Evolution happens in bursts when species are under pressure and a lot of animals are able to make quick adaptations within a few generations. Some animals don't even need to change because of prior adaptations they never lost and they just thrive.

For example, if a species had previously adapted to arid conditions but was living in a more hospitable region they don't tend to lose their ability to handle arid conditions without some kind of pressure. Access to water doesn't mean they will lose those traits, they just don't need all the water that is available. If the area turns arid they are not impacted by the lack of water like those that don't have those traits. Or they might not currently express their genes in a way that causes that adaptation to arid conditions to show up normally, but they don't necessarily need significant genetic changes to express those traits.

Look at dogs, who can have massive variation from breeding within a few generations.

[–] toynbee@piefed.social 3 points 6 days ago* (last edited 6 days ago)

That's how we get mutants.

edit: Link.