Question

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 reason for this?

Answer 1

This would be natural selection!

Natural selection is the process during which some individuals of a species survive and transfer their genes to heir offspring, and others don't. The ones that did survive mush have had some variation in their genotype that helped their survival.