Peppered moths and pigeon lice (Introduction)

by David Turell @, Thursday, March 14, 2019, 21:13 (102 days ago) @ David Turell

Color change in pigeon lice shows color change as in the moths:

"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.


"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.


"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."

Comment: This is probably a color adaptation, but it is possible minor color differences resulted in surviving lice that more closely mirrored their hosts. Only genetic studies will clear up the issue. This is not a species change, any more than the different colors of humans are different species.

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