In simple words what causes there to be additive genetic var

In simple words, what causes there to be additive genetic variance in a population (V_A)? Why is additive genetic variance (V_A) necessary for a trait to evolve in response to natural selection?

Solution

Additive genetic variance is caused due to average variance that results from an average or additive effect of alleles of parents.Additive genetic variance (VA)is necessary for a trait to evolve in response to natural selection because traits are determined by genes.An allele that is advantageous will be selected under natural selection only if it poses advantage additively.