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?
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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: Spot the oldest common ancestor split.
Right after the root, the tree forks into two branches: one holds Species 5 and Species 6, the other holds Species 1, 2, 3 and 4. This fork is the oldest, and so the longest, split on the whole tree.
Step 2: Place each island's species on the tree.
Island Y has Species 1, 2 and 4, and every one of them sits on the side with Species 1, 2, 3, 4. None of Island Y's birds is on the Species 5, 6 side. Island X has Species 1, 4 and 6. Species 6 is on the Species 5, 6 side, while 1 and 4 are on the other side. Island Z has Species 3, 4 and 5. Species 5 is on the Species 5, 6 side, while 3 and 4 are on the other side.
Step 3: Think of phylogenetic diversity as a travel distance.
Imagine tracing the shortest path along the tree that touches every species on an island. The total length of that path is the phylogenetic diversity of the island. For Island Y, since all three birds sit on the same side of the oldest fork, the path never needs to use that fork's long branch, so the trip stays short. For Island X and Island Z, one bird sits on the far side of the oldest fork, so the path is forced to walk all the way back through that long branch and out again, making the trip much longer.
Step 4: Draw the comparison.
Since both X and Z are forced to use the same long, deep branch that Y never touches, both X and Z end up with a bigger total path length than Y. Between X and Z themselves, both pay for the same deep branch, so this reasoning alone cannot say which of the two is larger.
Step 5: Pick the option that fits.
An option that ranks all three strictly, putting X above or below Z, claims more than the tree supports. The one that only says X is bigger than Y and Z is bigger than Y is exactly what the tree tells us.
\[ \boxed{X \gt Y \text{ and } Z \gt Y} \]
