In the comments below in regards to mixed-race children:
“Designer baby”? How about “genetic experiment”? Like sitting all day long, eating lots of sugar, etc., mixed race offspring are biological experiments we are doing on ourselves that very seldom happened among our ancestors in the environments of evolution. Ideological certainty notwithstanding, we don’t know what most of the biological consequences, good or bad, for mixed-race descendants are going to be.
On the one hand there is some truth in this is, insofar as contact between populations which are very genetically divergent was likely to be a rare event in the past. That is because even minimal gene flow between populations can homogenize between population genetic differences (population genetic theory usually gives 1 migrant per generation). Sustained contact between populations genetically and physically different to a magnitude which we would term racial probably required very specific events, such as trans-continental migrations, or the opening of virgin land due to deglaciation and demographic expansion from differing directions.
But, there are two caveats, a general and a specific. A general caveat is that those periods of contact were very important. This is trivially obvious when you consider the expansion of a recently African derived population over Eurasia and Oceania ~50,000 years ago, replacing earlier Homo lineages in less than 10,000 years. But there is a second critical point which has started to become evident over the past few years, and that is that it seems likely that the majority of populations extant today are the outcome of admixture events between rather isolated lineages which were genetically and physically distinct. I’ve posted on this before in a broad sense. But for Dr. Calvin Greene, and I suspect the reader below, this is probably the paper of relevance in terms of what they care about: Ancient human genomes suggest three ancestral populations for present-day Europeans. In short most of the world’s “races” are the products of admixture events in the range of 5,000 to 10,000 years ago.
Note: I use the image of Vendela Krisebom because she is the product of ancient and recent admixture.