What I personally do:
Have a breeding pair. When babies grow up, put the best one in the trade center, release/sell the others.
Everytime another litter grows up, if one of them is better than the one in the trade center, sell the one in the trade center and replace it with that new one.
Once the breeding pair dies of old age/gets rehomed, find a mate for your animal in the trade center in the market, don't forget to "compare mates" before buying: breeding strong size/longevity is very likely to give strong (or even stronger) size/longevity, but immunity/fertility is based on genome diversity, so sometimes, a 0% immunity animal would actually be your best choice. Here is info on how genes work: https://www.reddit.com/r/PlanetZoo/comments/e30gx7 Source: https://www.reddit.com/r/PlanetZoo/comments/e30gx7/data_how_genes_and_breeding_works_in_planet_zoo/
Have a breeding pair. When babies grow up, put the best one in the trade center, release/sell the others.
Everytime another litter grows up, if one of them is better than the one in the trade center, sell the one in the trade center and replace it with that new one.
Once the breeding pair dies of old age/gets rehomed, find a mate for your animal in the trade center in the market, don't forget to "compare mates" before buying: breeding strong size/longevity is very likely to give strong (or even stronger) size/longevity, but immunity/fertility is based on genome diversity, so sometimes, a 0% immunity animal would actually be your best choice. Here is info on how genes work: https://www.reddit.com/r/PlanetZoo/comments/e30gx7 Source: https://www.reddit.com/r/PlanetZoo/comments/e30gx7/data_how_genes_and_breeding_works_in_planet_zoo/