Some animals have subspecies. Humans don't. This is something that's widely misunderstood popularly but which is indubitable biologically. For example, many people are not aware that there is vastly more genetic variation between different breeds of dog than there is between any two groups of humans.
But regardless, the more important point is that whatever biologically-motivated categories humans might be grouped by would not correspond to 'races' as normally understood.
That article doesn't say anything about small populations. The word 'subpopulation' appears in the abstract, but it's talking about subpopulations of sperm in ejaculate.
I have to say, this is a super weird paper to suddenly pull out of nowhere in order to make an argument against the modern biological consensus that there are no subspecies of humans. It says nothing about species or subspecies, or even different populations of humans. Did you just Google some keywords and go with the first result? I'm genuinely puzzled.
> However, in this article, I show that female-mediated sperm selection can also facilitate assortative fusion between genetically compatible gametes. Based on this evidence, I argue that reproductive failure does not necessarily exclusively represent a pathological condition, but can also result from sexual selection (‘mate choice’) at the level of the gametes.
I'm haphazardly suggesting that the above is the same as:
> "There are some (small) genetic populations whose genetics diverged so much, from geographic separation, that they have fertility problems [1].
We have genetic populations that are the result of geographic separation, and we even have genetic divergence that makes reproduction difficult/impossible.
Again, what's left? Why can't we categorize human genetic populations to the same level? Please be specific in what's missing?
> It says nothing about species or subspecies
Why would it? If the categorization of humans included subspecies, I couldn't have responded.
Precise equality isn't required for a conceptual discussion, especially one that's so ill defined/subjective as the concept of subspecies [1].
It's an established political fact that classifying humans, at any level, could never be presented. That's not a bad thing, but it's also a mostly arbitrary thing.
But regardless, the more important point is that whatever biologically-motivated categories humans might be grouped by would not correspond to 'races' as normally understood.