How An Icon of Evolution Turned to the Dark Side

In the early 19th century, coal-fired factories and mills belched a miasma of soot over the English countryside, blackening trees between London and Manchester. The pollution was bad news for the peppered moth. This insect, whose pale speckled body blended perfectly against the barks of normal trees, suddenly became conspicuous—a white beacon against blackened bark, and an easy target for birds.

As the decades ticked by, black peppered moths started appearing. These mutants belonged to the same species, but they had traded in their typical colours for a dark look that once again concealed their bodies against the trees. By the end of the century, almost all the moths in Manchester were black.

As British air became cleaner and trees lighter in colour, the black moths faded back into obscurity. But in their brief reign they became icons of evolution. As geneticist Sewall Wright put it, they are “the clearest case in which a conspicuous evolutionary process has actually been observed.”

The story has endured a fair amount of controversy. Creationists asserted that the blackening of the moth was just a case of shifting gene frequencies rather than an outright change from one organism into another, ignoring that the former is the very definition of evolution. They also seized onto technical disputes between scientists themselves, over whether the moths’ colours really made any difference to their vulnerability to birds. The latter dispute was resolved through some groundbreaking experiments by the late Michael Majerus.

And now, Arjen van’t Hof and Pascal Campagne from the University of Liverpool have strengthened the peppered moth’s iconic status even further by identifying the gene behind its classic adaptation. And in a wonderful twist, the gene turns out to be a jumping gene—a selfish bit of DNA with the power to hop around its native genome.

I’ve written about jumping genes many times before (as has fellow Phenomena blogger Carl Zimmer). These mobile stretches of DNA can cause havoc by disrupting other genes and increasing the risk of cancers or developmental disorders. Or, they can provide opportunities by creating variation that evolution can act upon. Such genes have driven the evolution of mammalian pregnancies and our immune systems. And now, it seems that one of them helped the peppered moth to cope with the Industrial Revolution.

Back in 2011, the Liverpool team, led by Ilik Saccheri, bred and compared dark and light moths to identify the gene or genes responsible for the shadowy look. They narrowed their search down to a small section of the insect’s 17th chromosome—one containing 13 possible genes. Since then, having studied more moths, they’ve homed in on one particular gene called cortex.

In almost all the dark moths, the cortex gene contains a unique stretch of DNA that’s missing from all the light individuals. It has all the hallmarks of a jumping gene, including the ability to make an enzyme that cuts it out of its original location and pastes it elsewhere. The moth’s genome contains up to 255 copies of this gene, which the team calls carbonaria. It clearly gets around a bit.

And on one particular jump, it landed in the middle of cortex. This fateful event, which nestled one gene (carbonaria) within another (cortex), is what darkened the moth’s body. Van’t Hof and Campagne estimate that it probably happened somewhere around 1819—a couple of decades before entomologists first saw the dark moths in the wild.

The timing fits, but other details are less clear. For example, how exactly did carbonaria cause the dark colours? Genes encode instructions for building proteins—tiny biological machines that perform various jobs around an animal’s cell. You might guess that carbonaria changed the instructions in the cortex gene, leading to the production of a different protein with new capabilities. But not so—the jumping gene actually landed in a part of cortex  that gets discarded, and never contributes to building proteins.

Rather than changing what the cortex gene built, the team suspects that carbonaria changed when and where it is activated. Indeed, with the jumping gene in place, cortex switches on very strongly at the point in the larval moth’s life when it starts producing its adult wings. It’s unclear why that happens, or how it leads to dark wings, but for now, it seems that cortex affects the development of wings and that carbonaria changed how it did its job.

Indeed, in a separate study, Nicole Nadeau and Chris Jiggins from the University of Cambridge showed that cortex controls the patterns of the beautiful Heliconius butterflies, probably by influencing the development of their wing scales. By fiddling with this gene, natural selection has repeatedly tweaked the palettes and patterns of insects.