The tree of the lesson has four species…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The tree of the lesson has four species. P and Q join first. Then R joins the group PQ. At last S joins the group PQR.
- Which two species share the most recent common ancestor?
- Is R more closely related to P or to Q?
- Which species split off first from the others?
- Does it change the meaning of the tree if you swap the two branches at the join of P and Q?
Need a reminder? The key ideaShow
UPGMA joins the closest pair at half their distance, and averages the distances to the new group. The molecular clock gives , because both lines change. Closer relatives share a more recent common ancestor.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) P and Q. Their join is the first and the most recent.
Step 2. (b) R is equally related to both. R joins the group PQ as a whole, so it has the same common ancestor with P and with Q.
Step 3. (c) S joined last, so S split off first.
Step 4. (d) No. A fork can be turned around without changing which species share which ancestor.
Answer: (a) P and Q; (b) equally related; (c) S; (d) no.
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