"Social construct" implies an arbitrary choice -- our society decided to split humanity into races this way, but another society might do it in an entirely different way and all such ways are equally valid, which is to say, there are no underlying "real" differences.

Supposing someone wanted to split humanity into arbitrary races based on actual genetics (which is not how the concept of race originally started because genetics wasn't known at the time), it would make sense for most races to be African, since Africa has far more human genetic diversity than all the other continents combined do. The reason races are delineated the way they are now is due to social reasons. (It could possibly make sense when you consider the phenotype though, but due to the outgroup homogeneity bias, I have some doubts.)

Still, regardless of where you set the boundaries between races, there will be average biological differences between them (provided you don't do something biologically ridiculous like classifying whites and Asians as the same race but then classifying their half-white/half-Asian children as a different race).