Who is the closest relative?

Q: ...So closest related is same as asking who shares the more recent common ancestor, but which way do you go on the cladogram-up or down? For example, on lecture 5, slide 75, I don't understand why its the duck when the snail is closer to the fish? And wouldn't the snail be the most recent ancestor?

A: This is a common source of confusion and it's partly because there is some merit to your interpretation, especially when we start saying that certain fish (lungfish) are more closely related to cows than they are to other fish. But, the way cladistics defines relatedness has the advantage of being a clearly defined consistent definition. And it is, as you said, whoever shares the more recent common ancestor. In slide 75, lecture 5, note the time arrow. Any common ancestor higher up on the chart is a more recent common ancestor. To find the common ancestor you go down the cladogram (back in time), and never back up, from each of the taxa until you find a common point. For the fish and the snail that is the bottom of the chart (the first divergence = the oldest divergence = the oldest common ancestor). For the fish and the duck it is the node above, so the fish and the duck share a more recent common ancestor.

Q: what about "sister groups?" Is that the same as a common ancestor?

A: Yes, it's much the same thing. Sister groups share a common ancestor. It's the same as asking "who is the taxon's closest relative. Or, with whom does it share its most recent common ancestor.

I've posted an additional handout related to all this: branchingdiagrams.pdf.