the bee's live in a hive with lost of other bee that look alike, but are in danger of being eaten by a bird.
with the ones that have change color is because of gene variation, this happens because when they live in an environment with dark back round they would need to change to survive.
when the bee that survive they find a mate and the "mate" and when their children they will care that trait (mutation) and pass it on to their offspring .
when the bee becomes darker it has a higher chance of surviving because the lighter one stand out and have a higher chance of being eaten and the one who have evolved blend in and so see by their predator.
since the bird can't see the dark one they will have a higher chance of surviving by that doesn't mean that they will have offspring they will need to find the food. by the gene pool has change and with the ones that will mate will pass on those traits
with the bees they needed to change is they wanted to live, this happen through gene variation (mutation), since there was two kind of bees the light vs. the dark to live. the darker bees and the advantage because they blended more in with the environment so they lived longer and had more offspring to carry on those traits.