A number of epidemiologists and virologists did not expect to see significant adaptive evolution for increased transmission in covid-19, and continue to argue against that hypothesis. Vincent Racaniello, well-known virologist, takes this position. So does David Dowd, an infectious disease epidemiologist at Johns Hopkins.
They think that chance can drive a new variant with no transmission advantage to high frequency, even when there are many cases ( tens of thousands or more) . The B.1.1.7 variant went from ~1% to a big majority of cases in England.
I’ve just run some sims ( with the sort of transmission dispersion observed in cov-19). R (fitness) = 1, in both cases. I propagate 100 times in each run. Variant A starts out with 5,000 cases, B with 2000. How often did B catch up with A? 6 out of 100 runs.
How often did B catch up with A when A started out with 50,000 cases and B with 20,000 cases? zero, out of 100 runs.
Start out with 2000 B and 50,000 A, same fitness: how often did B catch up? zero out of 100 runs.
Start out with 2000 B ( with a fitness of 1.05) and 50,000 A with a fitness of 1: How often did B catch up? 100 out of 100 runs.
The top curve shows the relative frequency of B.1.1.7 as a function of time in Denmark. Dowd can look at that and believe it is a random fluctuation. Wow.