A new study by University of Pennsylvania researchers offers, for the first time, clear evidence that natural selection has acted on evolvability. (Agencies)
"It's not controversial that populations evolve and that some traits are more apt to evolve than others," Dustin Brisson, senior author on the study, said.
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 lay 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. If they don't adapt, they will die," Brisson said.
The researchers used this fact to seek evidence that natural selection had favoured 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 defences.
Though earlier studies had suggested that selection may directly favour the capacity to evolve, they could not definitively rule out that evolvability had arisen for other reasons.
In particular, it has been difficult in empirical studies to rule out the possibility that evolvability arises and is maintained as a byproduct of selection on organismal features more directly related to fitness.
In the Lyme disease system, the researchers got around this confounding factor by looking at diversity in the unexpressed cassettes, which would not have been the object of direct selection because they have no known function on their own; they simply exist as a way of increasing the potential diversity of the VlsE protein.
Thus diversity in the cassettes would offer a window into past natural selection for a more "evolvable" VlsE.
"Organisms with greater diversity among the cassettes will have a selective advantage as they will be more antigenically evolvable, or better able to repeatedly generate novel antigens, and will thus be more likely to persist within hosts," researchers said.
The study was published in the journal PLOS Pathogens.
A new study by University of Pennsylvania researchers offers, for the first time, clear evidence that natural selection has acted on evolvability.