I'm not an expert in biology -which is the reason I'm posting this, in the hopes someone more knowledgeable can elucidate. It seems to me that the terms "race" and "species" -when referring to aliens from other planets- are used interchangeably, perhaps irresponsibly, in Star Trek series, and even though I'm no expert, I believe both terms aren't synonymous nor appropriate. According to this wiki graphic, for example, a species sits at the bottom of a biological classification which consists of much larger groups such as Domains, Kingdoms, Classes, but these definitions pertain to earth organisms, therefore it cannot describe alien worlds. The term race also poses problems. According to wiki: "Race is a classification system used to categorize humans into large and distinct populations or groups by anatomical, cultural, ethnic, genetic, geographical, historical, linguistic, religious, or social affiliation." Since race is used to categorise humans, it can only be applicable to those populations, therefore it's incorrect to refer to aliens as "races", in my opinion. So, what could be the appropriate term to refer to alien inhabitants of other planets?
I say both can be used interchangeably in casual or everyday conversation. Specifics can be reserved for stuffy classroom lectures.
There are other senses of the word race than that cited in the OP. For example, when applied to humanity as a whole—as in the human race—it means, well, humanity as a whole, and at least in that case race is synonymous with species.
Until there's definitive proof of an intelligent lifeform on another planet comparable to humans, race and species work well enough. Then the biologists can invent new classifications.
We also use the term "race" in the context of "the human race," so it can be used to refer to the entirety of a species, at least if it's a sentient species. So in that sense, it can be a valid usage. (Edit: Just got beaten to it.)
Species should be any alien, sentient or not. Race should be applied to any sentient species. Worf's comment that "I like my species the way it is." not withstanding.
It's been my observation that because of the way Trek aliens are often presented on a gradient of most human like to least human like, the terminology helps the audience distinguish between the characters' understanding of such differences. Thus, you'll rarely hear them refer to aliens like Tribbles or Horta as 'races' rather than 'species', assuming they use either term (for instance 'creatures' or 'lifeforms' are frequent substitutes). Where as they're more likely to use 'race' to describe human-like aliens such as Bajorans or Vulcans. Star Trek is science-fantasy, not a clinical dissertation on alien life. The words merely have to function to give the viewer an approximate understanding of what they're talking about. As Melakon points out, we won't have a proper lexicon for this until we have to regularly identify and catalog real-world aliens.
For a lot of the people on Star Trek, I think "race" would be just fine. Especially if you look at the number of them who can interbreed with each other -- including Humans, Vulcans/Romulans, Klingons, Betazoids -- even with scientific help, it no longer makes much biological sense to call them separate "species" in the biological sense. It's a pet peeve of mine, though -- one of the scientific concepts that Trek gets absolutely wrong.
You can't really apply real-life scientific concepts to Star Trek in this area. Even with the "explanation", it makes absolutely no sense. Klingons, Humans, Vulcans, Cardassians, etc. regularly interbreed, which essentially makes them the same species, despite having evolved on different worlds. How could that possibly happen, even with transplanted DNA code segments (TNG: The Chase), when mutations are random? The chromosomes and genes would have to line up exactly - and even that assumes that sexual reproduction occurs the same way. The same goes for the non-human animals referenced during the various series. Mastodons? Emus? Hawks? All seem to be popping up everywhere throughout the galaxy. I wouldn't care all that much, except that most people already don't understand evolution at all. Pop culture isn't helping by completely screwing it up. Another beef I had: The Voth. Dinosaurs were well established to be warm blooded long before VOY aired, yet they had to make them cold blooded. Why? That one point nearly ruined the entire series for me.
Well, they had 65 million years of evolution on an alien world to change them. Maybe their ancestors were warm-blooded but the environment they were transplanted to gave them an incentive to evolve into ectotherms once again. After all, there's no "upward" direction in evolution, just adaptation to new environments. Perhaps their new environment had such a uniform, warm temperature that it was a waste of metabolic energy to generate internal heat, so they lost the ability.
And how could so many intelligent alien species have evolved to look exactly like humans, except for having funny ears, bumpy foreheads and weird skin conditions?
Are they used interchangeably? Race and species are not themselves interchangeable, the former carrying connotations of "civilization" that aren't present in the latter. When Azetbur says, "We are a proud race. We are here because we want to go on being proud," she is not talking solely of the biological continuation of the existence of Klingons, but of the survival of their cultural and social structures. ETA: Another example, from Odo, who says, "What better way to gauge another race than to see how it treats the weak and vulnerable," referring to the values rather than just the innate biological impulses.
This is why I've always regarded Star Trek as Science Fantasy: Veneer is a perfect way of describing Trek "science". It's at least internally consistent, as all fantasy should be, but it's by no means scientifically consistent.
Most SF in TV and film is science fantasy. Star Trek actually has a higher ratio of science than most of it.
Perhaps by the 24th century "race" as a social construct is so thoroughly outdated that it's become synonymous with species in Federation usage. That seems to be about right, since we never hear it being used to refer to ethnicity.
Fair point. You know you're right. I can only think of a few instances where ethnicity was referenced at all. That said, we don't know if any non-humans, who also frequently use this term, have had racial (or even ethnic) classification within their civilizations. I still think the likely (admittedly boring) explanation is a literary one rather than an in-universe one.