As natural selection acts on a trait in a population, how will the amount of variation associated with that trait in the population

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.  

metamorphism is the change of minerals or geologic texture (distinct arrangement of minerals) in pre-existing rocks (protoliths), without the protolith melting into liquid magma (a solid-state change). the change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. i hoped i !

maybe because its impossible for balloons to lift a whole entire house.