Evolution__3rd_Edition

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The fossil characters were
reinterpreted
Fruitflies have radiated in Hawaii
Their phylogeny is inferred from
chromosomal inversions
CHAPTER 15 / The Reconstruction of Phylogeny 463
other field). The controversy has now been settled (with a few dissenters) in favor of
the original molecular evidence. The morphological characters previously believed
to show a relation between Homo and Ramapithecus succumbed to reanalysis. The
dental arcade of Ramapithecus had been wrongly reconstructed (originally by combining
parts from different specimens). The reduced canine teeth may be because the
fossil Ramapithecus specimens were female. Martin (1985) finally removed the last
important character a thickened enamel a by reinterpreting it as an ancestral character.
Moreover, when Ramapithecus was compared with another fossil (Sivapithecus)
that was generally accepted to be a close relative of the orang-utan, and with the
orang-utan itself, it was found to show clear similarities to them. The specimens
formerly classified as Ramapithecus are now usually included in the genus Sivapithecus,
which in turn is thought to be a close relative of the ancestors of modern orang-utans
(Figure 15.25b).
In summary (simplifying things a little), molecular evidence helped to inspire a
reanalysis of the fossil evidence for human origins a with the result that a figure of
about 5 million years, and at any rate in the 4–8 million year range, is now widely
accepted for the time of origin of the hominin lineage.
15.14 Unrooted trees can be inferred from other kinds of
evidence, such as chromosomal inversions in
Hawaiian fruitflies
Many other individual techniques are useful for inferring the phylogeny of particular
taxonomic groups. We can look, for instance, at a particularly powerful technique that
has been used with fruitflies. For some reason, an extraordinarily large number of
species of fruitflies (Drosophila) live in the Hawaiian archipelago. There are probably
about 3,000 drosophilid species in the world, and about 800 of them appear to be confined
to this archipelago. The phylogeny of one subgroup of the Hawaiian fruitflies
is better known than that of any other equivalently large group of living creatures.
It was worked out, by Carson and his colleagues (see Carson 1983), from chromosomal
banding patterns. Chromosome bands are clearly visible in fruitflies (Section 4.5,
p. 82).
The banding patterns differ between species, and it soon becomes obvious that
regions of the chromosomes have been inverted during evolution: a segment of genes
within a chromosome has been inverted as a whole. The important event, for phylogenetic
inference, is when a second inversion takes place across the end of an earlier inversion
(Figure 15.26). When this happens, we can infer with near certainty that the
unrooted tree is 1 ↔ 2 ↔ 3, not 1 ↔ 3 ↔ 2. If species 1 had evolved directly into species
3, and then 3 into 2, the two inversions in Figure 15.26 would be needed for the evolution
of species 3; then, to go to species 2, the exact same two breaks (one at each end) of
the second inversion would have to happen again in reverse a which is much less probable
than evolution in the order 1 ↔ 2 ↔ 3. As more species are added, with more overlapping
inversions, the improbability of most alternative trees multiply to the point of
practical impossibility.