Evolution of small things like algae and the krill which feed on it and feed the whale is quite fast. Single celled organisms reproduce on the scale of 20 minutes and hold immense amounts of genetic diversity in their populations to facilitate the success of a better adapted line almost immediately. Additionally, they are adept at horizontal gene transfer from other well-adapted organisms.

Evolution has been found to be happening 2-4 times faster than the rate earlier thought: https://news-archive.exeter.ac.uk/2022/may/articles/fuelofev...