With respect to Lewontin’s well known “there is more genetic variation within groups than between groups” we need to clarify whether the 85:15 split has any meaning other than the fact that the bulk of human genetic variation is randomly distributed.
Comparing Danes vs. Nigerians: 85% variation within each group and 15% between. The same would be observed with Japanese vs. Iranians.
What if you considered a mixed group of Danes + Nigerians as a single population, and the same for Japanese + Iranians? If you then apportioned genetic variation between D+N vs. J+I you would still get more variation within than between.
If you went in the opposite direction, and considered Japanese from Tokyo as one population and Japanese from Kyoto as another population, the same within/between distinction would hold. If you compared one Japanese family to another, you would also see more genetic variation within the group (family) than between families.
As has been pointed out previously by others, a significant amount of genetic variation is found within single individuals; thus, if you were to compare one Japanese individual to another,~ half the genetic variation would be found within the single individual.
For any set of human groups, one would expect to find more genetic variation within the group than between groups.
Hence, the “within group” component of genetic variation is found within any defined set of individuals, and is randomly distributed among individuals. It cannot be used to assert that members of an ethny are more dissimilar than to other ethnies, nor can it be used as a legitimate argument against the reality of genetically distinct population groups.
And this doesn’t even touch upon the fact that with respect to many phenotypically relevant traits under selective pressure, racial differences in allele frequency is so great that there is actually greater genetic variation between compared to within groups.