New research provides evidence that evolution is much more like this simple game than first expected, and solves a 30 year old paradox by one of the great evolutionary biologists
A popular and instinctive approach to evolution is to see it as a linear process. One individual in a population gains an evolutionary advantage and reproduces more successfully. Say they have trait B, where most of the population has trait C. After a while you see more and more individuals in the population with trait B, because they’re reproducing more than those with trait C. Then after a while someone else comes along with trait A. Trait A allows that individual to reproduce more successfully than trait B. It follows, logically, that trait A will allow individuals to reproduce more successfully than trait C, right? This makes sense to us mathematically: if B is greater than C, and A is greater than B, then A is greater than C. This idea is called transitivity: relationships existing on a line.
But evolution isn’t like this. This has been most famously explained by evolutionary theorist Stephen Jay Gould described in a 1985 paper as ‘the paradox of the first tier’. This states that clear evidence of natural selection operating over successive short periods of time suggests that, if evolution is transitive, we should be able to identify broad patterns of progress over long periods of time. Only, we can’t do that.
A great deal of evolutionary change appears to be governed by nontransitive relationships, which has only just been experimentally shown for the first time in a study from Lehigh University. But what’s a nontransitive relationship? The best analogy is a game we all know: rock paper scissors. Teaching a young child rock paper scissors for the first time is always amusing in a condescending kind of way, and usually goes something like this:
“Is rock the best?”
“Actually, paper beats rock”
“oh…so paper is the best?”
“No, scissors beats paper”
“I get it, so scissors is the best”
“Well no, because rock beats scissors”
It can go on quite a while. These kinds of relationships aren’t obvious or instinctive to us, so it’s hard to imagine them. They’re also shown in some of the art of MC Escher.
Stairs going up, and up, and...back where we began? Image credit: MC Escher
The experiment
So how was nontransitive evolution shown by researchers at Lehigh? They used yeast, observing it over the course of 1,000 generations.* The yeast were bred in sync with a virus. At first, the population produced the virus’ toxins but also produced their own immunity to the toxin. Over time, the yeast gained many beneficial genetic mutations in its own genome and the virus lost its toxicity. The yeast then lost their immunity to the virus’ toxins, as it was no longer needed. Finally, these yeast in the 1000th generation were put in competition against yeast from the original generation (or at least their clones) and, since the original generation produced the viral toxin, the 1000th generation was wiped out.
Success and the future of evolution
The nontransitivity of evolution is a highly effective argument against people who describe organisms (let’s be honest, usually humans, or even a specific type of humans) as ‘more evolved’ or ‘more evolutionarily advanced’ as an argument for some kind of naturally divined superiority. Being further down the timeline in evolution means very little in terms of physical prowess or ability to adapt. It’s simply the right organisms being in the right places at the right times. Martin Luther King Jr. once said, paraphrasing Theodore Parker, ‘the arc of the moral universe is long, but it bends towards justice.’ This research shows that the arc of evolution by natural selection is long, and bends a lot, towards nowhere in particular.
References:
https://elifesciences.org/articles/62238 - source article
https://www.cambridge.org/core/journals/paleobiology/article/abs/paradox-of-the-first-tier-an-agenda-for-paleobiology/C739F209925B7E0E282BE6BADEB1847D - Gould explaining his paradox of the first tier
Cover image credit: rawpixel.com
*Given yeast’s generation time of 90 minutes – 2 hours, a 1000 generation experiment is possible in under six months. An equivalent experiment in humans would take over 20,000 years.
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