The phylogeny of six species of birds (Species 1 to Species 6) is illustrated below, drawn so that horizontal branch lengths correspond to genetic distance between the species. These six species are distributed across three islands: Island X harbours Species 1, 4, and 6; Island Y harbours Species 1, 2, and 4; and Island Z harbours Species 3, 4, and 5.
Which one of the following is true about the phylogenetic diversity of these islands?
Show Hint
Compare how much of the tree's branch length (genetic distance) is needed to connect each island's species; islands whose species span across the deep split near the root end up with higher phylogenetic diversity.
Step 1: Read the tree topology.
The tree splits at the root into two main branches. One branch leads to a shallow pair, Species 5 and Species 6, which are sisters. The other branch leads to a group holding Species 1, 2, 3, and 4, which itself splits into a Species 3 and Species 4 pair, and a Species 1 and Species 2 pair.
So the single deepest split in the whole tree, right at the root, separates Species 5 and 6 from Species 1, 2, 3, and 4.
Step 2: Recall what phylogenetic diversity means.
Phylogenetic diversity (PD) of a group of species is the total branch length of the tree needed to connect all of them, using the horizontal branch lengths as genetic distance.
Species that are close relatives sitting in the same small part of the tree give a small PD. A group that includes species from opposite sides of a deep split gives a large PD, because the long branch near the root gets added to the total.
Step 3: Check Island Y, which holds Species 1, 2, 4.
All three of these species sit inside the same branch of the tree that holds Species 1, 2, 3, and 4. None of them is Species 5 or 6.
So the smallest set of branches that connects them stays inside that one branch and never crosses the deep root split. This keeps the total branch length for Y small.
Step 4: Check Island X, which holds Species 1, 4, 6.
Species 1 and 4 sit in the branch with Species 1, 2, 3, 4, but Species 6 sits on the other side of the tree. Connecting Species 6 to Species 1 and 4 needs the long branch at the root that separates the two main groups. So the total branch length for X is bigger than the total for Y.
Step 5: Check Island Z, which holds Species 3, 4, 5.
Species 3 and 4 sit in the branch with Species 1, 2, 3, 4, but Species 5 sits on the other side, next to Species 6. Just like Island X, connecting Species 5 to Species 3 and 4 also needs the long root branch. So the total branch length for Z is bigger than the total for Y too.
Step 6: Try to rank X against Z.
Both X and Z include one species from the Species 5 and 6 side and two species from the Species 1, 2, 3, 4 side, so both cross the same deep root split once. The tree only lets us say both are bigger than Y. It does not give us a way to say whether X is bigger or smaller than Z. So ranking all three islands from largest to smallest goes further than the tree can support.
Step 7: Final answer.
Since Island X and Island Z both include a deep, cross group split that Island Y avoids, both X and Z are more phylogenetically diverse than Y.
\[ \boxed{X > Y \text{ and } Z > Y} \]
