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?