Finding Descendants on a Phylogeny
Lets say we wanted to know the inverse of the Most Recent Common Ancestor (MRCA). In other words, how do we find all the descendants of some common ancestor?
To do so, we travel from some internal node towards the tips of the tree. At each subsequent node, we we continue down all branches that lead towards the tips. Repeat this process for every node encountered "downstream" of the ancestor. Once we have exhausted all possible branches and tips we could run into, we have our answer!
On the below phylogeny those pathways from internal node to the tips is highlighted by the arrows. Each of the descendants from the node is highlighted.
Of course we can also see that the most recent common ancestor for the Bushmaster, Fer-de-lance, Eyelash Viper, Rattlesnakes, Cottonmouth and Copperhead is highlighted in red!