In all of the incarnations of D&D players can choose to be of different "races." Humans, Dwarves, Elves, and Halflings, from the beginning (putting on pause the race-as-class idea from ODD). In 1e we see Gnomes, Half-Elves, and Half-Orcs enter the fray, and the "Races" chapters of each later PHB contains even-greater variety. I think it's fair to say that the concept and its attendant terminology are in the DNA of most fantasy TTRPGs.
Some of these "races" are clearly just that: physically-distinct groups within the same species that, when in congress produce fertile offspring: Orcs+Humans or Humans+Elves. Looking at the various monster manuals we see other cross-breeds: Human+Giant, Ogre+Orc (Ogrillon), Giant+Goblin (Bugbear, according to original Greyhawk, I believe), Human+Goblin, Human+Dragon, &c.
But we never see "quarterlings" ("three-quarterlings?"), "dwelves," "minognomes," "dra-goblin-born," or plenty of other crosses from among the above constituents. Are some of these different species, such that they cannot co-procreate? Was the use of the term "race" intentional from the beginning, meant to convey that all of the races presented belong to one species?
Exclude from consideration cross-bred species created magically, such as centaurs, chimera, owlbears, &c. To put it bluntly, this is about a member from each of two groups coming together to produce an offspring with traits of each parent, which offspring itself can continue its lineage.
Excellent answers would refer to sources (published play-material, articles, interviews) from the 70s and 80s.