Adventures in the Pacific
Among these varied lands are some that originated as slivers wrenched from ancient supercontinents a hundred million years ago. Others are continental chips torn more recently from the great island of New Guinea. Yet others formed as volcanoes that belched forth from the ocean depths, arriving above the waves as virginal lands innocent of life until drifting seeds, spores and insects arrived. Krakatoa, which blew itself apart in a paroxysm of volcanic activity in 1883 and then grew anew from the sea, gives us some idea of the process. First ferns and insects, then flowering plants, birds and lizards arrived to colonize their new-found land. Krakatoa is just a few tens of kilometers from Java and Sumatra, and less than a century old. Imagine a volcano surfacing a thousand kilometers from the nearest land, then receiving its pilgrims over a million years.
There are other ways, too, for islands to form. Some are simply heaved above the waves by the movements of continental plates, while others, known as land-bridge islands, were, just 10,000 to 20,000 years ago, attached to much larger landmasses by connections now drowned in a rising sea. Some islands originated as combinations of several of these processes. But all islands, regardless of their origins, have one thing in common: transience. True, some islands are longer-lived than others, but all, in the unfolding of geological time, are destined to sink beneath the waves or amalgamate with larger landmasses. Over just the past few centuries, dozens of islands have been born and have died, and like us ultimately all of them will die, even as new islands are born.
On islands, evolution can be slowed down or speeded up. It can also take unlikely directions, fashioning novel creatures adapted to the particular conditions of the island. Why do islands have such peculiar powers over the evolutionary process? Imagine taking a species from the complex, rich continental ecosystem in which it has evolved, then releasing one or two randomly chosen individuals on an island where nothing like it has previously existed. If they survive, the individuals that begat the island population will have just a small subset of the species' genetic diversity, and this alone will wield an influence. To understand how, just imagine choosing two humans—say a red-head and a very tall person—and leaving them on a desert isle, then returning in a million years to examine the characteristics of their descendants.
But genetics is only the start, for when any creature reaches an island, it has effectively been transplanted into a new world. Its predators, competitors, diseases and even favored foods may not exist in its new home, and, instead of the boundless habitat of a continent, it finds itself part of a tiny population hemmed in on each side by sea. Such circumstances can greatly speed the evolutionary process. In the beginning the species is likely to breed up rapidly, for in the absence of predators and disease there is no check on its rate of increase. But soon it faces overpopulation: most individuals will die and only those with a particular advantage will survive. Perhaps those lucky few can utilize some food inaccessible to the rest, or perhaps they can conserve energy because they don't fly much, or perhaps they are smaller than average and can subsist on the slender resources the island affords them. Because the population is small and the selection of the survivors so rigorous, the evolutionary process is greatly accelerated. The influence of such powerful evolutionary pressure can be profound. As we see in the dodo, sometimes it creates beings which don't appear to belong here on Earth. Hardly anybody studies mammals on islands: it's those able colonizers the birds that usually get the attention. Yet some island mammals have, like the dodo, been as remarkably transformed. On islands, bats have been known to take on some of the characteristics of apes, and rats those of badgers, shrews or possums. Even humans and their behaviors are shaped by island life, and as a result island cultures have become as varied and novel as any on Earth.
Not all island life is established by vagrants arriving by raft, wind or wing. Islands that originate as slices of continents, severed from the mainland by powerful geological forces, carry with them a subset of continental life. When that happens, entire ecosystems are set adrift to adjust over millions of years to life in a small, isolated sphere. Inevitably, some species become extinct, unable to adjust to the limited circumstances they find themselves in. At the same time new species, which evolved in other parts of the continent, are unable to invade the now-isolated island. The result, for the island survivors, is often a slowing of evolutionary change. Because competition drives evolution, fewer competing species means less change, and in these circumstances evolution can come to a near halt. So it is that islands can become arks full of 'living fossils'—species whose relatives elsewhere are long extinct or transformed by evolution into very different kinds of creatures.
Evolution on islands also plays with the size of creatures. The world's islands are (or rather were) full of giant rats and tortoises, and oversized flightless birds. But there are island dwarves too. Before humans arrived, elephants, mammoths and hippos the size of Shetland ponies abounded on some islands in the Mediterranean and Arctic. There was even a miniature hominid, the hobbit, on the island of Flores in the Indonesian archipelago. Such gigantism among the tiny, and dwarfing of the great, means islands are great levellers, the species isolated on them converging on an ideal size.