Evolution by Pollution

Major evolutionary changes in most species of life take place at a barely perceptible rate, often requiring hundreds of thousands or even millions of years. One rare and classic exception to this rule, the peppered moth, underwent a major alteration during just 50 years in 19th century England--thus providing living proof of Darwin's still controversial theory of evolution. Now, three English scientists say, the peppered moth is at it again, changing rapidly once more in order to survive.

Before the smoke and soot from the burning coal and wood of the Industrial Revolution began to blacken the bark of England's trees, the predominant variety of peppered moth had light-colored, speckled wings that blended perfectly with lichen-covered tree bark and camouflaged the insects against predatory birds. A mutant form of the moth, with black wings, was easily spotted against the light-colored tree bark, picked off and eaten. The surviving black moths were so rare, in fact, that the first one was not captured until 1848.

As tree trunks grew progressively blacker, however, it became easier for birds to see and prey upon the speckled variety of the peppered moth. The once-vulnerable black moth, becoming indistinguishable against the darkening bark, found it easier to survive and multiply. By 1900, black peppered moths outnumbered the speckled variety by 99 to 1. Between 1952 and 1964, in a continuing survey made near one industrial town, not a single speckled moth was found. Eventually, this example of evolutionary forces at work became standard fare in college biology courses, illustrating the adaptability of organisms to a changing environment or--as Darwin called it--the principle of natural selection.

But man has intervened again to accelerate the pace of evolution. Since the passage of smoke-control laws in the 1950s, England's landscape has begun to emerge from its layers of soot. The cleaner trees thus provide a lighter--and safer--resting place for any surviving speckled moths.

Zoologists Laurence Cook and Richard Askew of Manchester University, and James Bishop of Liverpool University intermittently collected moths from 1966 to 1969 in the same area where the previous survey had been made. The results, they report in Nature, are "very highly significant." Of the 972 specimens collected, 25 were of the speckled variety, a clear indication that the peppered moth is again in the process of changing its hue.

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