Why Evolution Is True (19 page)

Animals and plants can also hitch rides to islands on “rafts”—logs or masses of vegetation that float away from continents, usually from the mouths of rivers. In 1995 one of these large rafts, probably blown by a hurricane, deposited a cargo of fifteen green iguanas on the Caribbean island of Anguilla, where they had not previously existed, from a source two hundred miles away. Logs of Douglas fir from North America have been found on Hawaii, and logs from South America have made it to Tasmania. Rafting like this explains the presence of the occasional endemic reptile on oceanic islands, such as the Galapagos iguanas and tortoises.

Further, when you look at the type of insects and plants native to oceanic islands, they are from groups that are the best colonizers. Most of the insects are small, precisely those that would be easily picked up by wind. Compared to weedy plants, trees are relatively rare on oceanic islands, almost certainly because many trees have heavy seeds that neither float nor are eaten by birds. (The coconut palm, with its large, buoyant seeds, is a notable exception, occurring on almost all Pacific and Indian Ocean islands.) The relative rarity of trees, in fact, explains why many plants that are short weeds on continents have evolved into woody treelike forms on islands.

Terrestrial mammals are not good colonizers, and that’s why oceanic islands lack them. But they don’t lack
all
mammals. This brings up two exceptions that prove the rule. The first was noted by Darwin:

And there are also
aquatic
mammals on islands. Hawaii has one, the endemic monk seal, and the Juan Fernández group has a native fur seal. If native mammals on oceanic islands were not created, but descended from colonists, you’d predict that those ancestral colonists must have been able to fly or swim.

Now, it’s clear that long-distance dispersal of a given species to a distant island can’t be a frequent event. The chance that an insect or bird could not only traverse vast expanses of sea to land on an island, but also establish a breeding population once it got there (this requires either an already fertilized female or at least two individuals of opposite sex), must be very low. And if dispersal were common, life on oceanic islands would be quite similar to that of continents and continental islands. Nevertheless, most oceanic islands have been around for millions of years, long enough to permit some colonization. As the zoologist George Gaylord Simpson remarked, “Any event that is not absolutely impossible ... becomes probable if enough time passes.” To take a hypothetical example, suppose that a given species has only one chance in a million of colonizing an island each year. It’s easy to show that after a million years have passed, there is a large probability that the island would have been colonized at least once: 63 percent, to be exact.

One final observation closes the chain of logic that secures the case for evolution on islands. And that is this: with few exceptions, the animals and plants on oceanic islands are most similar to species found on the nearest mainland. This is true, for example, of the Galapagos Islands, whose species resemble those from the west coast of South America. The similarity can’t be explained by the argument that the islands and South America have similar habitats for divinely created species, because the Galapagos are dry, treeless, and volcanic—quite different from the lush tropics that dominate the Americas. Darwin was especially eloquent on this point:

What is true of the Galapagos is also true of other oceanic islands. The closest relatives of the endemic plants and animals on Juan Fernández come from the temperate forests of southern South America, the closest continent. Most of the species on Hawaii are similar (but not identical) to those from the nearby Indo-Pacific region—Indonesia, New Guinea, Fiji, Samoa, and Tahiti—or from the Americas. Now, given the vagaries of winds and the direction of ocean currents, we don’t expect
every
island colonist to come from the closest source. Four percent of Hawaiian plant species, for example, have their closest relatives in Siberia or Alaska. Still, the similarity of island species to those on the nearest mainland demands explanation.

To sum up, oceanic islands have features that distinguish them from either continents or continental islands. Oceanic islands have unbalanced biotas— they are missing major groups of organisms, and the same ones are missing on different islands. But the types of organisms that
are
there often comprise many similar species—a
radiation—
and they are the types of species, like birds and insects, that can disperse most easily over large stretches of ocean. And the species most similar to those inhabiting oceanic islands are usually found on the nearest mainland, even though their habitats are different.

How do these observations fit together? They make sense under a simple evolutionary explanation: the inhabitants of oceanic islands descended from earlier species that colonized the islands, usually from nearby continents, in rare events of long-distance dispersal. Once there, accidental colonists were able to form many species because oceanic islands offer lots of empty habitats that lack competitors and predators. This explains why speciation and natural selection go wild on islands, producing “adaptive radiations” like that of the Hawaiian honeycreepers. Everything fits together if you add accidental dispersal, which is known to occur, to the Darwinian processes of selection, evolution, common ancestry, and speciation. In short, oceanic islands demonstrate every tenet of evolutionary theory.

It’s important to remember that these patterns do not generally hold for continental islands (we’ll come to an exception in a second), which share species with the continents to which they once were joined. The plants and animals of Great Britain, for example, form a much more balanced ecosystem, having species largely identical to those of mainland Europe. Unlike oceanic islands, continental islands were cut adrift with most of their species already in place.

