A Peppered Icon Enters the Genomic Era

In any field of scientific endeavor, there are certain ideas or examples that achieve iconic status, perhaps way beyond their individual importance. In evolutionary biology, iconic status has certainly been achieved by the humble peppered moth, Biston betularia. It is an unlikely candidate for celebrity status; it is a speckled black and white moth, common in northern temperate regions of Europe and North America. Like many species, its wing patterns impart an uncanny ability to blend into the background when it rests on mottled tree trunks. What has made this species famous, however, is a remarkably rapid evolutionary change in its coloration following the British Industrial Revolution, from the typical peppered form, typica, to a dark melanic form, carbonaria. A single genetic locus controls the switch from the typical to the melanic form, so this represented a simple evolutionary change at a single gene (Kettlewell 1973). Recent work has uncovered the genetic locus that controls this dramatic change in appearance, offering new insights into its evolutionary history (van’t Hof et al. 2011). Similar changes have been observed in many moth species, but what gained the peppered moth its iconic status was evidence that linked the undisputed evolutionary change that had occurred in the population to a causal mechanism—namely, natural selection by bird predators. A series of simple but compelling release-and-recapture experiments were carried out by Bernard Kettlewell in the 1950s that showed that dark moths had reduced survival in rural Dorset (an unpolluted area), whereas in polluted Manchester, the pattern was reversed, with the typical peppered form showing lower survival (Kettlewell 1973). These field studies were combined with caged-bird experiments showing attack patterns consistent with the survival patterns seen in the field. In combination with striking photographs of moths resting on lichen-covered and sooty tree trunks (the dark moths are alarmingly conspicuous on the former), this seemed to be one of the best documented examples of recent evolutionary change and became a textbook case of natural selection in action. Textbook examples deserve scrutiny every few years, if only to ensure that the original evidence holds up. The peppered moth came under such scrutiny about 10 years ago. In a book on melanism, Michael Majerus highlighted some aspects of Kettlewell’s experiments that were potentially unrealistic (Majerus 1998). The moths were placed on tree trunks, rather than under branches where they were thought to rest. The moth densities tested were unrealistically high. The different morphs were released at unnatural frequencies. Subsequently, Kettlewell has even been accused of fraud, although such accusations seem entirely unfounded (Hooper 2002). In the 10 years between the publication of his book and his tragic death in 2009, Majerus worked to rebut criticisms of Kettlewell’s experiments. He documented natural resting places of wild moths and showed that a significant proportion do indeed rest on tree trunks. More importantly, he also carried out a long-term experiment near Cambridge, United Kingdom, in which moths were allowed to choose their own resting sites and were then observed during daylight hours for direct evidence of predation. In an experiment that took six years, involved nearly 5000 moths and countless hours of observation, differential predation was observed and was, furthermore, strong enough to explain the natural observed rate of decline in the carbonaria morph during the experiment. Sadly, Majerus died before these results were published. However, he presented the data at the European Society A Peppered Icon Enters the Genomic Era