“…light species of moths that got dark in 19th century England are reverting to light forms as air gets cleaner.”
—J. A. Bishop and Laurence M. Cook, entomologists, 1975
Who cares what happened to a local species of moth in the region of Manchester, England in the 1800s? Or what happened to the same bugs after World War II?
Well, evolutionary biologists care a great deal. That’s because the changes in the peppered moth population — from one of nearly all-white moths with small dark markings (peppered) to one of uniformly dark, almost coal-black moths, then back to the original mix — is seen as one of the best examples of evolution in action. Intensely studied for more than a century, these insect population shifts were described in 1978 by geneticist Sewell Wright as “the clearest case in which a conspicuous evolutionary process has actually been observed.”
But unless our personal agenda involves stepping into the “debate about evolution,” or just learning about science, why should the rest of us care? Simply put, because the rate at which these population shifts took place demonstrates the astonishing speed at which nature can and does respond to changing environmental conditions — including changes in atmospheric carbon levels.
Of the two types of peppered moths (a single species), typica, the light one with dark markings, is considered to be the “typical” representative of the species before the industrial revolution came to the English Midlands in the 1800s. That’s because typica is extremely well camouflaged as it rests on naturally light-and-dark-colored bark of the region’s oak and beech trees. (Technically, lichen that colonize the bark give the trees their coloration.) Their camouflage protected typica from being spotted and eaten by birds. However, the birds had little trouble spotting the Peppered Moth’s dark genetic variant carbonaria (named after carbon) against the same background.
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Thus, for thousands of years preceding the early 1800s, carbonaria represented only about two percent of the total peppered moth population. Then coal-powered industrial technology came to the textile mills of the midlands. With the coal came soot — not just from the factories, but also from the home fires of the factory workers who caused Manchester’s population to swell from approximately 70,000 in 1800 to 300,000 by 1850 and half a million by 1900.
The soot settled on the region’s trees, discoloring and killing the whitish lichen and turning the moths’ resting environment a dark gray. Birds now overlooked carbonaria but easily spotted typica. So during the five decades between 1840 and 1898, when a scientific survey was taken, carbonaria grew to represent an astonishing 98% of the region’s peppered moth population.
This is a virtually instantaneous change on a geological time scale — representing much less time than a snap of your fingers is to your lifetime. Then, in a dramatic reversal, the typica population rebounded between 1946, when Manchester’s air pollution was finally addressed and the region’s trees began to lighten, and the mid-1970s when several confirming population studies were conducted. Within 30 years, typica represented over half the region’s peppered moths.
While these phenomena speak for themselves in verifying evolution by natural selection, what concerns us here is the speed at which they demonstrate nature reacting. Just as Charles Darwin mistakenly believed that shifts in speciation (the origin of species) was an extremely gradual process, affected over hundreds of thousands or millions of years, most climatologists who began to sound the alarm about global warming in the 1980s and '90s thought significant effects would be experienced in a century or two.
They were wrong. More-frequent and intense superstorms have already become commonplace; global ice melt and dying oceans are a fact of life. Soon we will learn what pre-scientific peoples always knew: nature can strike almost without warning at our ecological house.