The venerable peppered moth (Biston betularia) has popped up a couple of times in recent posts. It seems that some of our Darwinian commenters (see, e.g., qwerty017 in comment  here) have not gotten the memo – the peppered moth myth has been completely exploded. Don’t take our word for it. Uber-Darwinist Jerry Coyne says in the November 1998 edition of Nature: “For the time being we must discard Biston as a well-understood example of natural selection in action.” Why else would the popular school text Biology pull its discussion of Biston as an example of “evolution in action”?
My purpose in this post is not, however, to re-hash the vast debunking literature. Instead, when I did a brief internet search on the subject I was intrigued by an astonishing display of Darwinian “It’s just gotta be” psychology on display in this New Scientist article. After rehearsing the history of the controversy and admitting that Kettlewell’s iconic experiments were seriously flawed, the author reports on recent attempts by Michael Majerus to rehabilitate the myth. But having been burned once, many scientists are reluctant to re-admit Biston to the pantheon of evolution icons. And this is where it gets interesting. Near the end of the article the author admits:
Majerus’s study also leaves a long-standing problem unsolved. For reasons that are not clear, the frequencies of dark and light moths do not always correlate with the level of pollution. In East Anglia, for example, dark moths have always been relatively common despite low pollution levels.
But, says the author, while “these are legitimate problems that require scientific explanations,” they “do not point to a fundamental problem with the peppered moth story,” and he concludes:
Anti-evolutionists will continue to suggest [the peppered moth story is controversial], of course, but as far as Majerus and others are concerned their claims have been debunked and the peppered moth should be reinstated as a textbook example of evolution in action.
The fact that experimental data that is admitted by all observers flatly contradicts the myth is not a “fundamental problem,” and only benighted “anti-evolutionists” would resist re-admiting the peppered moth as a textbook example of evolution.
The peppered moth story, even had it been true, never demonstrated macro-evolution unless one was already a true believer, but it did serve to confuse generations of schoolchildren. The point of this post has nothing to do with whether the story is true though. As I said, the de-bunking literature is already vast.
No, this post is about the psychological phenomenon known as “cognitive dissonance,” the ability to hold two mutually exclusive ideas at the same time. Look at the quoted language again or, better yet, go back and read the whole story. In one paragraph the author admits that nowadays the peppered moth story is controversial even among Darwinian scientists. Then, only a couple of paragraphs later, the author boldly petitions the scientific community to readmit the story to the canon, because only knuckle-dragging, snake-handling fundies have the least doubt about it. Rarely are we privileged to see such a crystalline example of cognitive dissonance in action.