If you look up what has classically constituted species and subspecies, you will know that there are geographically isolated groups of H.sap that cannot interbreed naturally between both genders.
I can name many, but for this discussion we only need two.
Species aren’t, or haven’t classically, been defined as populations whose members
can be genetically compatible, but as those who in nature, interbreed to produce viable offspring. If two populations cannot do that, M1-F2 and M2-F1, they are not members of the same species. If they do it M1-F2 but not F1-M2, or vice-versa, they are subspecies of one another.
We also classify as different species, those who can mate and produce viable offspring, but in nature do not, usually because the populations are geographically separated. But some groups have changed so much physically, that in sexual selection, the other group is not recognizable as “self” and they simply
won’t mate with one another. This is how Darwin’s finches speciated when they were all confined to a small set of islands.
So, by example: The BiAka people of the Western Congo Basin are a very ancient nomadic pygmy tribe. The Abatutsi people of Central Africa average over 7’ in height. These two peoples would not encounter each other for mating, but if they did, while it is theoretically possible that BiAka males could produce viable offspring with Abatuitsi females, it is not possible for Abatutsi males to produce viable offspring with BiAka females. The woman’s anatomy precludes carrying the infant to term. These two groups not only do not, but cannot produce viable offspring in nature and so, if we use pure science and treat H. Sap like we do any other species, they are subspecies.
Because human young are so very altricial already, nature pushes the biological envelope and makes our babies as big as possible when they are born, which is why you can confidently sex a skeleton with half a pelvic girdle. Indigenous African peoples are at much greater genetic distance from one another than all other peoples. A Norseman is genetically closer to an Australian aborigine than a BiAka is to an Abatutsi.
The whole of the human race really makes up what is called a “ring species.” A whole series of subspecies that often interbreed at the edges of their populations, but as they become more physically, geographically and genetically distant, no longer do or no longer can. What is unscientific, is the refusal to simply recognize this, which keeps some important work in genetics from being done because the politics means:
you cannot even write a grant to study it.
(Of course, we are rapidly erasing almost all of these geographic boundaries in modern times and many of these boundaries are being erased. )