Planet Earth Online

Moth's black colour traced to single mutation

15 April 2011, by Tamera Jones

Using a detailed genetic analysis, they found that just one mutation in a single ancestor led to the striking adaptation back in the 1840s. It's a classic example of natural selection, with the mutation responsible for the change in colour causing increased dark pigment, or melanism.

'There are very few examples of where we can trace the genetics of an adaptation in a natural population to environmental change; this study expands our understanding of how evolutionary change happens,' says Ilik Saccheri, an evolutionary biologist from the University of Liverpool, lead author of the study published in Science.

And it turns out that the small bit of chromosome responsible for the change in colour also controls different wing patterns in some butterflies.

Blended in

Before the Industrial Revolution, the light-coloured Peppered Moth blended into its environment, because its speckled wing pattern is difficult to see on tree bark.

But soon after coal-burning chimneys started belching out black, sooty smoke during the Industrial Revolution, a black version of the peppered moth spread rapidly across northern England. In the early 1900s, the moth – called carbonaria – then spread south and west across the UK.

Scientists realised that as the sooty smoke turned the trees black, birds could easily spot the insects. So when a black mutant came along, it had the advantage.

The peppered moth has been well-studied, but until now, knowing which part of the moth's chromosome was responsible for the change in colour has eluded scientists.

'We didn't know if industrial melanism in the peppered moth is due to a single recent mutation or several, or if it came from outside Britain,' explains Saccheri.

To understand the genetic basis of the change in colour from peppered to black, Saccheri and colleagues from the universities of Liverpool and South Bohemia used a clever technique called linkage mapping.

The idea is that traits that sit close to each other on the chromosome are more likely to be passed on to the next generation together than those that are far apart.

So Saccheri and his colleagues mated a black male moth with a light-coloured female moth and looked at which traits were inherited by their offspring along with the black colour. They then traced those traits back to a small region on one of moth's chromosomes.
The researchers then expanded their study to a sample of peppered moths collected from all over mainland Britain between 1925 and 2009, and found that all the black moths in the sample had the same genetic fingerprint as the offspring in their first experiment.

Their results confirm that all carbonaria-type moths are derived from a single mutant ancestor.

'Once the mutation happened, it spread rapidly through the population,' says Saccheri. 'Our study shows that a large chunk of the chromosome swept through the population.'

Keywords: Biodiversity, Environmental change, Europe,

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