Thread by @mbeisen

it's a cartoon explaining a classic result in microbial evolutionary biology that (largely) resolved the question of whether selection acts on preexisting variation or if the selection induces mutations to occur (it won Salvador Luria and Max Delbruck a Nobel Prize)

https://twitter.com/Rossputin/status/1465487200640851972

the idea is as follows - you take a population of cells and divide them equally into a bunch of tubes and let them grow for several generations - then you pour the cells onto plates, apply some selective pressure to the cells, and count the number of colonies that grow

in the original experiment the selective pressure was exposure to a lethal virus, but it can and has been repeated with almost any condition where the growth of the bacteria requires a mutation not found in the original cell

the left side of that image shows the expectation if such mutations arise as a result of applying the selection pressure: that the independent replicas of the experiment should have roughly the same number of colonies, with variation explained by the Poisson distribution

the right side shows the expectation if mutations occur randomly in each tube before exposed to the plate - in this model you could end up with anything between 1 mutant cell or the entire population being mutant - with predictable frequencies of each outcome

when they did the experiment they found that the variance among numbers of colonies on each plate was way too high to be explained by Poisson, and was much more consistent with the pre-existing mutation model

this was, you can probably tell at this point, posted in response to people who claim that new variants are a result of vaccination - the point here is that this is bogus - the variants are the result of having lots of infections

with high levels of infection, lots of new variants emerge - most don't take hold for purely random reasons - but ones that are better at infecting people have an advantage and hence you get things like delta

if one of these new variants also happens to be less sensitive to the vaccine, it will spread more than ones that are vaccine sensitive - these are the equivalent of colonies on the plate

but these new variants aren't arising *because* of the vaccine - we're just noticing them because the vaccine is suppressing other strains