Cladistics involves using shared, derived plant characteristics as a means of hypothesizing evolutionary relationships. To give an example, if two species of plants share the derived characteristic of toothed leaves, while a third lacks such teeth, we could hypothesize that the two species with teeth were most closely related to each other. It sounds very straightforward, but cladistics has only been used for about twenty years. DNA testing, which is even newer, allows us to make the same kind of analysis, but using many, many more characteristics from the plants. We've probably learned more about the evolutionary relationships of plants in the last ten years than in the previous 100.
Some people think plant taxonomy is long-finished with naming the earth's organisms.
After all, we've been doing this for several hundred years. Previously, if a botanist wanted to find out which species were most closely related to each other in terms of common ancestry, there was no way to really test this. But now if I make an evolutionary hypothesis, my ideas can be tested and verified. For example, a few years ago I examined a small tree in the higher elevations of Puerto Rico, a particular species called the Calycogonium squamulosum. So I published a paper proposing the plant be renamed Henriettea squamulosum.