G.B. Johnson, et al. This means that variation is irrelevant to a population's capacity to respond to new forces of selection. Genetic variants do not differ in fitness and do not constitute raw material for adaptation to new selective regimes. Molecular evolution is mostly caused by drift. Some genetic variants may be neutral under specific conditions or at particular times but show selective values as conditions change.
Selection can operate without creating impossible genetic loads.
Kimura's original case for neutral theory is not supported by more recent results. The controversy is not yet settled. We'll probably settle on some holistic view, but there is still plenty of room for argument over the details. Some loci show major polymorphisms , having two or more relatively common alleles. Other loci have a number of very rare alleles and, therefore, show minor polymorphisms .