Researchers find that a single ancestor is responsible for the 'best example' of natural selection.
A genetic analysis is unpicking the mysteries of one of the best-known examples of natural selection — the dark form of the peppered moth, which spread rapidly in nineteenth-century Britain's industrial, sooty environment.
Ilik Saccheri, an evolutionary biologist at the University of Liverpool, UK, and his colleagues have used molecular genetics to show that one mutation from a single ancestor causes increased dark pigment, called melanism, in the typically light-coloured moth. Their results are published today in Science1.
"It's a big breakthrough as far as peppered moths' industrial melanism is concerned," says Laurence Cook, a retired population geneticist from the University of Manchester, UK, who has been studying the peppered moth since the 1960s. "We've been going on for an awfully long time knowing just the classical Mendelian genetics."
The typical form of the peppered moth (Biston betularia) has light-coloured wings speckled with small dark spots, camouflage well-suited to hiding from predatory birds on the bark of birch trees common to their habitat. But following the industrial revolution in both Britain and North America, the air became polluted with dark soot, which covered many surfaces, including trees.
It was then that a dark form of the peppered moth saw a population boom, probably because predatory birds couldn't see it clearly against sooty surfaces and ate the light-coloured form instead. When regulations reduced air pollution in the 1970s, dark-moth populations began to fall and the lighter form resurged.
Such obvious population changes over a relatively short period of time have made the peppered moth a textbook example of natural selection, but the genetic basis of the rise and fall of the dark form has not been understood. Previous evidence2 shows that the dark colouring isn't associated with any of the genetic pathways already known to cause melanism in insects, and it was unclear whether the moth's dark form had arisen several times or just once.
Saccheri's team used a genetic technique called linkage mapping to search for the gene responsible. A linkage map puts traits in groups according to how often they are passed on to the next generation together, which indicates how close together they sit on a chromosome. The closer the traits are in the genetic sequence, the less likely it is that they will be separated during sex-cell division, and the more likely it is that they will be passed on together.
To make the map, Saccheri and his colleagues twice crossed a dark male moth with a light-coloured female; the result was 132 offspring with varying traits. The traits most often inherited alongside dark coloration were matched up with genes of the silkworm (Bombyx mori) — a closely related moth species with a sequenced genome.
The locations of the genes for the traits pointed to a narrow region on chromosome 17, where the scientists say that a single gene variant is probably responsible for the peppered moth's melanism, although they don't yet know exactly which one it is.
Once the chromosome region was identified, the researchers examined moth samples collected all over Britain between 1925 and 2009. The same group of gene variants huddled in the chromosome region closest to the mutation in the dark moths, providing strong evidence that natural selection had acted recently on an advantageous mutation from one individual. If a mutation had been in the population for a longer time, or had come from multiple individuals, the selection of traits that were inherited together would vary more widely.
"It's not just the one mutation that has been swept through the population, it's that whole chunk of chromosome that has hitch-hiked," says Saccheri.
Now that the researchers have closed in on a chromosome region, they can use genetic sequencing to search for the gene responsible for the peppered moth's melanism, and then determine its broader function. They can also compare it with the same gene in North American peppered moths and see whether the same mutation is responsible for the parallel rise and fall of the dark moth there.
In addition, a few butterfly genes that control colour are found in the same chromosome region as the melanism mutation, so the researchers may be onto an as-yet unidentified pigment-controlling pathway common to moths and butterflies, they say.
A greater understanding of the peppered moth's genetics will "complete the package" of research on "the best example of adaptation involving natural selection that we have", says Bruce Grant, a retired population geneticist from the College of William and Mary in Williamsburg, Virginia.
van’t Hof, A. E., Edmonds, N., Dalíková, M., Marec, F. & Saccheri, I. J. Science doi:10.1126/science.1203043 (2011).
van't Hof, A. E. & Saccheri, I. J. PLoS ONE 5, e10889 (2010).
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Dickey Zakaib, G. The peppered moth's dark genetic past revealed. Nature (2011). https://doi.org/10.1038/news.2011.238
Philosophy & Technology (2012)