The Island of the Colorblind

Biologically, as well as geologically, continental islands (such as, for instance, New Zealand, Madagascar, or New Guinea) are entirely different from oceanic ones. For continental islands are broken-off pieces of the main and (at least initially) may have all the species of the parent continent. Once broken off, of course, they become as isolated as any other island, and their isolation (and altered conditions) may promote the most extravagant speciation, as with the unique primates of Madagascar or the flightless birds of New Zealand.

There are also diseases endemic to islands, diseases which have emerged or persisted because of their isolation, and are thus analogous to an island’s endemic flora and fauna. This too was recognized more than a century ago, by the great German epidemiologist Hirsch. The study of such diseases, he thought, would constitute a ‘geographical and historical pathology’ and such a science, he wrote, ‘in an ideally complete form, would furnish a medical history of mankind.’

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More than forty varieties of banana are grown on Pohnpei, and some of these seem to be unique to the island. The banana has a remarkable tendency to somatic mutation, to ‘sports’ – some of these are disadvantageous, but others may lead to plants which are more disease resistant, or fruit which is more delectable in one way or another; and this has stimulated cultivation of some five hundred varieties worldwide.

The major banana sports are regarded as species (and given binomial, Linnaean names), the minor sports as varieties only (which bear only local names). But the difference, as Darwin remarks, is only one of degree: ‘Species and variations,’ he writes in the Origin, ‘blend into each other by an insensible series; and a series impresses the mind with the idea of an actual passage.’ In time, many varieties will diverge sufficiently to become distinct species.

The importation of bananas onto islands, as it happens, has also shown us something of the rate of evolution in sympatric species. Thus, as H.W. Menard notes, ‘Five new species of banana moths have evolved in Hawaii since the Polynesians introduced the banana to Hawaii only about one thousand years ago.’ For islands are forcing grounds for evolutionary change, whether of plants or animals, insects or microbes; under the special conditions of island life, the slow processes of mutation and specialization may be amplified and accelerated to a spectacular degree.

J.B.S. Haldane once proposed a way of quantifying the rate of change of any variable – a bird’s beak, an ammonite’s whorl – as it evolved, suggesting that a change of one percent per million years be called a ‘darwin.’ Evolution generally proceeded, he thought, in ‘millidarwins,’ and he imagined (as Darwin himself did) that with this infinitesimal rate evolution could never actually be seen. But we are now finding (as Jonathan Weiner recounts in The Beak of the Finch ) that evolution can occur at a very much faster rate when selection pressures are high. This has been studied by Peter and Rosemary Grant, with the very finch populations Darwin himself observed, on the small Galapagos island of Daphne Major. Following a catastrophic drought, the finch population showed clear evolutionary changes (in beak and body size) in a matter of months, an ‘evolutionary rate,’ Weiner calculates, of 25,000 darwins.

One does not need to deal only with rare and catastrophic circumstances to see evolution in action. A beautiful example has recently been observed by Martin Cody and Jacob Overton with the seeds of some daisies, which are blown by the wind to small islets off the Pacific coast of Canada. A fluffball or pappus holds the seed aloft, and its size determines, other things being equal, how far the seed is liable to be carried. Once the plants have settled on an island, their pappi become shorter, so they are less liable to be dispersed. These changes, like those of finches, have been observed within the span of a year or two.

But the most astounding example of very rapid, massive evolution relates to the more than three hundred species of cichlid fish unique to Lake Victoria. DNA studies (by Axel Meyer) have indicated that these species diverged very recently in evolutionary terms, and there is now strong geologic evidence that the lake itself is only 12,000 years old. While Darwin’s Galapagos finches evolved perhaps twenty different species over four million years, the cichlids of Lake Victoria have shown a rate of speciation more than five thousand times greater.

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Jack London, in Uaitape, found Bora-Borans dancing ‘with strange phosphorescent flowers in their hair that pulsed and dimmed and glowed in the moonlight.’

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