A family tree becomes a tangle if you account for adopted children and remarriages. Evolution often resembles such a tangle: although the Standard Model, the traditional view of divergent evolution where species branch like a tree, paints a neat tree of species, in reality genes travel not only by inheritance but also directly between unrelated organisms — a process called horizontal gene transfer. The result is a tangled network where lineages intertwine, forming not a simple tree but a complex graph with multiple connections.
To measure how far a network is from a simple tree, scientists have devised a yardstick: how many extra connections need to be severed or imaginary branches added to untangle the web into a tree-like structure. In other words, they introduced quantitative metrics of 'treeness' that assess the minimum number of editing operations. They studied three approaches: one prescribes cutting only 'incorrect' connections: those that contradict the tree hierarchy (so-called non-tree edge removal); another cuts any connections to break the network into components without cycles; the third draws in ancestors, adding hypothetical nodes that restore branching. It turns out that the measures do not always agree: a network may look simple but require more edits due to tricky nodes — local configurations where connections form dense tangles.
Horizontal gene transfer is like suddenly receiving a gene from your neighbor and becoming better at digesting milk. In nature, this happens all the time, especially among microbes.