system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What triggers evolution in a species?

I recently learned about cat's not drinking water near their food because of natural instincts they had developed. 

This means that at some point, felines drank water and had to have died right? So how was that instinct developed/passed on if it involved something that would kill the host?
gpt
There is no real trigger... Evolution is the survival of the individual who is best adapted to surroundings. Let's say a pack of mammoths ends up in the desert. Then the amount of hair determines how well a mammoth regulates its body temperature. The overheating mammoth dies, giving the rest of the mammoths more chance to procreate and pass their amount of hair to the next generation. Basically we always evolve, but random mutations that give an individual benefits or not appear randomly, so there's no guarantee that there will be change.

You could say that change of surroundings triggers evolution to cause change but evolution itself is always there.