WEIT: Island Evolution

Last time I covered Coyne's evidence from the geographical distribution of fossils and current life on the world's continents. The rest of the chapter is based on the distinction between types of islands. Oceanic islands are land masses that rose up from beneath the sea—for example, the Hawaiian and Galapagos islands. In contrast, continental islands such as Japan (at right) and Madagascar began as part of the nearby continent but later broke off and drifted away.

When we study the life on these islands, here are the trends we find:

• Continental islands generally have every major type of animal just as the mainland does: insects, birds, amphibians, mammals, and so on.
• In contrast, oceanic islands often have native birds, plants and insects, but generally lack amphibians, reptiles, mammals and freshwater fish.
• There tend to be a huge number of bird, plant and insect species on oceanic islands, which are related to one another, but fill different niches.
• The species existing on oceanic islands tend to be the most similar to species that live on the closest mainland continent.

And now for the big question: why exactly do we see this unusual pattern? If you're a creationist... well, no real reason. God just felt like it, maybe. But even leaving aside the fact that this explanation lacks any evidence, it could be used to explain literally any pattern we happened to see. It is completely devoid of what scientists call explanatory power. A good explanation is detailed, non-arbitrary, makes testable predictions, and can be falsified. "God did it" has none of these traits.

The real answer is much more interesting. Unsurprisingly, the ecosystems of continental islands look normal because they were once part of the mainland. But what about oceanic islands? Their species are most similar to the nearest mainland species—for example, species from the Galapagos Islands (at right) resemble those from South America—because that's where they originated. And why do we see only certain forms of life on oceanic islands? Because there are practical limits to crossing large bodies of water. Birds can fly, while plant seeds and insects can attach to their feathers or be carried in droppings. Some seeds and bugs are even light enough to drift on the wind, and other seeds can survive for long periods floating in salty oceans. In contrast, amphibians and freshwater fish can't handle the saltwater, while reptiles and mammals can't swim far enough. Mammals like seals and bats are the lone exceptions and are found on oceanic islands, elegantly confirming this trend.

Finally, in the absence of competition, the species that are found on oceanic islands undergo rapid speciation to play a wide variety of roles. Coyne gives several examples. Of the 28 bird species in the Galapagos, half are finches—all descended from one ancestor. He says:

"[D]ifferent species specializ[e] on foods as different as insects, seeds, and the eggs of other species. The "woodpecker finch" is one of those rare species that uses tools—in this case a cactus spine or twig to pry insects from trees. ...And there's even a "vampire finch" that pecks wounds on the rear ends of seabirds and then laps up the blood."

There are also about 60 species of honeycreepers (with diverse feeding habits and beak shapes) and almost a thousand species of Drosophila fruit flies have been discovered on the Hawaiian islands. And on the South Atlantic island of St. Helena, members of the daisy and sunflower family have evolved into "small woody trees."

These examples are powerful displays of evolution's ability to mold both behavior and physical traits to suit the environment. Again, creationism is at a loss to explain these phenomena. Why did God make it so that there are nearly 1,000 species of Drosophila on one island group, and 14 finch species that variously feed on seeds, insects, fruit, eggs, nectar and blood on another? Well, now we know: he didn't.