I'm not native in English speaker, so I looked up 'race' in Merriam-Webster. None of the results use 'race' in the sense of species. They use race as a group within a species by different aspects.

So why are elves, orcs, dragonborns races? Honestly I found it bizarre these very different fantasy beings would belong to a single species, and being its subspecies.

Calling them species wouldn't hurt mixing. Horses and donkeys are different species, but they still can produce mules. Other mixings also happen sometimes, like tiger-lion mix (tigon, liger) or the zebroid, the zebra-horse mix.

Mostly it's just because the original game designers used "race" and it stuck.  Though "species" doesn't precisely fit either, when you've got a bunch of magically-created beings their relations to each other can't be accurately described in modern taxonomical terms.

Pretty much spot on. Race really doesn't fit...but species doesn't apply very well either when you have lizards quite happily producing healthy offspring with humans. The English language didn't evolve with some of the concepts present in D&D and certainly not in the form they take in the game - as a result, the words don't fit very well.

Quote from Csatadi >>

I'm not native in English speaker, so I looked up 'race' in Merriam-Webster. None of the results use 'race' in the sense of species. They use race as a group within a species by different aspects.

So why are elves, orcs, dragonborns races? Honestly I found it bizarre these very different fantasy beings would belong to a single species, and being its subspecies.

Calling them species wouldn't hurt mixing. Horses and donkeys are different species, but they still can produce mules. Other mixings also happen sometimes, like tiger-lion mix (tigon, liger) or the zebroid, the zebra-horse mix.

 Ok first... Definitions in English. 

Species -a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding. The species is the principal natural taxonomic unit, ranking below a genus and denoted by a Latin binomial,

Demi-Human - A member of a human-like race; in fantasy, sometimes distinguished from the more bestial humanoid races.

Race - any one of the groups that humans are often divided into based on physical traits regarded as common among people of shared ancestry.

I'm including Demi-Human here as in the earliest editions Elves, Dwarves, and Halflings were called Demi-Human.  

Now, in D&D and most fantasy settings, all the races are created by the god of that race, but their "genetic" templet is often said to be based on the gods, who originally were massive humans in the sky. As such all the original gods are the same species. with a small handful of notable exceptions ie Dragons. Due to the all the races made by the gods tend to look like the god that created them, and as such their children could intermix with each other. (A few notable exceptions which seem to have been lifted as of the One D&D announcement. (Gnome, Dragonborn, Lizard Folk)  Basically elves, dwarves, Halflings, humans, giants, and many others are a subspecies of the divine humanoids. 

Ironically, Orc & Tiefling are not counted among this number. Tiefling is a mutation caused by Demonic or Infernal powers, and all children born of someone with this mutation will be a Tiefling. Orcs, are empowered by their god Gruumsh to be able to mix with all races and produce more Orcs (note - Half-Orc was a lie, all Half-Orcs are Orcs who happen to grow up in a human area.) I should also say Gruumsh hates Elves due to him having a fight with Corellon the god of the Elves. As such No Orc was able to mix with elves... until One D&D. 

tl;dr 

All the Player Races (for the most part) are subspecies of the same race. 

Though the above answered the in-universe justifications rather well, it is worth also explaining how the word came to be used in the game.  For that, we will turn to the Oxford English Dictionary - English's foremost dictionary, which includes etymology and history of words.  

Under an archaic definition dating back to the 1500s, Race means "Any of the major divisions into which living creatures may be separated."  Here, there is not a biological differentiation--under this archaic definition, race means what we scientifically use "species" for today.  For example, a writing from 1549 uses the word "race" to refer to the "race of gods" as a distinct entity from mortal man.  Flash forward four hundred years and we reach the forerunner of all modern fantasy--Tolkien.  Tolkien, a scholar of the formation of English, was a pretty big fan of using a more historical version of English in his writing.  He referenced Elves, Men, Hobbits, Dwarves, etc. as different "races" in reference to the 1500s definition of the word, which had mostly fallen out of favour by the time of his writing in the 20th century.  Though it is notable that these races can intermingle and produce viable offspring, he was not trying to say they were the same species--more so trying to invoke mythology (itself predating modern genetics) where creatures of different species might intermix and create children.  

