"Long before it's in the papers"
January 27, 2015


“Evolvability” may itself evolve

Nov. 18, 2013
Courtesy of the University of Pennsylvania
and World Science staff

The abil­ity to evolve can it­self evolve, a study sug­gests.

Ev­o­lu­tion­ary the­o­ry holds that as or­gan­isms mu­tate, those with help­ful muta­t­ions thrive at the ex­pense of oth­ers. Those muta­t­ions then spread through popula­t­ions and grad­u­ally change en­tire spe­cies.

Ev­o­lu­tion does­n’t op­er­ate with a goal in mind. But or­gan­isms with more ca­pa­city to evolve may fare bet­ter in fast-changing en­vi­ron­ments, sci­en­tists note. This raises the ques­tion: does ev­o­lu­tion fa­vor char­ac­ter­is­tics that in­crease a spe­cies’ abil­ity to evolve?

For sev­er­al years, bi­ol­o­gists have tried to pro­vide ev­i­dence that ev­o­lu­tion­ary pres­sures have acted on evolv­abil­ity. Now a pa­per by Uni­vers­ity of Penn­syl­va­nia re­search­ers of­fers what they call clear ev­i­dence that the an­swer is yes.

“It’s not con­tro­ver­sial that popula­t­ions evolve and that some traits are more apt to evolve than oth­ers,” said Dus­tin Bris­son, sen­ior au­thor of the stu­dy, pub­lished in the jour­nal PLoS Pathogens. “What we were ask­ing is wheth­er the abil­ity of an or­gan­ism to evolve is a trait” that is it­self sub­ject to ev­o­lu­tion­ary pres­sure.

For vi­rus­es, path­o­gen­ic bac­te­ria and par­a­sites to sur­vive long-term, they must be able 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,” or pres­sure to evolve, 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. It has a gene called VIsE needed for es­tab­lish­ing long-term in­fec­tion.

In the bac­te­ri­a’s ge­nome, near the VlsE gene are sec­tions of ge­net­ic ma­te­ri­al called “cas­settes” that are nor­mally not ac­ti­vat­ed, or made in­to pro­teins like nor­mal genes. But they can mix, or re­com­bine, with VlsE to al­ter the pro­tein that it makes.

The re­search­ers looked at di­vers­ity in the non-ac­ti­vat­ed cas­settes, which would pre­sumably not have been the ob­ject of di­rect ev­o­lu­tion­ary pres­sure. Thus di­vers­ity in the cas­settes would of­fer a win­dow in­to past nat­u­ral se­lec­tion for a more “e­volv­able” VlsE.

The re­search­ers eval­u­at­ed 12 strains of B. burgdor­feri for signs that nat­u­ral se­lec­tion had acted to in­crease the di­vers­ity of the cas­settes. “The ev­i­dence was re­markably strong in fa­vor of ev­o­lu­tion for more di­vers­ity among cas­settes and thus great­er evolv­abil­ity in the ex­pressed [pro­duced] pro­tein,” Bris­son said. It’s not clear wheth­er the re­sults apply to free-living an­i­mals, he added, but “we can now say that evolv­abil­ity can be the ob­ject of se­lec­tion in the face of en­vi­ron­men­tal pres­sure.”

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The ability to evolve is something that itself can evolve, a study suggests. Evolutionary theory holds that as organisms mutate, those with helpful mutations survive and spread at the expense of others. Those mutations spread through whole populations and gradually change entire species. Evolution doesn’t operate with a goal in mind. But organisms with more capacity to evolve may fare better in fast-changing environments, scientists note. This raises the question: does evolution favor characteristics that increase a species’ ability to evolve? For several years, biologists have tried to provide evidence that evolutionary pressures have acted on evolvability. Now a paper by University of Pennsylvania researchers offers what they call clear evidence that the answer is yes. “It’s not controversial that populations evolve and that some traits are more apt to evolve than others,” said Dustin Brisson, senior author of the study, published in the journal PLoS Pathogens. “What we were asking is whether the ability of an organism to evolve is a trait” that is itself subject to evolutionary pressure. For viruses, pathogenic bacteria and parasites to survive 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,” or pressure to evolve, 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 a molecule called VIsE needed for establishing a long-term infection. In the bacteria’s genome, the VlsE gene is preceded by sections of genetic material called “cassettes” which are normally not activated, or made into proteins like normal genes. But they can mix, or recombine, with VlsE to alter the produced protein. The researchers looked at diversity in the non-activated cassettes, which would presumably not have been the object of direct evolutionary pressure. Thus diversity in the cassettes would offer a window into past natural selection for a more “evolvable” VlsE. The researchers evaluated 12 strains of B. burgdorferi for signs that natural selection had acted to increase the diversity of the cassettes. “The evidence was remarkably strong in favor of evolution for more diversity among cassettes and thus greater evolvability in the expressed protein,” Brisson said. It’s not clear whether the results apply to animals like humans, but “we can now say that evolvability can be the object of selection in the face of environmental pressure,” Brisson said.