times in unrelated lineages.