Trees, bundles or nets?

Organisms are classified hierarchically. The reason why such a classification is appropriate is that organisms have arisen by a branching process: as Darwin realized, the best image of the evolutionary process is a tree. But the exchange of genetic material, in the sexual process and in more distant transfer events, means that sometimes a net is a more suitable image. Given the characteristics of a set of objects, can we decide how they arose, and whether a hierarchical classification is appropriate? A geometrical approach to this question has recently been suggested by Manfred Eigen and his colleagues.