杏吧原创

There has been much theoretical debate about whether and how organisms can evolve to become better at evolving, but now a team of researchers has made it happen in a lab 鈥� and documented almost every single mutation along the way.

at the Max Planck Institute for Evolutionary Biology in Germany and his colleagues put bacteria in an alternating environment where they had to evolve to survive. Many lineages of bacteria died out, but some became better and faster at evolving.

These lineages 鈥渞eplaced those that were less proficient,鈥� the team writes in a paper awaiting peer review. 鈥淥ur results鈥� clarify conditions for the adaptive evolution of evolvability.鈥�

Evolution relies on variation. When organisms reproduce, some of their offspring may have variants that make them more or less likely to survive than others. Beneficial mutations spread because individuals that have them are more likely to survive and reproduce.

There is no doubt that some species have properties that make them better at evolving than others 鈥� that they are more evolvable. However, evolvability isn鈥檛 a property of an individual but of species or lineages 鈥� and the orthodoxy among evolutionary biologists is that natural selection acts at the individual level rather than at the lineage or group level.

To put it another way, being good at evolving is something that benefits groups rather than individuals, which makes evolvability a controversial subject. Some think it can come about as a byproduct of selection for other characteristics, but that it isn鈥檛 selected for directly.

In the experiment, Barnett and his colleagues grew a soil bacterium called Pseudomonas fluorescens in small tubes of water containing essential nutrients.

Bacteria that sink to the bottom of the tubes run low on oxygen, limiting their growth. So some strains of P. fluorescens have evolved the ability to spread across the surface by forming mats.

The mats form when cells produce cellulose to link themselves together, but producing cellulose is costly. So in these mats, bacteria that cheat by not making cellulose tend to evolve sooner or later.

The researchers selected for lineages that could quickly evolve between the mat-making and cheating states. They put mat-making strains in 32 tubes. After six days, they isolated any cheater bacteria that had evolved from each tube and put them in a clean tube. The cellulose-producing bacteria were discarded.

In the clean tubes, the cheater bacteria sank to the bottom and there was a chance for mat-forming bacteria to re-evolve. After three days, any mat-forming bacteria were isolated and put in fresh tubes. This process was repeated up to 40 times.

If no mutants could be isolated, that lineage died out and one of the others took its place by being divided into two tubes. Along the way, the genomes of each lineage of bacteria were sequenced.

Most of the starting lineages died out sooner or later. Two of the surviving lineages evolved overall mutation rates up to 350 times higher than they had initially, meaning they generated more mutants of all kinds.

A higher overall mutation rate has its downsides because it generates lots of harmful mutations too. One lineage of bacteria in the experiment evolved an even better solution, becoming 10,000 times more likely to generate mutations affecting just one key gene involved in switching between mat-forming and cheating.

This is known as local hypermutation. Some disease-causing bacteria exploit local hypermutation to help them evade immune defences, but this is the first time it has been seen evolving from scratch.

In this case, the local hypermutation is a result of the evolution of a repeated sequence whose nature means the number of repeats is likely to change whenever the DNA is replicated. An odd number of repeats effectively switches off the key gene, while an even number turns it on.

鈥淭his is undoubtedly a highly original and very important paper,鈥� says at the University of Oxford.

at Michigan State University says that in his team鈥檚聽long-running evolution experiment, some bacteria did become more evolvable by increasing their overall mutation rate,聽but none developed local hypermutation.

鈥淚 don鈥檛 know of another experiment that has demonstrated the de novo origin of this phenomenon,鈥� says Lenski. 鈥淲e tried something similar many years ago, but we failed.鈥�

While there is no doubt about how the bacteria evolved in this experiment, the team鈥檚 explanation for why is likely to be more controversial. The way the experiment was done imposed selection for lineages rather than for individuals, the researchers write.

What鈥檚 more, this could happen outside the lab, too, they argue. While it may be rare, it is likely to happen among disease-causing pathogens, the paper states: 鈥淲e further suggest that selection on lineages is likely more important than generally appreciated.鈥�

鈥淭here is little doubt that the claim of lineage selection will prove controversial and will require substantive discussion,鈥� says Moxon.

New 杏吧原创 contacted the paper鈥檚 authors, but they didn鈥檛 want to discuss their findings publicly ahead of publication in a peer-reviewed journal.