Ultra-violet confirms “Darwin’s moths”

New research published in the journal Communications Biology has provided the final piece of evidence needed to confirm the most famous case of evolution-in-action: that of Darwin’s moths.

William Blake referred to the coal-powered factories of the Industrial Revolution as “dark Satanic mills”, and they did have diabolical effects on Britain. Not only did they establish catastrophic working conditions for most, but pumped out vast plumes of pollution, changing the environment for animals and humans alike.

The plight of a particular insect, Biston betularia or the peppered moth, became one of the earliest examples of anthropogenic environmental change affecting not only the behaviour, but the very biology of organisms. In the process, it also provided the first observed instance of natural selection, helping to confirm Charles Darwin’s theory of evolution that was first aired in 1858.

And it has been a battleground for the validity of that theory ever since.

Prior to intense industrialisation, the peppered moth had pale colouration. But by 1811 a rare new form, or morph, appeared: a black winged type later dubbed carbonaria. Between 1848 and 1895 the carbonaria form eclipsed the original morph, now known as typica, growing from 0% to 98% of the population in just 47 years.

So how is this linked to the Industrial Revolution?

As pollution blackened trees in industrial areas such as Manchester, it also began to kill common lichens that grew on their previously light-coloured bark.

The typica morph had been wonderfully suited to these pale lichen-clad trees: it was seamlessly camouflaged from avian predators such as sparrows, robins, and black birds. But as the trees darkened, typica became more conspicuous and thus eaten more often, while the carbonaria’s camouflage became ever better. This phenomenon of the evolution of darker forms in polluted areas is known as “industrial melanism”.

Big names in biology have weighed in on B. betularia. JBS Haldane, the great English polymath, worked on the problem himself, calculating that the change in morph frequencies could only be explained in terms of changed fitness. The American geneticist Sewall Wright described it as “the clearest case in which a conspicuous evolutionary process has actually been observed”.

However, a British geneticist and moth specialist, Bernard Kettlewell, conducted extensive research in the 1950s, demonstrating experimentally selection against typica in polluted forests and selection against carbonaria in unpolluted ones. Kettlewell also observed that with the fall in pollution that followed the introduction of the Clean Air Acts in the 1950s, the frequency of carbonaria declined while typica went into the ascendant. However, doubts remained.

“This is one of the most iconic examples of evolution, used in biology textbooks around the world, yet fiercely attacked by creationists seeking to discredit evolution,” says author Martin Stevens, of the Centre for Ecology and Conservation of the University of Exeter in Cornwall, UK.

Stevens, along with Olivia Walton, also from the University of Exeter, have now provided the missing piece of the puzzle in the story of the peppered moth.

Scientific understanding of the moth’s camouflage has been centred around a human perspective: the two morphs look pale or dark to us, as do the trees. However, the visual systems of people and birds are quite different, with the latter able to perceive ultraviolet (UV) light. What has been missing, it turns out, is a bird’s eye view.

Walton and Stevens drew upon Kettlewell’s original collection of specimens and, using a model of bird vision based on the predicted response of the type of cones in avian eyes, they attempted to quantify the camouflage of peppered moths.

What they discovered is that under UV light typica exhibits a speckling that is strikingly like the UV patterns of lichen that grow on unpolluted trees. Similarly, they found that carbonaria showed UV patterns like that of plain bark, confirming the camouflage hypothesis for avian vision.

The pair then took to the forest for experimental confirmation, placing artificial prey items matching the UV signature of both morphs onto the trees of an unpolluted woodland. As predicted, the survival rate of the typica targets was substantially larger, about 21% more, than carbonaria.

“Our findings confirm the conventional story put forward by early evolutionary biologists – that changes in the frequency of dark and pale peppered moths were driven by changes in pollution and camouflage.”

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