Science News: Victory for Peppered Moth

Image by Виталий Гуменюк

For decades science teachers have used the Peppered Moth (Biston betularia) as a prime example of natural selection. Prior to the Industrial Revolution in Europe, the Peppered Moth population in England consisted of a high percentage of light-colored salt and pepper moths (known as typica) and a low percentage of dark moths (known as carbonaria). At that time, the majority of trees were also light-colored due to the growth of lichens on their bark, providing typicas camouflage. Dark moths, it was presumed, were disproportionately spotted by birds and eaten, thus keeping their population low.

As the Industrial Revolution progressed, the lichens began to die and the tree trunks became darker as they collected soot from the growing number of factories. At the same time, the percentage of light moths began to decline, while carbonarias increased and became the majority. The assumption of scientists was that now the darker moths had better camouflage and the best adaptation to avoid predation, while the lighter moths were now easy prey.

This is classic Natural Selection: A given population (members of the same species living in the same place and time) always contains a variety of phenotypes (traits), some of which are better than others at helping an organism survive long enough to reproduce and pass the adaptation to their offspring. However, environmental changes (like pollution) can alter the balance, shifting greater fecundity to organisms with a different phenotype, causing their proportion in the population to increase.  

The example was so classic, so cut and dry, so obvious, that few scientists and even fewer science teachers ever questioned its validity. The hypothesis was proposed as early as 1896, according to The Scientist (May, 2012) and validated in the 1950s by Bernard Kettlewell, who collected compelling evidence that bird predation was in fact the selective force at work and that moth camouflage was affected by pollution, by placing light and dark moths directly on trees in polluted and unpolluted tracts.

However, in the 1980s, Peppered Moth experts started to identify flaws in Kettlewell’s experiments. Perhaps most compelling was their finding that tree trunks might not be the moths’ preferred resting place, thus calling into question the whole camouflage/bird predation hypothesis. It also threatened to make fools of the thousands of science teachers who were still using the Peppered Moth as a prime example of Natural Selection. Worse than this, however, was the field day it created for creationists, who called the Peppered Moth story a fraud and an example of scientists’ fallibility.

Enter Michael Majerus, an evolutionary biologist from the University of Cambridge, and a 50-year expert on Biston betularia. Starting in 2001, according to The Scientist, he set out to confirm Kettlewell’s findings using a more robust and convincing protocol. First, through years of direct observation, he discovered that the moths’ preferred resting site was the lateral branches of trees (not their trunks). Then, rather than artificially placing moths in a desired setting as Kettlewell did, he released thousands into an unpolluted tract covered with nets (so they couldn’t escape and confuse migration with predation).

Over the course of seven years he found a 9% lower survival rate for carbonaria moths, indicating that they indeed had a lower fecundity in an unpolluted setting and suggesting that they were in fact being consumed at a higher rate.

Majerus died in 2009 from an aggressive mesothelioma before his results could be published. However, a detailed account of his work was published this year in Biology Letters (February 2012) by some of his peers.

The lesson for K-12 science teachers is that they need to stay up to date on scientific discoveries and debates. In my experience, this does not happen often enough. Many (if not the majority) of those who teach science have little, if any, practical experience doing science in a real-world setting and few read scientific journals with any regularity. Furthermore, it is very rare that they have time to meet with colleagues to discuss new discoveries, review journal articles, analyze methodologies, or debate controversies, something that is an important cornerstone of university research.