On a large scale, there can not be loops, unless we accept the existence of time travel. But there are indeed some complications that make the concept of a tree

On a large scale, there can not be loops, unless we accept the existence of time travel. But there are indeed some complications that make the concept of a tree of life surprisingly subtle: - ring species - species complexes - hybrid species (turning the tree into a Directed Acyclic Graph) - horizontal gene transfer via plasmids (between bacteria) or viruses (edited for formatting)