Last weekend I completed my second annual Memorial Day survey of moths the Rocky Gap State Park. I got better pictures this year, and these are posted up on Flickr where they await identification.
One of the first surprises is that it turns out I misidentified something from last year’s survey. In this year’s survey, Willapalens identified the photo on the left below as Biston betularia. I recognized the dark wavy line on the wing as something I’d seen before. On the right is the exceedingly worn individual from last year. We agree they are the same species — the peppered moth that is a famous example of natural selection in action.
In 1849, a coal-black mutant was found near Manchester, England. Within a century, this black form had increased to 90% of the population in this region.
The moth flies at night and rests by day on tree trunks. In areas far from industrial activity, the trunks of trees are encrusted with lichens. The light form is practically invisible against this background.
In areas where air pollution is severe, the combination of toxic gases and soot has killed the lichens and blackened the trunks. Against such a background, the light form stands out sharply.
The moth is preyed upon by birds that pluck it from its resting place by day. In polluted woods, the dark form has a much better chance of surviving undetected. When the English geneticist H. B. D. Kettlewell released moths of both types in the woods, he observed that birds did, indeed, eat a much higher fraction of the light moths he released than of the dark.
Since pollution abatement programs were put in place after World War II, the light form has been making a comeback in the Liverpool and Manchester areas.