Evolution does not operate with a goal in mind; it does not have foresight. But organisms that have a greater capacity to evolve may fare better in rapidly changing environments. This raises the question: does evolution favor characteristics that increase a species' ability to evolve?
For several years, biologists have attempted to provide evidence that natural selection has acted on evolvability. Now a new paper by University of Pennsylvania researchers offers, for the first time, clear evidence that the answer is yes.
The senior author on the study, published in the journal PLOS Pathogens, is Dustin Brisson, an assistant professor in the School of Arts and Sciences' Department of Biology. His coauthors include Penn's Christopher J. Graves, Vera I. D. Ros and Paul D. Sniegowski, and the University of Kentucky's Brian Stevenson.
"It's not controversial that populations evolve and that some traits are more apt to evolve than others," Brisson said. "What we were asking is whether the ability of an organism to evolve is a trait that natural selection can pick."
For species of viruses, pathogenic bacteria and parasites to survive over the long-term, they must possess an ability to rapidly adapt and evolve, enabling them to stay one step ahead of their hosts' immune systems. But these pathogens don't need to foresee what conditions lie ahead of them. They only must change into something that the immune system has never seen before.
"Pathogens face a very strong selection pressure from the host's immune system," Brisson said. "If they don't adapt, they will die."
The researchers used this fact to seek evidence that natural selection had favored increased evolvability, focusing on the Lyme disease bacteria, Borrelia burgdorferi.
B. burgdorferi possesses one protein that is essential for establishing a long-term infection of a mammalian host: VlsE.
In the Lyme bacteria's genome, the VlsE gene is preceded by "cassettes" which are normally not expressed, or made into individual proteins, but can recombine with VlsE to alter the expressed protein and thus present a novel challenge to a host's immune defenses.