I think it's honestly better if you use Mosquitoes and DDT as an example for chemical conditions in the environment.
Mosquitoes and DDT:
When DDT (dichloro-diphenyl-trichloroethane) was first used as an insecticide to kill malarial mosquitoes, low concentrations were effective.
In subsequent doses, higher concentrations were needed and the sprayings became less effective.A select few from the population were naturally DDT-resistant that had survived, these then reproduced and passed on their resistance gene to their offspring, as a result the majority of the mosquito population is mainly resistant to DDT
OR you can use bent grass and heavy metal toxic waste as a different example, (whatever you find easiest to understand):
In mining areas of Wales, some areas of soil got contaminated by heavy metal waste. The bent grass grew in both the unpolluted and polluted areas. Over a number of generations, the populations on polluted areas became a whole different species.