Answer:
The amount of variation associated with a trait in a population changes because natural selection selects those alleles that are beneficial for the survival of the individual so it increase the frequency of beneficial alleles of a trait and eliminates those allele which reduce the reproductive fitness of an individual.
So the population which have traits that are non-beneficial will be eliminated by natural selection and the population with the beneficial variant trait are selected to live and reproduce to pass their offsprings.