When creating D&D, Gygax used Tolkien's definition of race (I highly doubt Gygax was all that knowledgeable of 16th century etymology), and, by proxy, used a more archaic definition of race, giving D&D that more medieval/fantasy feel Gygax was going for.

In my mind, it's a fairly simple explanation: "Race" is a substitute for "species" or other because most D&D settings are generally themed to be set in a pre-scientific world, it could have just as easily been "people" or "bloodline" or something to that effect, but it wasn't.  Thus the quite scientific sounding "species" doesn't integrate itself as organically into use; which is probably why the former stuck and the latter didn't.  "Species" is often used in science fiction-based settings/rulesets precisely because it does then fit into use organically.

One of the problems in nomenclature in D&D is that there needs to be a way of grouping and classifying the intelligent, civilised people's together. In English, we don't have this nomenclature because for as long as we've had written language and historical records, we've only had one such type of people - homo sapiens, so humans covered it. That doesn't work in fantasy, a Lizardfolk or plasmoid isn't and can't be homo sapiens, but in some of the most useful ways of categorisation, they would have more in common with us than an chimp - the ability of civilization, to form treaties, complex trade, etc. How do you succinctly say that "all the intelligent and civilizable species of the world must unite!"? "Race" and "peoples" bear that burden in many fantasy franchises, but they use them in a sense that is pretty idiosyncratic to the genre that can contribute.tom confusion when we don't realise just how idiosyncratic those meanings are.

As the lines between the playable races and "monsters" have blurred over the editions, I am of the firm opinion that the use of the word species really would be better altogether. 

I'm not sure that it is a requirement for something to be a playable species that they can interbreed with other playable species - but cross-species breeding do indeed occur as the OP points out. (E.g. Horse and Donkey, while you have difference breed/races of cows for example)

Putting breeding aside, I think Tolkien assigned it to the respective creation by the gods of the world instead - but that's Middle earth only of course. And with the increase in playable species, I am not sure if this simply wouldn't mean that all creatures created by a god in your world should then also be playable as a character. 

But then again, I am a biologist by training, so race is a poorly defined term compared to species (having been used to denote subspecies based on genetic "distance" as well as physiological or geographical divisions)

Under all circumstances, the term race have connections to topics of racism, which we are generally trying to remove from the game, and the loose definition that can be based on either culture or physical distinctions.

Quote from CaptainCorvid >>

In my mind, it's a fairly simple explanation: "Race" is a substitute for "species" or other because most D&D settings are generally themed to be set in a pre-scientific world, it could have just as easily been "people" or "bloodline" or something to that effect, but it wasn't.  Thus the quite scientific sounding "species" doesn't integrate itself as organically into use; which is probably why teh former stuck and teh latter didn't.  "Species" is often used in science fiction-based settings/rulesets precisley because it does then fit into use organically.

 
While “species” does sound more scientific and is a specific scientific term (whereas “race” is more of a sociological term in modern parlance), it is interesting to note that “species” is starts making its appearance in English during the 16th century—congruent with “race” entering English.  With his On the Origin of the Species, Darwin really cemented “species” as a scientific term, leading to Tolkien using the similarly-aged “race” in his work, in turn influencing every fantasy writer to follow.

It is fairly fascinating that little quirks of linguistic history can take two similarly-aged words and add myriad complexities to how they are used contextually. 

The semantics of a fantasy setting didn't fully align with the English used to create it. So, they chose something similar and went with it. I would not recommend overthinking it.

I don't like "species" in D&D, it sounds too sciency. I also was uncomfortable with "race" but somehow the easy they handle biracial and multiracial characters in the One D&D playtest makes me more comfortable with it. 

Thanks for the explanation. It's surprizing to know how many things go back to Tolkien, even today.

