What's to say that species will adapt all the time? It could be that terns won't be able to adapt fast enough, and will die. There's die-offs of individual species all the time, and historically there's been several major multi-species die-offs when conditions have changed too rapidly.

Mutations don't have to occur at the point of their necessity. Mutations that survive can be survivorship-neutral (or nearly so), but suddenly become useful many years later during condition changes, and thus become dominant.

Take eye colour. As far as I'm aware, aside from random sexual preference, there's no real survivorship benefit to have a particular colour eyes. So we have populations with various different colour eyes. If aliens who hate people who don't have blue eyes show up and kill everyone without blue eyes, that mutation suddenly becomes selective.

The mutations that help species survive changes in conditions are usually already there when the change shows up.