For whatever reason, this morning I was remembering back to 10th grade Biology class, and the frustration I felt when trying to correct a classmate who said "well, bees aren't animals, right? They're insects!" I tried in vain to explain how different groups fit within the group called "animals," but I believe the moment I gave up was when she said that "well, not all mammals are animals, because humans are mammals but we're not animals." *sigh*
Perhaps I should have whipped out some paper and sketched an evolutionary tree. But trees are confusing to people who are new to the idea of cladistics. However, there is another option, and that is the Venn Diagram.
Now, Venn Diagrams are usually reserved for data sets that intersect. With cladistics there are no intersections (unless you get into hybridization, which I'm not going to do), but rather, data sets embedded within other data sets. They're also not the most efficient way to show clades, especially if you want to include a lot of groupings, but they can be excellent in introducing the idea of grouping.
Case in point, here's one by Ray Troll explaining why we are fish:
This view is also rather humbling, to see our group "Hominids" so tiny and so deeply embedded within so many stacking groups. Primates to mammals to amniotes to tetrapods to lobe finned fish to bony fish to vertebrates to chordates. We are all those things because we are grouped within them.
So how does a Venn clade compare to a normal branching one? Here's an example found here, at the website for the Universidad Autonoma de Madrid:
And here another example from an entry on Clades from the blog Evolving Thoughts comparing a branching clade to a Venn diagram.
These two illustrations also demonstrate a couple of important ideas in cladistics. The first is the idea of a paraphyletic group, represented here by both "Invertebrata" and "Reptilia." The other is a polyphyletic group, represented by "Homothermia" and "Crossopterygii."
And since 1. I'm already on the topic of Venn diagrams, and 2. I don't make fun of intelligent design nearly enough on this blog, and 3. I've been on a roll lately making trouble with people, I leave you with this!: