How these moths evolved to disguise themselves as soot in Industrial Revolution

Two new studies investigated color changes in moths and butterflies, and discovered a single gene behind their rapid and extreme color variations.

Two new studies, both investigating the evolutionary histories of fluttering insects, have pinned down a genetic explanation for rapid and radical wing color changes.

One gene, formerly believed to have nothing to do with coloring, is responsible for the huge range of colors in butterflies and at least one moth, say researchers.

“It’s amazing that the same gene controls such a diversity of different colors and patterns in butterflies and a moth,” said Dr. Nicola Nadeau, a University of Sheffield researcher, in a press release.

The first of the paired studies, led by researchers at the University of Liverpool, focused on the transformation of the Biston betularia, or peppered moth. One of the classic examples of evolutionary transformation, the moths were observed in early 19th-century Britain changing color from speckled to black due to melanism – a contrasting adaptation to albinism, in which dark pigments develop in animals – in response to the increasing pollution of the industrial revolution.

The second study, based on work by Dr. Nadeau and international academic team, highlighted the likely genetic cause of the pigmentation shift in both peppered moths and a wide range of butterflies: the cortex gene.

When a transposable element, or jumping gene, is inserted into a cortex nucleotide sequence, it can affect wing variations in most lepidopterans, the scaly-winged insect order made up of around 160,000 moth and 17,000 butterfly species.

Both research papers were published in the journal Nature.

The black peppered moth, or carbonaria variant, was first noted in 1848 as a departure from the lighter typica form common at the time. The new research confirmed “industrial melanism” to have developed and proliferated among the B. betularia population through years of the advancing manufacturing and technological period, resulting in the pitch-black moth.

“Our best estimate of 1819 shows that the mutation event occurred during the industrial revolution and that it took around 30 years for it to become common enough to be noticed,” said co-author Dr. Pascal Campagne in a Liverpool press release.

The study also confirmed that the cortex shift was not caused by coal and smoke output, but rather in line with it, allowing the previously light-colored moths to blend in with their newly sooty surroundings to avoid predators seeking an easily identifiable bright white meal.

The peppered moth’s microevolutionary changes helped it hide, but researchers investigating the cortex effect in heliconian butterflies – a long-winged genus native to the Americas – found that the gene pushed the insects’ wings to become even brighter and more colorful to avoid predation.

Heliconians’ famous capacity for mimicry is also linked to cortex, say the researchers, who were surprised by the gene's versitilty.

“This is highly unexpected, both because the butterfly and moth polymorphisms appear very different to the eye, and [because] the species are separated by over 100 million years,” said Liverpool’s Dr. Ilik Saccheri in the university release. “What this suggests is that the cortex gene is central to generating pattern diversity across the Lepidoptera, and more generally that adaptive evolution often relies on a conserved toolkit of developmental switches.”

Former investigations into butterfly and moth coloration had ignored cortex, as the gene was thought to control only cell division, unrelated to pigmentation.

With cortex’s evolutionary role now revealed, the gene is due for closer examination, say researchers.

“It’s a different gene to the one we might have expected and we still need to do more to understand exactly what it’s doing, and how it’s doing it,” said Chris Jiggins, a Cambridge evolutionary biology professor who contributed to the second study.

The discovery raises a fascinating question, said Dr. Jiggins. "Given the diversity in butterflies and moths, and the hundreds of genes involved in making a wing, why is it this one every time?”