Monday, February 22, 2010

Amateurs on Islands

A lot of new species have evolved on islands. Sometimes these are metaphorical islands, such as spots of tundra at the tops of mountain peaks, separated from other such spots of tundra by figurative oceans of forest and plain. But usually they are literal islands. I refer not to islands that are actually chunks of continent which, like New Caledonia, carry with them entire floras and faunas. I am talking about new islands that have arisen out of the sea, whether from Darwinian coral atolls or from volcanoes. I have visited such islands: Hawaii, like hundreds of millions of other people, and the Galapagos Islands, like hundreds of thousands of other people. I got up close to some of the Galapagos inhabitants, such as a giant tortoise that I kissed (see the photo).

Islands are evolutionary laboratories, where populations can “try out” new adaptations in isolation from mainland populations. But island evolution is a lot more than just isolation. The populations of plants and animals on islands experience very different conditions from those on the mainland. In many cases this is because the animals and plants that happen to arrive on islands can “try out” new adaptations, and be amateurs. These amateurs would be soundly defeated by experts—but the experts, for whatever reason, did not arrive on the island.

One example is the woodpecker finch, one of Darwin’s finches on the Galapagos islands. This species (Cactospiza pallidus) uses sticks to pry into holes in trees and pry out insects. They do a mediocre job of it, compared to woodpeckers. But the Galapagos islands have no woodpeckers, so the woodpecker finch is able to be successful in this ecological niche.

Another example is the tree-sized sunflowers on St. Helena, and other such species on some of the California channel islands. Sunflowers, like other members of its plant family, are herbaceous plants. Their stems are not built for supporting a lot of weight, which is one reason they never grow very big. If any population of sunflowers on the mainland “tried out” growing as tall as a tree, the expert trees with strong wood (such as oaks) would shade them into extinction. But the seeds that arrived on these islands were of herbaceous plants; there were no trees to compete with the large herbaceous plants, which have evolved into trees (small ones).

If the world was one big habitat, the expert species would rule. There would be lots of species but not much room for newcomers. Because of islands, there are spaces for newcomers, like coffee house performers not having to take on Hollywood. And once in a while, new talent emerges from such islands and changes the world.

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