By Ardea Skybreak

Revolutionary Worker #1159, July 21, 2002, posted at

The evolution by natural selection of industrial melanism* in the peppered moth species Biston betularia (as well as in many other insect species) is a well-known and well- documented phenomenon which is discussed and illustrated in just about every biology textbook. Recently, however, some articles have appeared in the press reporting that some scientists have challenged the validity of some of the original experiments done by H.B.D. Kettlewell in England in the 1950s, making it seem that this classical example of evolution by natural selection rests on shaky ground. Those of you who detect the smell of Creationists in this "controversy" would be right: for instance, one of the main people stirring up a fuss about the peppered moth example is none other than Jonathan Wells, a Berkeley-based Intelligent Design Creationist (what a blight on Berkeley!!) and a senior fellow at the Discovery Institute's Center for the Renewal of Science and Culture--a small Seattle based "nest" of IDCs that sends speakers around the country to debate evolutionists and promote Intelligent Design Creationism in opposition to evolution. In other words, when you hear that the classical example of evolution in action in the peppered moth is being "challenged," it might be important to consider the source!

Nevertheless, since even a broken clock can be right twice a day, we owe it to ourselves to always keep an open mind and recognize the possibility that new information could reverse prior scientific findings. But this seems unlikely in the case of the evolution of industrial melanism by natural selection in the peppered moth or in other insects. No doubt the very last word has not been written on this story--for instance, there may still be much to learn about how other selective factors (in addition to bird predation) may lead to the observed changes in gene frequencies in the moth populations. But the basic story holds: there is absolutely zero doubt that peppered moth populations evolved (from being made up mainly of light moths to being made up mainly of dark moths) over just a few decades and in direct correlation to changes in the level of industrial pollution. There is also no doubt that this evolutionary trend reversed itself when the levels of industrial pollution began to decrease. This whole trend was shown through careful surveys of moth populations in England between the 1950s and the 1990s. And, more recently, the biologist Bruce Grant has documented a striking case of parallel evolution of industrial melanism in peppered moths in the United States, comparing the relative proportions of light and dark variants in moth populations near Detroit, also between the 1950s and the 1990s. Just as in England, the proportion of dark variants increased with increases in pollution and then began to significantly decrease once again as air quality improved. There is also clear evidence that predation by birds has significant impacts on moth populations--birds are known to consume large proportions of adult moths in any population.

While Kettlewell's experiments in the 1950s may not have been absolutely perfectly designed (few experiments ever are) the basic picture which emerged from his work seems to be holding up: a) the systematic surveys of the proportions of the different variants in moth populations over several decades conclusively show that moth populations definitely did evolve over time, first in one and then in the other direction . Again, there is absolutely no doubt about this. b) Kettlewell's classical mark/release/recapture experiments demonstrated that when live light-colored and dark-colored moths were released together in polluted areas (where the trees were darker), twice as many dark variants were recaptured in succeeding days than light variants, indicating that, for whatever reason or combination of reasons, significantly more dark variants were managing to survive in polluted environments; and his experiments also showed that the exact opposite was the case with releases of both types of moths in unpolluted areas (where the trees were lighter): in that situation twice as many light variants as dark variants were recaptured, indicating that some factor was significantly favoring the survival of light variants in the unpolluted areas. c) In addition, Kettlewell and other scientists actually observed birds preferentially picking off light moths in polluted environments, and dark moths in unpolluted environments. d) Finally, in an attempt to further confirm this differential bird predation experimentally, Kettlewell pinned fresh specimens of both light and dark moths to dark tree trunks, and of both light and dark moths to light tree trunks. He then observed and recorded how the birds in fact much more rapidly removed the more readily visible forms in each case--the light moths on the dark background or the dark moths on light backgrounds.**

The Creationists have tried to sow confusion about the example of evolution in the peppered moth by saying that moths do not naturally rest on tree trunks during the day (when they actually do!) and by suggesting that Kettlewell's experiments were "unnatural, and "staged" because birds would not normally encounter moths pinned to tree trunks. This kind of thing is typical of the misleading tactics of anti-evolution Creationists. Every scientist knows that it is perfectly valid to conduct experimental manipulations of nature as part of trying to figure out what's going on. All such experimental manipulations are obviously "unnatural" in some respects, but they nevertheless provide us with valuable information (in this case confirming that birds tend to more readily pick off the more conspicuous variant in each environment). Such experimental manipulations are a perfectly legitimate and necessary part of scientific research, and for the Creationists to suggest that they represent some kind of semi-fraudulent "staging" is a gross mischaracterization of fact.

