You can use "one-snip test" to identify monophyletic groups meaning that if you "cut" any branch on a tree, everything that "falls off" is a monophyletic group. why is this valid?
O one snip gets an ancestor and all species that share a similar trait.
O one snip gets an ancestor, all its descendants, and nothing else.
O one snip gets all species that share a similar trait.
O one snip gets an ancestor and some of its descendants, but not all.