Paper Rock Scissors

How Rock-Paper-Scissors Could Shape Bacterial Evolution

“Imagine a nontransitive community in which, for convenience, we call the players Rock, Paper, and Scissors.” It’s a rather unusual line to find in a scientific paper, but the study it comes from is part of a special edition of the Proceedings of the National Academy of Sciences (PNAS) on the evolution of social behavior. And rock-paper-scissors is not only a good way to settle an impasse; it provides a way of understanding a biological phenomenon that has been termed “survival of the weakest.”

To understand this phenomenon, you have to understand what a non-transitive community is. The easiest way to understand this is to examine the three different types of bacteria used in the experiments described in the paper.

One of them produces a protein, colicin, that’s toxic to other cells; it also makes a set of proteins that allow it to survive the toxins it makes. Producing all these proteins exacts an energetic cost, so these bacteria can be outcompeted by cells that don’t face the same burden.

A second strain has mutations that eliminate the proteins colicin binds to. This is a big help if the first strain is around, but these proteins perform helpful functions, so these cells can also be outcompeted by healthy, wild-type cells. And those healthy, wild-type cells are the final strain involved. The result of mixing all three is a rock-paper-scissors-like situation, with each strain growing better when faced with competition from only one other. So, the resistant strain will outcompete the toxin-producer, which will kill the wild type cells, which will in turn out-compete the resistant one.

What happens when you throw them all together? That’s where the survival of the weakest comes in.

If one of the strains of bacterial successfully wipes out the one it grows better than, then it will be left with nothing but the third choice — which necessarily will grow better than it can. So, the selective pressure ends up favoring strains that don’t effectively wipe out any competition they can; instead, it favors the evolution of restraint. As the authors phrase it:

Next, imagine a less-restrained variant of Rock, called Rock*, that displaces Scissors at a faster rate. In a Rock*-Paper- Scissors community, the abundance of Scissors decreases because of the increased prowess of Rock*. As a consequence, Scissors’ victim (Paper) is liberated, which can displace Rock*. In an ironic twist, the improved Rock* decreases in abundance because of the expansion of its victim’s victim.

sectionals: CHALLENGE 47 - ROCK, PAPER, SCISSORS.

Specifics: Most of you should be familiar with Rock, Paper, Scissors! Paper covers rock, rock smashes scissors, and scissors cut paper. You must comment with one of those 3 choices, and you'll be matched with the person that commented before or after you. If you win the matchup, you'll win 10 points. If you lose, you'll earn 0 points, simple as that!

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