Evolution__3rd_Edition

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Fruitflies and lizards show a
contrasting pattern
Younger species inhabit younger
islands
CHAPTER 17 / Evolutionary Biogeography 503
The fruitflies of the Hawaiian archipelago provide an instructive contrast. The phylogeny
of this group is known from chromosomal inversions (Figure 15.27, p. 465).
The phylogeny in Figure 15.27 is drawn on a map of the Hawaii archipelago. Inspection
of it shows that, although many of the species evolved from ancestors on the same
island, a large number of speciation events occurred following dispersal between
islands. For example, at the bottom right of Figure 15.27, species 99 on Maui appears to
have given rise to species 100 and 101 on Hawaii. 1 The reason for the difference between
the Hawaiian fruitflies and the Caribbean lizards is unclear. It could be because the
Hawaiian fruitflies on each island are not a regular set of ecological types, unlike the
lizards, and did not evolve because of ecological competition.
A further hypothesis can be tested with the Hawaiian frutiflies, because in this case
we also know something about time. The islands were geologically formed successively,
perhaps as the tectonic plate moved from east to west over a volcanic “hot spot” that
threw up one island after another. The oldest islands are in the west; the youngest island
is Hawaii in the east. The fruitflies may have radiated as colonizing fruitflies gave rise to
new species after the younger islands emerged from the ocean waves. If so, we can predict
a tendency for descendant species to be on younger islands, and ancestral species
on older islands. Figure 17.6 shows the inferred colonization events, which mainly conform
to the prediction. The tarweeds, a group of plants, show the same pattern. The
pattern has also been shown elsewhere. On the Galápagos Islands, for instance, the
youngest species tend to be on the youngest islands, and to have evolved from ancestors
on the older islands.
17.6 Species of large geographic areas tend to be more
closely related to other local species than to
ecologically similar species elsewhere in the globe
In the Caribbean Anolis lizards, a twig-dwelling species on (for instance) Cuba is more
closely related to a Cuban grass-dwelling Anolis than to a Haitian twig-dwelling species
(even though the two twig-dwelling species look more alike). A similar principle can be
seen at work on a much larger geographic scale. For instance, Mediterranean-type
ecosystems can be found at five places around the globe: the Mediterranean itself,
California, Chile, South Africa, and Western Australia. In all five places, plants have
evolved with a similar set of adaptations to local conditions. Mediterranean plants can
resist drought and fire, but not frost. Many of the plants are shrubs, and are hard and
spiny. Animals, too, have evolved distinct Mediterranean types.
1 The phylogenetic test in Figure 17.5 can also be compared with the test of sympatric speciation in African
lake cichlids (Figure 14.4, p. 414). In the Caribbean lizards, as in the African cichlids, the closest relatives of a
species are usually found on the same island (analogous to a lake). However, Figure 17.5 is probably weaker
evidence for sympatric speciation than is Figure 14.4. Lizards may be more likely to form local populations
within an island, and speciate allopatrically, whereas the fish roam more widely through the lakes a but it is
also possible that the cichlids speciate by microallopatric speciation.