Well, I said race is "less real" not that it is "not real". Here are other things that are more real than "race": "blood type", "whether one has Down syndrome", "the number of limbs one has".

These kinds of properties/classifications are, I claim, more "natural", as far as biology. These groupings into classes are more natural (based on biological properties) than groupings into "races".

Maybe choose a more precise and less judgmental adjective than "real." The opposite of "real" is "fake," not "difficult to substantiate."

But “difficult to substantiate” isn’t what I mean? I mean something more like whether something is there independent of what people think of it. (“Reality is that which, when one stops believing in it, doesn’t go away.”)

Distinctions between categories are often somewhat fuzzy, but in some cases there are processes independent of peoples’ opinions that behave largely like there are distinct buckets. Such as “has a left arm”.

With “race”, while there are certainly correlations between various genes and where one was born (and where one’s grandparents were born), and correlations between genes and other genes (which is partially explained by the correlations with location), any lines one may draw to split humanity into “races” will be fairly arbitrary, and at least substantially more arbitrary than splitting by whether someone is male or female (even though there are edge cases there as well; like I said, categories are often a little bit fuzzy).

Now, there may be other concepts that are even less real than race, but I don’t know if any of them are cared about enough to - oh, astrological signs! A person’s astrological signs are probably even less real than race, and people care enough about them to give them names.

That a function is continuous instead of discrete doesn't make it any less real. The fact is, humans like to categorize things because it makes things easier for them to process and communicate. Categorization is frequently imperfect; accepting that is a hallmark of being an mature adult.

Sure, but discrete categories people come up with are less real the less they align-with/derive-from the actual how-things-are .

I’m willing to talk about the radius of a helium atom even though there is no sharp cut-off in distance beyond which the amplitude for an electron being found there becomes zero.

But not every k-means clustering on a dataset reflects a real separation into types. Just because people draw lines in their model of the world, and just because these lines are sometimes useful, doesn’t mean those lines cleave reality at the joints.

One can observe people’s skin color and other outward appearance attributes. Those are undeniably real. It doesn’t mean that the conclusions one makes from those observations are necessarily correct or useful, which I think is what the OP is getting at.

And yet sometimes they are useful. Prople whose lineage originates in different places (race, if you will) do have medically significant differences (see the literature).

Yes, “amount of melanin content” is a real thing. And it is in large part attributable to genes which have correlations with both other genes (with medically relevant differences) and to geographic locations of ancestors.

So, if by “race” one means “melanin content and geographic location of ancestors”, then that is a real thing. But the way these vary do not come in any particular natural grouping into categories; “race”, in the sense of a categorical variable (not just the concept of ancestry more generally), isn’t real, or, at least, it is less real than the distinction between male and female.

If one has a random real-valued variable which has a continuous distribution, it may be useful to chunk it up into discrete buckets even if the way of chunking ends up being fairly arbitrary. Having a way of chunking it up into intervals being useful doesn’t imply that the arbitrary chunking is actually a natural categorical variable.