the harsh conditions could cause a narrowing of the gene pool. Say some of the species "Organism A" have a mutation that causes them to be less susceptible to to the chemicals found in drains. This means those survive while others do not. This means this trait is passed on until it become normal. Now say this mutation also causes them to be a different colour (or a recessive gene passed on by the progenitor of the chemical resistance mutation also passes on this recessive gene/mutation) then it is more likely that "Organism A" found is sewers will also be displaying this recessive gene / mutation.
So basically an organism found in harsh conditions may be considered a mutant but its not because of the chemicals per se but natural selection and a narrowing of the gene pool...... and that doesn't happen over night.
Edited by Cynical Sounds, 26 December 2012 - 12:05 PM.