Melanism gene found in peppered moth

Well, the gene was actually located but not yet identified.  Nevertheless, it soon will be.

This story is about what many evolutionists consider our best-documented example of natural selection in real time: the evolution of “industrial melanism” (dark color in response to environmental pollution) in the peppered moth, Biston betularia.  The normal appearance of the moth is whitish with black specks (accounting for its name; it’s also called the “typica” form):

Photo by Bob Fredrick from Arkive

In the early nineteenth century, a few all-black variants of this form (called “carbonaria”) had been found by British collectors (the first was described in 1848), but they were in very low frequency: only a few hundredths of one percent. (The Brits are diligent amateur lepidopterists, and so the records are pretty good).  Later genetic tests showed that the difference between carbonaria and typica was due to a single gene, with the dark color being dominant.

By 1898, the frequency of the dark form had skyrocketed, reaching 98% in the woods around Manchester.  It rose as well in other parts of England, particularly the industrialized parts. In rural areas the frequency of the dark variant was lower.   This concerted rise in such a short time surely indicated the operation of natural selection.  Although there were several theories about how this operated, the most likely seemed to be based on camouflage: as industrialization darkened the tree trunks with soot, and killed the lichens, the typica form was no longer cryptic on the formerly light-colored trees (especially birches), and now the dark form was camouflaged instead. Here are some photos showing how the dark form is conspicuous on darkened trunks and the light form on normal, non-sooty trees:

Note that there are two moths in each picture.

When Britain passed Clean Air Acts in the 1950s, the pollution began to abate and the trees lost their darkness: the soot washed off, and lichens began to return.  Sure enough, the color change reversed itself: the light-colored moths began to increase in frequency, so that now the typica form is present in frequencies of 90% in areas, like Manchester, where it had almost vanished. Here are what the two color morphs look like in a contemporary unpolluted woodland:

As I said, the concerted rise and fall of color genes throughout Britain indicated the action of natural selection, a hypothesis supported by similar changes in the American subspecies of B. betularia, particularly in polluted areas in Michigan and Pennsylvania.

The hypothesis that the selection on color was based on predation—especially by visual predators like birds, who love to eat moths—was tested by British biologists, particularly Bernard Kettlewell at Oxford.  He released mixtures of dark and light moths in both polluted and unpolluted woods.  As expected, in each type of wood he recaptured more of the moths that matched the trees, suggesting that the conspicuous moths had been eaten more often.  His estimate of selection against conspicuous moths was quite strong: they appeared to have had only half the reproductive output of  the camouflaged moths.

I had been a severe critic of Kettlewell’s experiments, which were cited in all the textbooks as proving natural selection in real time.  His experimental design had several fatal flaws. But more recent work has shown that dead moths of different colors pinned to trees of different colors do show the expected differences in attack by birds, and that in nature moths indeed rest on tree trunks and limbs.  Even more recently, Michael Majerus of Cambridge University repeated Kettlewell’s release experiments, but did them correctly. Sure enough, he found the expected differential recapture of light and dark forms.  Sadly, Majerus died of mesothelioma before his data could be published, and so the results of the strongest test of selection in this system still reside, as far as I know, on websites rather than the pages of reviewed scientific journals.

At any rate, the Biston betularia story stands, next to the Grants’ work on medium ground finches in the Galápagos, as one of the best-understood instances of selection occurring in “real time.” (“Industrial melanism” is not limited to Biston betularia, by the way: around 70 species of Lepidoptera also showed similar changes.)  Only one piece of the story was missing: the genetic basis of differences between light and dark forms. It was, as I have said, known to result from a single gene, with the dark “allele” being dominant, but the nature of that gene wasn’t known.

This week, however, a paper in Science by Arjen van’t Hoff et al. took a large step to identifying the “melanism” gene.  The authors mapped this single genetic factor to a small chromosome region by crossing light and dark moths that differed by many genetic markers, and noticing which genetic markers were associated with wing color in subsequent generations.  The “color gene” resided on a small section of chromosome homologous to chromosome 17 in Bombyx mori, the silkworm moth, whose genome has been mapped.  The authors haven’t yet narrowed down this small region to one specific gene, for that is very hard to do, and there are no obvious “candidate genes” in that region that affect color.

What is most interesting, however, is that the genetic analysis of various color forms collected throughout the UK gave information about the evolutionary origin of the black form.  It could have originated in two ways: either a single unique dark mutation, in a single carbonaria moth, could have spread throughout Britain, or there could have been multiple origins of the mutation (each, perhaps, kept at low frequency by natural selection against the dark color), all of which began to rise in concert when the environment changed.

Genetic studies showed that the first hypothesis was the most likely.  All the carbonaria individuals carried a unique genetic signature around (not in) the “dark” gene, suggesting that that signature was the unique DNA of a single mutant individual that, because the DNA was physically close to the dark gene undergoing selection, “hitchhiked” to high frequency along with the dark allele itself. (Remember that nearby genes reside on a single stretch of DNA, so if one part of that DNA rises in frequency due to selection, it carries with it nearby regions as well.  More distant regions, however, lose this association because of genetic recombination between DNA strands.)  If the dark coloration in British moths had stemmed from several or many different dark mutations at an initially low frequency, you wouldn’t expect the “dark” region to have a unique genetic signature shared by all currently dark individuals.

So this settles one aspect of the melanism story.  The rest of the story will follow soon:  the identification of the precise gene distinguishing light and dark forms, and the sequencing of that gene to determine if the color difference is due to a “structural” difference residing in a protein, or to a “regulatory” difference affecting whether or how a protein is expressed. But this molecular-genetic work is just icing on the cake, for regardless of the precise genetic basis of the color difference, the broad outlines of the story—and the validity of B. betularia as a case of natural selection in “real time”—are clear.

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A.E. van’t Hof, N. Edmonds, M. Dalíková, F. Marec, and I. J Saccheri.  2011. Industrial melanism in British peppered moths has a singular and recent mutational origin,” Science 332:958-960.