“Believing that it is always best to study some special group, I have, after deliberation, taken up domestic pigeons,” wrote one Charles Darwin in On the Origin of Species. Four years earlier, Darwin had taken to raising pigeons in his own dovecote, hobnobbing with other pigeon fanciers, and carefully measuring the birds. In the diverse breeds, with their fantails, feather-duster feet, and frilly backs, Darwin saw validation for his ideas about evolution. If people could artificially select for such astonishing diversity in just a few generations, nature was surely capable of far more over longer timescales.
Now, 160 years after Darwin published his opus, a team of biologists from the University of Utah have once again turned to pigeons to demonstrate evolution in action. But instead of focusing on the birds themselves, they turned their attention to the pigeons’ parasites.
In a simple experiment, Sarah Bush and Scott Villa placed feather-eating lice on different-colored pigeons and left them there to breed and evolve—for four years. Over that time, the insects adapted to better match the color of their host, which made them harder to spot and pluck off.
For Bush, this was “incredibly exciting”—an experiment that reminded her of the peppered moths she learned about in high school. Those insects normally have speckled white-and-black wings to camouflage against tree bark. But in 19th-century England, when coal factories blanketed trees with soot, the peppered moth quickly evolved into all-black forms. In doing so, it became a textbook example of evolution. Now perhaps Bush’s feather lice can join them.
Feather lice are small, wingless insects that spend their whole lives among the plumage of birds, eating feathers and flakes of skin. The discovery of a 44-million-year-old fossil louse with feathers in its gut suggests that “they’ve been doing the same thing since forever,” says Bush. Today, “there’s pretty much one species of louse per species of bird.” Their presence isn’t welcome, though, and birds will try to preen them from their plumage. The lice, in turn, hide through camouflage: In 2010, Bush and her husband, Dale Clayton, showed that lice tend to match the color of their host’s feathers.
To see how quickly the lice can adapt, Bush, Clayton, and their colleagues captured regular urban pigeons from around Salt Lake City and fumigated their feathers with carbon dioxide. Lice fell off them in droves, and the team transferred 2,400 of these insects onto 96 captive pigeons—some white, some black, and some gray.
For four years, the pigeons did whatever pigeons do. Meanwhile, for the lice, oceans rose, empires fell, and 60 generations came and went. Over that time, their colors changed. The lice on black pigeons became slightly darker, the ones on white birds became much brighter, and the ones on gray birds stayed the same.
But these changes occurred only if the pigeons could preen themselves. Bush stopped half the birds from doing so by fitting them with poultry bits—plastic clip-ons that prevented them from closing the very tips of their beaks. On those birds, the lice suffered no risk of removal, and their colors stayed the same. (The birds that couldn’t preen also ended up with 20 times as many lice—a clear sign of the strong evolutionary pressure that a beak can exert.) This clearly shows that the lice don’t automatically blend in when they arrive in a new environment. They do so specifically to avoid the attention of their hosts.
All the lice that Bush used belonged to the same species—Columbicola columbae. But by the end of the experiment, these individuals began to resemble other species that have been parasitizing different pigeons for millions of years. For example, the lightest individuals were just as light as Columbicola wolffhuegeli, a species that lives on one of the whitest pigeons—the pied imperial of Australia.
C. columbae and C. wolffhuegeli have been evolving independently for at least 20 million years, but in just four years, the former had changed enough to resemble the latter in color (although many other differences separate the two species).
This study reminds me of another ambitious evolutionary experiment that I wrote about earlier this year. In the hills of rural Nebraska, Rowan Barrett and his colleagues placed mice in large outdoor enclosures, built on light sand or dark soil. Over time, individuals that better matched their backgrounds were less likely to be eaten by owls—just as lice that blended in among their host plumage were less likely to be preened off.
Barrett’s team went one step further, though. It identified a gene that’s responsible for the rodents’ fur color, and it showed how variations in that gene became more or less common over the course of the experiment. It would be great if Bush and her colleagues could do the same for their lice, says Jessica Light from Texas A&M University. Still, as it stands, their experiment is already groundbreaking: Biologists “rarely, if ever” do evolutionary studies of this kind with parasites, says Light.
Which is odd because, as Bush says, “parasitism is the most common lifestyle on this planet.” It has evolved hundreds of times, and it’s likely that the majority of animal species are parasites. Many of these are confined to specific hosts. Feather lice, for example, are usually spread through direct contact between, say, parent birds and their chicks. But occasionally they can hitch a ride on more mobile parasites, such as flies, and end up on an entirely new host.
For them, that’s an event akin to a bigger animal arriving on a new island. Evolution tends to run wild on islands, as newly arrived animals quickly adapt to the new opportunities on offer and diversify into a riotous range of forms. The finches of the Galápagos, the tree snails of Hawaii, and the anole lizards of the Caribbean all arose in this way. But for a parasite, a host species can be an island—an isolated chunk of flesh with new opportunities to explore, and new challenges to overcome.