Natural selection and the pressure to evolve

Some genes might not mind a bit of extra pressure when it comes to evolution.

A Swiss team led by Andreas Wagner of the University of Zurich has demonstrated evolution of a yellow gene to green in Escherichia coli – a common bacteria that lives in the gut. Strong selective pressure caused the gene to evolve more quickly, because it developed a robust protein that helped it to do so efficiently.

This could be one of the first experimentally demonstrated examples of selection helping a gene to be better at evolving, instead of crippling it. This is very hard to observe because of how long evolution usually takes.

“To our knowledge, this is the first experimental proof that selection can drive the ability to adapt in a Darwinian sense and increase evolvability,” says Wagner. “There are still people out there who question whether evolution is real. But we don’t just look at fossils where we have a historical record. We observe evolution in the laboratory.”

The findings are described in a paper in the journal Science.

Strong selective pressure occurs when only a few individuals in a population can reproduce, usually because the environment is harsh, and a very specific set of genes is needed to survive to adulthood. However, this can often mean that the genome doesn’t get a chance to collect useful mutations, and proteins can become weak.

However, the researchers found the opposite was the case in their E. coli experiment, where strong selection led to more robust proteins that were less likely to be damaged by harmful mutations.

“This discovery was a real surprise to me because it showed that selection for fitness didn’t conflict with selection for robustness, which contrasts with previous work,” says co-author Jia Zheng, also from the University of Zurich.

“While most mutations that proteins encounter harm their stability or ability to fold correctly, the robustness-improving mutations actually mitigate such deleterious effects. Robust proteins have a higher chance to function and thus evolve new traits.”

The team took a gene from a jellyfish that glows fluorescent yellow in certain light and put it into the E. coli to observe whether the resulting protein evolved to be a new colour. The use of a foreign gene meant they could observe the change in proteins without there being any influence from interacting genes elsewhere in the genome.

They then watched how the gene in E. coli evolved over four generations. To simulate natural selection, they chose greener glowing E. coli and subjected the gene to genetic mutation, as would happen in nature. They found that most of the E. coli in the final generation glowed green, despite the extra mutations, when the green protein was strongly selected for.

The team compared two experiments. The first tested strong selective pressure by only choosing the top 0.01% of green glowing E. coli each generation, and the second tested weak selective pressure by choosing any cells that glowed green to carry on.

Interestingly, the genes that experienced strong selection to be green also evolved robust proteins that kept their shape and function well. This might be because the selective pressure did not allow for detrimental mutations to really become established in the population, so proteins ended up being stronger and/or resisting the effects of mutations.

“It shows that natural selection can play a crucial and active role in creating standing variation that is both beneficial and enhances evolvability—for example, by increasing robustness to deleterious mutations. This contrasts with some theoretical and experimental work, in which first-order selection for fitness conflicts with second-order selection for robustness,” the researchers say in their paper.

This experiment was done under laboratory conditions and might not reflect all instances of natural selection in the wild, especially as it tested a single gene instead of a whole genome. Nevertheless, this is an exciting demonstration of selective pressure because of how difficult it is to replicate evolution.