Don't even start on D&D terminology for playable thingies. They mostly use 'race' to mean 'species' (fade to short classroom slide presentation).
Different concepts exist to determine what is a species. Under the Biological Species Concept, all members of a species are capable of breeding to produce fertile offspring; populations which cannot interbreed are therefore seperate species. Under the Phylogenetic Species Concept (& I'm breaking into lay definition now, as there are actually a cluster of slightly different named concepts here which I'm lumping together under the most frequently used term), all members of a population that is genetically distinct from all other populations form a species. For the PSC, seperate species can interbreed & produce fertile offspring so long as genetic transfer from one population to the other is rare enough not to alter their genetic distinctiveness (e.g. interbreeding is rare, offspring possess traits which make them less likely to survive &/or breed).
Forms possessing genetic &/or morphological differences insufficient to regard as species are termed 'subspecies' (i.e. they can interbreed, but they look rather different, or have some consistent genetic differences). Some 'species' under PSC would be considered subspecies under BSC if they interbreed (since the ability to interbreed is the defining criterion for the BSC), even though genetic interchange might be greatly restricted (the main criterion for the PSC). Theoretically a situation could exist where two BSC species were considered subspecies under PSC (if they had diverged very recently but developed barriers to interbreeding very quickly), but in practice PSC usually accepts all BSC species & adds more because of its different conceptualization of the term.
*lights come up*
Thus in D&D, most 'races' are actually species, as they do not interbreed. Humans & elves can, & under BSC would be considered the same species; under PSC I suspect that they would be considered seperate species, as interbreeding is relatively rare & there are surely some rather distinctive genetic differences behind the distinctive longevity, ears, vision, etc. 'Subraces' in D&D would mostly be considered subspecies by BSC (they are mostly capable of interbreeding AFAIK); for the PSC it would be an area for future research & genetic testing. I don't think most of them are simply 'cultural variants'; I think most are, at least, subspecies -- note the differing stat adjustments, for example, a clear sign of morphological differences.
Now, why they didn't just put in a simple explanation like that & call things 'species' & 'subspecies', I'll never know!
Comparing to RW human cultures &/or races is very difficult, as RW research into humanity as a species (by which I mean human taxonomy & phylogeny -- naming & relationship) is about as 'Hotbutton' of a topic as you could attempt to find. Older research is horribly tainted by racism (not to mention outdated theories of human migration patterns; the term 'Caucasian' derives from a time when it was believed that ancestral humans migrated into Europe from the area around the Caucasus Mts., while current evidence does not support this). I'm not even sure there are many modern attempts to classify humans in the way we do other species for fear that a) the research will be viewed as inherently racist (even though 'classify' != 'claim superiority for
any group), & b) even unbiased scientific results could be twisted by racist lay groups. Heck, even the term 'subspecies' would probably be misconstrued as 'less than the real species' instead of 'one group within the same species'. In a way, this is unfortunate; what studies I have seen have pointed to genetic relations that don't even remotely match any visual differences which may currently carry 'race' labels, & furthering this understanding of our true interrelatedness could, IMO, help to break down the RW 'race' concept.
