When we think of evolution, we think natural selection, survival of the fittest. But that’s not all. Primal forces lurk work in the background. When selection disappears, they finally break free.

Ok, perhaps that’s a bit overdramatic.

But seriously, chance is a hidden force nudging evolution around. While selection points to a trend (who’s more likely to survive), it is chance that actually decides who lives or dies. An incredibly fit, genetically perfect bug could still end up splattered on your windshield before bringing its perfect genes to the next generation.

 

This randomness has an effect, called genetic drift, on the overall distribution of genes in the population. While the force of selection pushes for the fittest to survive and the others to go extinct, genetic drift is a force of chaos, just pushing for the population to become perfectly homogeneous, regardless of its fitness.

Genetic drift should be the stronger, the smaller the population. Take two bacteria, it only takes one of them to die for its entire lineage to go extinct. With 1000 of each type, the impact of each single birth and death is much less. So as the population grows, drift should weaken. But enough to stop completely? I worked on this with biologists, who grew actual bacteria in a lab, while our theory team worked on math to describe it. We wanted to see whether growth was strong enough to stop genetic drift.

 

 

We found was that, as populations grow larger, drift really becomes weaker. Its action slowed more and more, until it comes to a complete stop: the distribution of genes set forever. Growth freezes genetic drift.

We looked at a simplified version of a bigger problem: from algal blooms to the first bacteria to colonize a newborn baby, examples of few cells growing with little selection abound. Our research indicated that, by growing, these community can maintain the genetic diversity that will allow them to tackle many diverse conditions in the future.

If you want more

• It turns out bacterial reproduction looks a lot like drawing marbles from an urn. So an old model for that was our starting point.
• This work was actually published. You can check out what the paper looks like here.
• This post is but the first installment of a whole series on what biophysicists (or, in this case, I) do. You can find all of it here.

 

Cover photo: CC0 Myriam-Fotos/pixabay