Neither species nor race has a universally self-consistent definition period, let alone one that correctly aligns with the categories we have in D&D. They had to just pick a word to describe something that doesn’t exist in real life,  because human society doesn’t really acknowledge any species other than humans as “people.”

“Race” has problems, but I haven’t seen anything better in common use. Species doesn’t fit with our expectations of the tone of the game, regardless of its actual historical use. I’ve seen “ancestry” and “lineage” used, and I find them unbearably pretentious.

“Kind” is the best word I can think of, honestly, though I haven’t seen anyone suggest its use.

If I remember correctly (IIRC), Tolkien was talking to or knew of the work of Watson and Crick, ie DNA and that is how the idea of combining of "races" was inserted into his work.

Also IIRC in Greek mythology a lot of monster(s) came into being by Gods having monstrous offspring. (Cleaned up for young posters) I do not remember a horse sitting next to a person on a park bench and then a stork delivering a centaur baby to them. Also in Greek mythology they often used animal features to relate some trait. So centaurs were people who were very good at riding horses and satyr's were people who loved drinking and song thus more bestial then others. 

I do not think any "race" should be a able to reproduce just so you can have an easy game mechanic that allows a player to pick any options they want from both parent races in D&D 5e and in most other games I have seen. In general I am also not a fan of very cartoonish race creation offered by such systems, ie intelligent shark and intelligent bird people produce an offspring by sitting on a bench and then you have a flying bird-shark that takes the best of both sharks and birds. 

Also in general being able to reproduce and that reproduction being viable in every day living and reproduction can be a long time coming, ie just because you sit on a bench it does not mean that the stork will drop off a living baby. 

The English language can be confusing and terms can shift around in meaning as well as a term that has a specific meaning in a field of study that then is used by the broader populace in a different way.

Quote from Csatadi >>

Thanks for the explanation. It's surprizing to know how many things go back to Tolkien, even today.

 Well; it is and it isn't: Tolkien is the archetype after which msot fantasy settings pattern themselves.  There's an equivalent in science fiction in Frank Herbert's Dune; in both cases the works are so influential that elements of their specific worlds have become default elements for an entire genre.

Because it was a good simple word back in the 70s and people knew what it meant. And today it is a good game jargon word. 

It’s 2022 and race is a problematic term.

I’ve been a DM for decades and currently run 2 campaigns.  One group is 20 somethings and the term is very problematic for them so we’ve decided to use ancestry.  My other group is all in their 50s - I explained the perceptions from my younger group, and they have also decide they prefer ancestry.  

I think it’s time for a change, it’s an outdated word, with too much baggage - we shouldn’t use a term that needs a paragraph to explain the lexicon of it to try and make it more palatable for our current times.   

That’s just my 2cp.  

I think it sounds a little better to speak of all the races in the game instead of species, which sound too scientific, but Species would have to be the correct term.

Also Tolkien described the races of elves, dwarves, men and Hobbits, so I think that gave Races a head start.

I'm afraid if the issue is "race is offensive to me!" then well... that problem isn't going to go away by changing the word; because quite simply it has to convey the same concept regardless.  The phrase "offence is taken and not given" exists for a reason: because this particular Ouroboros circle once started never stops.  To take but one example: the word "fat", when describing someone has been deemed to be insulting to the point of offensive... despite it being a descriptor.  Thus the word went from "fat" to "overweight", which then became "obese", which then became "plus-size", which then became "person of size"... and will presumably in future change again... to "gravitationally oppressed" maybe.  The word changed but the "offence" stayed regardless because at the end of the day; the word had to convey the same information, the same concept.  Ultimately the exercise strikes me as rather pointless when we have a perfectly concise word already that quite frankly a vanishing minority of people get unreasonably worked up over.

And before someone jumps up me backside about "not getting it"; I happen to BE one of these "BIPOC" people I keep hearing so much about and all this language policing and sensitivity reading is supposedly partially done on my behalf... and frankly I'd rather WotC put some content in their books for a change instead.