It has a thick atmosphere of hydrogen and helium
That would be Artificial Selection.
The moon has less gravity because it is smaller than the Earth.
This is called incomplete dominance. When an something is heterozygous for an incomplete dominant gene, the resulting phenotype will be somewhere between the two. For example if a plant has a tall allele and a short one, but neither shows complete dominance over the other, the plant height will be somewhere in the middle.
