It is biological. It can be detected visually and on a finer level, it is inherited, it correlates with aggregate trait differences. It I something biological even if you think it's "arhitrary" so we can discuss what it means
 
And the arbitrariness is never shown to matter on the issues being discussed. I often hear things like, "Africa is diverse" and  more specifically that only some Africans are good sprinters. Okay.