system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do breeders ensure diversity among their animals' offspring? Wouldn't they have to constantly buy new breeding pairs?
gpt
Sometimes they don't.

There are things called line breeding in which breeders intentionally breed related animals (such as ones with the same grandparent).  

People will also sometimes breed daughters to their fathers..  one mating of such a combination usually isn't a problem but if that offspring was then bred to a close relative that is when problems might show up.

You didn't really say what kind of animal you were thinking about.. cats.. dogs.. horses.. cattle.. chickens..

It's quite different for dogs as it would be for horses because people might only own one pair of dogs at a time but with horses they might own 30 mares (females) and 2 stallions (males).. with cattle they might own 300 cows and 0 bulls (they can get frozen semen) so could pick different bulls each year.

Additionally most breeders don't keep the offspring - they sell them - and might on occasion keep 1 or 2 animals that they produced but then would breed to a different animal.