The Return of the Peppered Moth
In your high school biology class, you probably saw a picture of a peppered moth with a caption about how during the industrial revolution, trees that were previously light in color turned black because of pollution, conferring an advantage on the dark variety of the moth over the light colored version because the dark version was more difficult for birds to see and therefore eat. Color differences are due to differences in a single allele (with the darker coloring dominant to the lighter variety), and research by one Bernard Kettlewell was taken to show that the increase in frequency of the darker variety (called melanics) was due to reduced predation by birds.
It's all there, in black and white
So that all seems pretty simple except that it wasn’t. The case was touted as a very beautiful example of evolution, which might partially explain why the case attracted the attention of people who were not fans of evolution, though it’s important to note the claim was also challenged in the scientific literature. Geneticist Michael Majerus wrote a book on the topic (Melanism: evolution in action), which was reviewed by Jerry Coyne in Nature, who argued that the case wasn’t quite as tight as one might like.
There do seem to have been some reasons to worry about the methods used by Kettlewell. For instance, he placed moths on tree trunks, which critics noted were not, typically, where moths liked to alight, preferring branches instead. The concern about where moths were placed as well as other worries about the research led to much ado. These moths, literally a textbook case of evolution in a natural ecology, became a topic not just of scientific debate, but a source of discussion by people hostile to evolution.
Which brings us to a piece recently published in Biology Letters by Cook et al. Majerus, in response to such worries, ran a large study of this issue, releasing 4,864 moths, this time not on trunks, but on branches of trees. Majerus presented the results at a talk, but never wrote them up. The new paper, Selective bird predation on the peppered moth: the last experiment of Michael Majerus, reports the results of the study. To conduct the study, Majerus identified about one hundred spots that a moth might choose, placing a net around the site so that a moth placed on it could not escape, but could be eaten by a bird. Every night for six years, he released one moth into each of twelve sites out of the 100 selected for that night. The dependent measure was whether or not the moth was there in the morning. If the net was empty, this was recorded as a score of moth: 0, bird: 1. The prediction was that a larger fraction of black moths would be taken compared to the fraction of white moths (because the branches were light again, now that heavy industry has moved from England to China.)
The data were clear enough; black moths didn’t survive as well as their lighter cousins. The authors conclude, quite strongly that “it is virtually impossible to escape the previously accepted conclusion that visual predation by birds is the major cause of rapid changes in frequency of melanic peppered moths.”
The authors note that Majerus witnessed some of the predation events by birds, reducing the uncertainly about what was happened to the darker moths still further.
So, after all these years, the textbook story seems to be restored. Moths do better when they are more similar in color to the trees they’re resting on, and this is due to the effect on predation by birds. Happy ending. Not, obviously, for the moths left to die in the nets every night for six years, but still.
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