Following on from my previous post about visualising the mammalian classification in Wikipedia, I've extracted the largest component from the graph for all mammal taxa in Wikipedia, and it is a tree. This wasn't apparent in the previous diagram, where the component appeared as a big ball due to the layout algorithm used.
What this suggests is that Wikipedia contributors are quite capable of generating trees, it's just that not all the bits of the tree are connected (hence all the components in the previous post.
As Cyndy Parr suggested in her comments, it would be useful to compare the Wikipedia-derived tree with other trees, say from Mammal species of the World or ITIS.