Evolution__3rd_Edition

464 PART 4 / Evolution and Diversity
Figure 15.26
Overlapping inversions, in
different species, can be used
to infer their phylogenetic
relations, in the form of an
unrooted tree. With this pattern
of inversions, the tree must be
1 ↔ 2 ↔ 3, and not 1 ↔ 3 ↔ 2
or 3 ↔ 1 ↔ 2.
Different inversions show no
conflict
The tree can be rooted
Species
1
2
3
a b c d e f g h i j
Inversion 1
a b g f e d c h i j
Inversion 2
a b g f e i h c d j
Unrooted tree 1 2 3
Genes on chromosome
To apply the technique, we first have to work out the chromosomal banding patterns
of the group of species. Then one species is picked, more or less arbitrarily, as the
“standard” against which the other species are compared. Starting with the species
that have a chromosomal banding pattern most like the standard, we gradually work
outwards through the tree until all have been included. Carson concentrated on the
“picture-wing” group of Hawaiian drosophilids. Figure 15.27 is a phylogeny, based
on 214 inversions, of 103 of the 110 or so known species in this group. It is a marvellous
piece of work. We can get some idea of how certain the inference is from the fact that
none of the 214 inversions contradict the phylogeny; the characters all agree.
Figure 15.27 is an unrooted tree. The root can be located by two independent lines
of evidence. One is to look outside the archipelago for the nearest outgroup and see
what banding pattern it has. The fruitflies that are thought to be the nearest outgroup
of the picture-wing group live in South America and are most similar to Drosophila
primaeva (species number 1) and D. attigua (species number 2) among the fruitflies
in Figure 15.27. These species are therefore probably closest to the root of the tree. The
inference is supported by the geological history of the archipelago. Kauai is the oldest
island and Hawaii the youngest, so the ancestor of the group would probably have
colonized Kauai. If that ancestral species still survives, it is probably still on Kauai,
because almost every Hawaiian drosophilid is confined to a single island; for example,
D. primaeva and D. attigua live on Kauai. Indeed, much of the phylogenetic history of
the picture-wing group consists of populations of species on the older islands moving
to the younger islands, where they form new species by allopatric speciation. There are
no examples of species on older islands that are derived from a species on a younger
island. Thus the youngest island, Hawaii, has the most recently evolved species and the
oldest islands in the west have the most ancient species (Figure 17.6, p. 504).
..