The argument goes something like this, and is not the property of philosophy, but anthropology (cultural and biological) and genomics: The one race (human) has been separated into discrete groups based on observable phenotypic variance (eye color, skin color, hair texture, among other things). One defining trait of race is the inability to reproduce with those who are outside of it. If there really were biological races, then we couldn't interbreed- that's part of the argument, anyway.
Also, regarding classification- Linnaean taxonomy may still be the standard, but a lot of Linnaeus's original human classifications have been dropped over time because they were well, racist: ****** Americanus; ****** Asiaticus, ****** Europeanus etc. Now only "Human" remains. In any case, the mainstream scientific community at large stopped validating the idea of there being biological "races" a while ago. Compare racial discussion before and after the Human Genome project and you can definitely see a difference. http://harvardmagazi...etic-world-html
But none of this negates that race is a social and cultural reality, with actual ramifications. And now I'm out, because tbh, I don't find Solas very interesting. But jeebus OP, different strokes, etc.