Now try to think of a theory that explains the patterns we’ve discussed by invoking the special creation of species on oceanic islands and continents. Why would a creator happen to leave amphibians, mammals, fish, and reptiles off oceanic islands, but not continental ones? Why did a creator produce radiations of similar species on oceanic islands, but not continental ones? And why were species on oceanic islands created to resemble those from the nearest mainland? There are no good answers—unless, of course, you presume that the goal of a creator was to make species
look
as though they evolved on islands. Nobody is keen to embrace that answer, which explains why creationists simply shy away from island biogeography.

We can now make one final prediction.
Very old
continental islands, which separated from the mainland eons ago, should show evolutionary patterns that fall between those of young continental islands and oceanic islands. Old continental islands such as Madagascar and New Zealand, cut off from their continents 160 million and 85 million years ago, respectively, will have been isolated before many groups like primates and modern plants had evolved. Once these islands parted from the mainland, some of their ecological niches remained unfilled. This opens the door for some later-evolving species to successfully colonize and establish themselves. We can predict, then, that these old continental islands should have a
somewhat
unbalanced flora and fauna, showing some of the biogeographic peculiarities of true oceanic islands.

And indeed, this is just what we find. Madagascar is famous for its unusual fauna and flora, including many native plants and, of course, its unique lemurs—the most primitive of the primates—whose ancestors, after arriving in Madagascar some 60 million years ago, radiated into more than seventy-five endemic species. New Zealand too has many natives, the most well-known being flightless birds: the giant moa, a thirteen-foot-tall monster hunted to extinction by about 1500, the kiwi, and that fat, ground-dwelling parrot, the kakapo. New Zealand also shows some of the “imbalance” of oceanic islands: it has only a few endemic reptiles, only one species of amphibian, and two native mammals, both bats (though a small fossil mammal was recently found). It too had a radiation—there were eleven species of moas, all now gone. And, like oceanic islands, the species on Madagascar and New Zealand are related to those found on the nearest mainland: Africa and Australia, respectively.

THE MAIN LESSON OF BIOGEOGRAPHY is that only evolution can explain the diversity of life on continents and islands. But there is another lesson as well: the distribution of life on earth reflects a blend of chance and lawfulness. Chance, because the dispersal of animals and plants depends on unpredictable vagaries such as winds, currents, and the opportunity to colonize. If the first finches had not arrived in the Galapagos or Hawaii, we might see very different birds there today. If an ancestral lemurlike creature hadn’t made it to Madagascar, that island (and likely the earth) would have no lemurs. Time and chance alone determine who gets marooned; one might call this the “Robinson Crusoe effect.” But there is also lawfulness. Evolutionary theory predicts that many animals and plants arriving in new and unoccupied habitats will evolve to thrive there, and will form new species, filling up ecological niches. And they will usually find their relatives on the nearest island or mainland. This is what we see, over and over again. One cannot understand evolution without grasping its unique interaction between chance and lawfulness—an interaction that, as we’ll see in the next chapter, is critically important in understanding the idea of natural selection.

But the lessons of biogeography go further, into the realm of biological conservation. Island plants and animals adapt to their environments isolated from species that live elsewhere, their potential competitors, predators, and parasites. Because species on islands don’t experience the diversity of life found on continents, they aren’t good at coexisting with others. Island ecosystems, then, are fragile things, easily ravaged by foreign invaders who can destroy habitats and species. The worst of these are humans, who not only chop down forests and hunt, but also bring with them an entourage of destructive prickly pears, sheep, goats, rats, and toads. Many of the unique species on oceanic islands are already gone, victims of human activity, and we can confidently (and sadly) predict that many more will vanish soon. In our lifetime we may see the last of the Hawaiian honeycreepers, the extinction of New Zealand’s kakapos and kiwis, the decimation of the lemurs, and the loss of many rare plants that, while perhaps less charismatic, are no less interesting. Each species represents millions of years of evolution and, once gone, can never be brought back. And each is a book containing unique stories about the past. Losing any of them means losing part of life’s history.

Chapter 5

The Engine of Evolution

—Robinson Jeffers, “The Bloody Sire”

 

 

 

O
ne of the marvels of evolution is the Asian giant hornet, a predatory wasp especially common in Japan. It’s hard to imagine a more frightening insect. The world’s largest hornet, it’s as long as your thumb, with a two-inch body bedecked with menacing orange and black stripes. It’s armed with fearsome jaws to clasp and kill its insect prey, and a quarter-inch stinger that proves lethal to several dozen Asians a year. And with a three-inch wingspan, it can fly twenty-five miles per hour (far faster than you can run), and can cover sixty miles in a single day.

This hornet is not only ferocious, but voracious. Its young larval grubs are fat, insatiable eating machines, who insistently rap their heads against the hive to signal their hunger for meat. To satisfy their relentless demands for food, adult hornets raid the nests of social bees and wasps.

One of the hornet’s prime victims is the introduced European honeybee. The raid on a honeybee nest involves a merciless mass slaughter that has few parallels in nature. It starts when a lone hornet scout finds a nest. With its abdomen, the scout marks the nest for doom, placing a drop of pheromone near the entrance of the bee colony. Alerted by this mark, the scout’s nestmates descend on the spot, a group of twenty or thirty hornets arrayed against a colony of up to thirty thousand honeybees.