Most moths of a particular population died when the chestnut blight fungus that they were dependent on got wiped out but some moths did survive which mechanism explains the reasons for this

The answer is natural selection. 

Natural selection is one of the fundamental components of advancement, alongside change, movement, and hereditary float. Natural selection is the procedure amid which a few people of an animal types survive and exchange their qualities to their posterity, and others don't. The ones that survived more likely than not had some variety in their genotype that helped their survival.

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