Kettlewell and his colleagues explained how they had designed these experiments, and there was nothing improper or willfully misleading about their approach. But the Creationists insist on confusing people by tossing out a few lies (such as saying that moths don't rest on tree trunks, when they do), making a few out of context references to how Kettlewell "staged" some experiments (to make it seem as if they'd uncovered some dirty secret or evidence of fraud instead of an openly described and valid experimental technique), and then picking out a few points which might in fact be worth further exploring (such as whether Kettlewell overemphasized the role of lichens in moth camouflage) while conveniently managing to leave out any discussion of the fact that the totality of observations and studies of the peppered moth populations leave absolutely no doubt that these moth populations have in fact evolved , and that people have actually been able to observe this evolutionary change taking place over just a few decades. Again, this is typical of creationist methodology: they often combine lies, distortions, and very selective discussions of only small parts of the overall picture to give people the false impression that the evidence of evolution must not be such a well established fact after all, when in reality nothing could be further from the truth. As others have pointed out, for every one of the handful of Intelligent Design Creationists at the Discovery Institute, for instance, there are at least ten thousand scientists who consider the basic facts of evolution to be among the most solidly established and demonstrated as any in science. And not only is there absolutely clear evidence of evolution in peppered moths, but there are literally thousands of others studies which have documented natural selection in action in other populations of animals and plants in the wild .

There is, of course, always more to learn about any scientific subject, including peppered moth evolution. But when legitimate scientists debate some questions among themselves as part of the ongoing deepening of our understanding of evolution, that doesn't mean evolution itself is in question. For instance, when evolutionary biologists debate whether or not large-scale evolutionary changes always take place gradually or may sometimes be relatively more accelerated; or the relative importance of natural selection vs. genetic drift in overall evolutionary change; or why there are bound to be gaps in the fossil record; or how the role of lichens may or may not have been overemphasized in discussions of moth camouflage--this doesn't mean that evolution itself is in doubt! Such questions and debates among evolutionists further advance our understanding of evolution's mechanisms, but they in no way upset the basic applecart. The evolution of industrial melanism in Biston betularia and other insects--the back and forth shifts in the proportion of different genetic variants in relation to environmental changes-- is unquestionably real and has been readily observed in both England and the United States. So, while we should always keep our minds open to any new scientific insights which may end up enriching the basic story, we shouldn't let the Creationists pull the wool over our eyes!



* This refers to the increase in the proportion of darker variants in a population of moths or other insects in relation to an increase in industrial pollution. 

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** Kettlewell referred to the light moths being particularly well camouflaged when they rested on the whitish lichens covering some tree trunks in unpolluted woods. Lichens are living "crust-like" organisms, composed of a fungus and alga in symbiotic association. They are often found covering rocks and tree trunks, and they are very sensitive to air pollution. As industry developed in England, the whitish lichens covering tree trunks disappeared in many areas, resulting in darker tree trunks. Some recent studies have suggested that Kettlewell may have made too much of the presence or absence of these lichens because there is some suggestion that the proportion of light colored moths in a population may begin to increase again whenever air quality improves, even if the lichens which had been killed off by industrial pollution have not yet returned to the area. This is an interesting question to further explore in its own right, but, regardless of the presence or absence of lichens, it still seems that tree trunks in relatively soot-free unpolluted areas are generally lighter in color than in polluted areas. 

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[Return to "The Science of Evolution: Part 2" article]

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