On the left we see culture amplifying genetic differences - while on the right is an example of culture obscuring them.
There's a strong theoretical reason to expect cultural differences to be correlated with genetic differences: since both are correlated with distance between the populations being compared. Of course, in the modern world, egalitarian memes do their best to reverse this effect.
Probably cultural variation within groups is minimised by transmission processes and pro-egalitarian memes. However on larger scales, drift probably dominated over most of the course of human evolution. In this case, cultural variation seems likely to have acted as a significant divisive force between humans from different tribes. The effects of cultural variation can be large. Cultural differences may well represent a more powerful stimulus to kin recognition mechanisms than genetic variation does.
The divisive influence of cultural variation may go some way to explaining the human tendency towards xenophobia. Cultural variation may be acting as a superstimulus to kin-recognition mechanisms, causing strangers to appear especially alien.