But NB that for the virus to escape our immunity, that selection pressure is not enough.

We also need diversity. A new virus type that better evades immunity needs to exist or be created

And here's where we get into the weeds. Because in reality, you can't always completely decouple the question of diversity from the question of selection pressure

To stick with viruses, within a single infected individual, killing off viruses with an immune response creates selection pressure for immune escape, but it also limits the virus's ability to generate diversity through mutation

And it gets more complicated because some kinds of immunity may affect all the viruses equally (reducing diversity generation without creating meaningful selection pressure), while others affect some more than others (creating selection pressure)

@colinrussell, @veli_pet, colleagues, and I have modeled what we think is a pretty cool potential example of this in flu antigenic evolution: https://twitter.com/dylanhmorris/status/1326906383569186816

So when you think about adaptive evolution, remember that both diversity and selection are necessary, but also remember that certain forces can affect both the level of diversity and the level of selection pressure

/end