[lotsofpulp]

>Both parts can happen concurrently and gradually, like those butterflies who turned black to hide in coal smoke.

Do you mean the black ones survived and procreated?

[inigyou]

Yes, but gradually. The blacker ones reproduced more. There weren't any black ones at the beginning

The opposite is the Black Death. Some people had more aggressive immune systems that were able to destroy the bacteria before it destroyed them. This is thought to have caused a jump increase in the "immune aggressivity" of European populations, causing more autoimmune diseases in subsequent generations.