Darwin meets the citizen scientists

Charles Darwin was the original crowd-sourced scientist. He may have a reputation as a recluse who hid away on his country estate, but he actually turned Down House into the headquarters for a massive letter-writing campaign that lasted for decades. In her magisterial biography of Darwin, Janet Browne observes that he sometimes wrote over 1500 letters in a single year. Darwin was gathering biological intelligence, amassing the data he would eventually marshall in his arguments for evolution. In the letters he wrote to naturalists around the world, Darwin asked for details about all manner of natural history, from the color of horses in Jamaica to the blush that shame brought to people’s cheeks.

Given the skill with which Darwin used the nineteenth-century postal system, I always wonder what he would have done with the Internet. A new paper offers a clue: he might have enlisted thousands of citizen scientists to observe evolutionary change happening across an entire continent.

Darwin used his Victorian crowd-sourcing to collect evidence that was consistent with his evolutionary theory; he didn’t expect that he could actually document evolutionary change happening in his own lifetime. Ironically, he probably could have. Gregor Mendel worked out the basic rules of genetics around the time Darwin published The Origin of Species. At the time, pollution from England’s coal was turning trees dark, giving an evolutionary edge to dark moths over light ones. A naturalist even wrote directly to Darwin in 1878 to raise the possibility that natural selection was driving the shift in moth color. But it wasn’t until 14 years after Darwin’s death that a naturalist explicity put this idea into print.

In the decades after Darwin’s death, biologists translated Darwin’s ideas into the language of statistics. They figured out how to make measurements on animals and plants in the wild, and how to discover in those measurements the traits that led to the most reproductive success. Evolutionary biologists have now made thousands of measurements of natural selection in the wild. But each of those measurements has been hard won. To see natural selection, researchers must study dozens or hundreds of individuals. To get a sense of how tough this work is, read The Beak of the Finch: A Story of Evolution in Our Time, in which Jonathan Weiner chronicles the adventures of Peter and Rosemary Grant, who have traveled to an isolated island in the Galapagos Archipelago each summer for forty years in order to measure natural selection on Darwin’s finches.

But Jonathan Silvertown of The Open University in England and his colleagues have found a way to spread this kind of work far and wide. They set up a web site where volunteers could sign up to become amateur evolutionary biologists. They ende up with over 6,000 volunteers, who sent them measurements from across Europe.

Their measurements came from an animal that’s at once humble and iconic. The land snail Cepaea is common in gardens, ditches, forests, and meadows throughout Europe. The snails come in a beautiful variety of colors, as this photograph from Poland demonstrates. The patterns are encoded in genes, which the snails pass down to their offspring. In the early 1900s, many naturalists considered the patterns to be pretty but insignificant. They were just the result of random mutations that cropped up and then spread through the snail population thanks to chance.

Starting in the 1930s, a team of Oxford scientists took a close look at which snails lived and died. They could do so because the snails are a favorite meal for thrushes, which like to pick up their prey and carry the snails into the air, whereupon they drop the snails onto rocks below to crack the snails. The Oxford researchers were able to catalog these smashed shells, noting their colors and stripes.

The researchers found that some colors and stripes were more common among the shell debris than you’d expect from chance alone. It turns out that the birds are more likely to pick out the snails that stand out against the background. So snails that are better camouflaged are more likely to survive. Which pattern works best depends on where a snail lives; what hides a snail crawling over a dark forest floor doesn’t work so well in a field grazed short by cattle. The researchers found that the most common patterns were, indeed, well-matched to where the snails lived.

This research on Cepaea snails helped establish natural selection as a powerful force in evolution–although bird-driven natural selection turns out to be not the sole force at work. Studies across Europe revealed, for example, that southern European snails are more likely to have yellow shells than their darker northern cousins. The difference is probably due to the climate: yellow shells bounce sunlight away and keep the southern snails cool.

A few years ago, Silvertown and his colleagues set out to gather a new batch of observations on Cepaea to compare with these historical records. They wondered, for example, if the warming that Europe has experienced might have made northern snails more yellow. Through the web site Evolution Megalab, they enlisted people from 15 different countries, who set out into their own neighborhoods to find the snails and note their colors. Combining the new observations with the old, the scientists ended up with half a million snails, organized in a geographical database along with information such as the habitat where the snails lived, as well as the temperature and rainfall at each location. Nothing quite like it has ever been achieved by evolutionary biologists–professional or otherwise.

The collection chronicles fifty years of snail evolution, over the course of about twenty generations. In that time, the researchers didn’t detect a continent-wide change in the frequency of yellow shells. Only in populations that lived on beach dunes did yellow shells become more common. Silvertown and his colleagues suspect that most snails have been coping with the warming temperatures in Europe by spending more time in the shade. On the treeless dunes, however, that’s not an option. As a result, natural selection has favored the yellow snails, which can stay cooler without the help of foliage.

But Silvertown and his colleagues did find other shifts that took place across all of Europe. Snails without a stripe on their shell declined by about 10%, while snails with a mid-line band increased by 5%. The scientists doubt that the rise of striped snails has anything to do with a shifting climate. Indeed, the striped snails have become more common in southern Europe than in northern Europe–the opposite of what you’d expect if stripes were a defense against heat.

The scientists don’t know for sure what’s behind this evolution, but they have an idea. The song thrushes that eat the snails have been declining in some places for the past thirty years. It’s possible that the change in the predatory pressure of birds is shifting the force of natural selection.

It’s the sort of idea you could imagine Darwin coming up with as he sat in his study, paging through letters from his farflung correspondents. But now Silvertown and his colleagues are going to test it, taking advantage of the original citizen scientists: birders. And if you’re reading this in Europe, you can be part of the investigation.

PS: In case you don’t know how to hunt for a snail, here is a charming video from the Megalab: