In fantasy and sci-fi, there is a lot of talk about “races”. In Lord of the Rings, there are humans, elves, dwarves, orcs. In Star Trek, there are humans, Klingons, Andorians, Vulcans. In Star Wars, there are humans, . In Warhammer, there are humans, orcs / orks, elves / Eldar and so on. In Inheritance Cycle (a LotR / Star Wars ripoff) there are elves, dwarves, orcs Urgals and so on. There is a bit of a problem with this, though.
With the exception of races in Lord of the Rings, none of these are actually “races”.
Race, just like species, is a biological category. Species is a group of organisms which can produce viable offspring. Race is a group of organisms within a species which share most of biological characteristics, but differences are lesser and more mutable than within a species.
So what about “races” in sci-fi and fantasy?
In Lord of the Rings, “races” are actually races. There are human-elf hybrids, human-orc hybrids, and possibly even human-troll hybrids which suggest that humans and ents might have offspring as well. And this offspring is biologically viable: half-elves Elrond and Elros both have children of their own. Uruk-hai are produced by breeding Uruks with humans, and are themselves clearly capable of reproducing. As such, they would be equivalent of mulattos in our world, not of a fictional cross of humans with apes or dolphins.
In Star Trek “races” are a more complex matter. Evolution would dictate that they would actually be different species, having evolved on different worlds, and thus utterly incapable of producing any children (genetically, humans should be closer to amoebas than to Klingons). Yet that is not the case, for which a bullshit “progenitor species” explanation is provided. Most races look no more different from humans than different human races are from each other. More importantly however they are capable of producing viable offspring, as presence of multiple hybrids confirms. Spock is a Human/Vulcan hybrid, while Tora Ziyal is Cardassian/Bajoran hybrid and Deanna Troi is Human/Betazoid hybrid.
But many later works simply jumped on the “races” bandwagon without considering the science behind it. Races in Warhammer Fantasy and Warhammer 40k are not truly races, but in fact different species – or at least they should be species, seeing a) how different they are (Orks breed through spores) and b) how there is no common ancestry in evidence, and is in fact in most cases impossible. Thus any hybridization between the “races” should be impossible, making them into truly different species. Yet such hybrids are (through rarely) in evidence.
Why is that?
Easiest answer is that everybody followed Tolkien but nobody had his understanding of biology. Thus “race” came to mean merely “vaguely humanoid thing”, with no regard for biological basis of terminology. With this came disregard for implications of the same, with result that children were had by pairings more removed from each other than humans are from shellfish.