Textbook moth mutation located
Identifying the genomic region that gave peppered moths their darker coloration during the Industrial Revolution lends molecular support to a classic example of evolutionary change
A single mutation in the genome of the peppered moth is responsible for shifting populations to a darker phenotype that helped camouflage the insects during the Industrial Revolution in England, when pollution blackened the trees in the moths' habitat.
|The dark (carbonaria) and light (typica) forms of the peppered moth|
Photo by Ilik Saccheri, © Science/AAAS
The results, published today (April 14) in Science Express
, support the widely-cited example of adaptation to environmental change, which became the subject of controversy over the last couple decades.
"I think it's really exciting," said evolutionary biologist linkurl:Chris Jiggins;http://www.zoo.cam.ac.uk/zoostaff/jiggins.htm of the University of Cambridge, who was not involved in the research. "This really quite nice population genetics support for the classic story that a single mutation arose and then spread rapidly through natural selection."
"It's such a classic, well known, well documented example, but the molecular stuff has been absent," added linkurl:Bruce Grant,;http://www.wm.edu/as/biology/people/emeritus/grant_b.php a population geneticist who studied American peppered moth populations while at the College of William & Mary. "We've needed this for a long time."
The peppered moths of England have for decades served as textbook examples of environmental variation and natural selection driving evolutionary change in a population. Prior to the country's Industrial Revolution, the moths were light in color, blending in with the pale trees and lichens that dominated their woodland habitats. But in the 19th
century, when coal-burning factories sprouted up across the landscape and blanketed the UK countryside in soot, a new dark variation of the moth, known as carbonaria
, predominated. When clean air laws passed in the 1950s and 60s resulted in decreased pollution, and the lighter typica
morph once again became the norm.
Studies by geneticist Bernard Kettlewell in the middle of the 20th
century suggested that darker moths alighting on soot-covered tree trunks were less likely to be eaten by avian predators, acting as agents of selection. Earlier breeding experiments had shown that the trait followed an autosomal dominant inheritance pattern, demonstrating the genetic basis of the trait. The data seemed clear cut -- a genetic mutation spread through the population as a result of an environmental change -- and the story quickly became gospel. But criticisms of Kettlewell's work began popping up in the last 10 or 15 years, causing many creationists and even some biologists to argue that "the classic evolutionary story was flawed," Jiggins said.
To determine the molecular genetic basis of the dark coloration, ecological geneticist linkurl:Ilik Saccheri;http://tulip.liv.ac.uk/portal/pls/portal/tulwwwmerge.mergepage?p_template=bio&p_tulipproc=staff&p_params=%3Fp_func%3Dteldir%26p_hash%3DA154687%26p_url%3DBS%26p_template%3Dbio of the Institute of Integrative Biology at the University of Liverpool and his colleagues first mapped the key variation to an orthologous region on chromosome 17 in the silkworm. The researchers then compared carbonaria
peppered moths at several loci in the region, further narrowing the search for the responsible gene to a smaller section of the chromosome.
Furthermore, the patterns of linkage among the genes within the chromosomal region suggested that the mutation responsible for the darker coloration arose a single time in the relatively recent past and spread rapidly through the population. "It's molecular data that's consistent with the classic story," Jiggins said.
But there's a big remaining question: What gene is responsible for the changes?
"That region is a bit obscure to be honest," Saccheri said. "We're in the process of sequencing it now, but from the available evidence, there are no candidate genes in this region. There's nothing that stands out as having anything to do with patterning."
Interestingly, it is the same region of the genome that Jiggins and his colleagues linkurl:identified in 2006;http://www.plosbiology.org/article/info:doi%2F10.1371%2Fjournal.pbio.0040303 as responsible for the complicated wing patterns of Heliconius
butterflies. "[That] really sort of blew me away," he said. "Maybe it's just coincidence but it seems a bit too good be true."
It raises the question, Jiggins added, of why the same gene has potentially mutated and been selected for in two very different systems. "That would seem to suggest an extraordinary predictability about evolution," he said. "We've only just started to think about why that would be."
A.E. van't Hof et al., "Industrial melanism in British peppered moths has a singular and recent mutational origin," Science Express, 10.1126/science.1203043, 2011.
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[7th April 2011]*linkurl:Evolution outside the lab;http://www.the-scientist.com/news/display/58097/
[31st March 2011]*linkurl:Second Thoughts about Peppered Moths;http://www.the-scientist.com/images/yr1999/may/opin_990524.html
[24th May 1999]