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Evolution__3rd_Edition

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..<br />

Fruitflies and lizards show a<br />

contrasting pattern<br />

Younger species inhabit younger<br />

islands<br />

CHAPTER 17 / <strong>Evolution</strong>ary Biogeography 503<br />

The fruitflies of the Hawaiian archipelago provide an instructive contrast. The phylogeny<br />

of this group is known from chromosomal inversions (Figure 15.27, p. 465).<br />

The phylogeny in Figure 15.27 is drawn on a map of the Hawaii archipelago. Inspection<br />

of it shows that, although many of the species evolved from ancestors on the same<br />

island, a large number of speciation events occurred following dispersal between<br />

islands. For example, at the bottom right of Figure 15.27, species 99 on Maui appears to<br />

have given rise to species 100 and 101 on Hawaii. 1 The reason for the difference between<br />

the Hawaiian fruitflies and the Caribbean lizards is unclear. It could be because the<br />

Hawaiian fruitflies on each island are not a regular set of ecological types, unlike the<br />

lizards, and did not evolve because of ecological competition.<br />

A further hypothesis can be tested with the Hawaiian frutiflies, because in this case<br />

we also know something about time. The islands were geologically formed successively,<br />

perhaps as the tectonic plate moved from east to west over a volcanic “hot spot” that<br />

threw up one island after another. The oldest islands are in the west; the youngest island<br />

is Hawaii in the east. The fruitflies may have radiated as colonizing fruitflies gave rise to<br />

new species after the younger islands emerged from the ocean waves. If so, we can predict<br />

a tendency for descendant species to be on younger islands, and ancestral species<br />

on older islands. Figure 17.6 shows the inferred colonization events, which mainly conform<br />

to the prediction. The tarweeds, a group of plants, show the same pattern. The<br />

pattern has also been shown elsewhere. On the Galápagos Islands, for instance, the<br />

youngest species tend to be on the youngest islands, and to have evolved from ancestors<br />

on the older islands.<br />

17.6 Species of large geographic areas tend to be more<br />

closely related to other local species than to<br />

ecologically similar species elsewhere in the globe<br />

In the Caribbean Anolis lizards, a twig-dwelling species on (for instance) Cuba is more<br />

closely related to a Cuban grass-dwelling Anolis than to a Haitian twig-dwelling species<br />

(even though the two twig-dwelling species look more alike). A similar principle can be<br />

seen at work on a much larger geographic scale. For instance, Mediterranean-type<br />

ecosystems can be found at five places around the globe: the Mediterranean itself,<br />

California, Chile, South Africa, and Western Australia. In all five places, plants have<br />

evolved with a similar set of adaptations to local conditions. Mediterranean plants can<br />

resist drought and fire, but not frost. Many of the plants are shrubs, and are hard and<br />

spiny. Animals, too, have evolved distinct Mediterranean types.<br />

1 The phylogenetic test in Figure 17.5 can also be compared with the test of sympatric speciation in African<br />

lake cichlids (Figure 14.4, p. 414). In the Caribbean lizards, as in the African cichlids, the closest relatives of a<br />

species are usually found on the same island (analogous to a lake). However, Figure 17.5 is probably weaker<br />

evidence for sympatric speciation than is Figure 14.4. Lizards may be more likely to form local populations<br />

within an island, and speciate allopatrically, whereas the fish roam more widely through the lakes a but it is<br />

also possible that the cichlids speciate by microallopatric speciation.

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