I’ve had tanks where shrimp will match their surroundings quite closely within a year or so. This would be due to some micro predator being present and picking off any babies that are easy to see. Shrimp have babies frequently and the only ones that survive in those conditions are able to blend in really well. Every 2.5 months or so a new generation becomes sexually mature and has dozens of babies every 8 weeks or so.

It’s a fun hobby because you can actually develop your own morphs in a matter of only years if you want to.

So it’s not necessarily evolution, but pigments developing (or not) due to environmental pressures.