Human races in science fiction
One thing that seems blatantly stupid in most far-future science fiction is the racial types of humans that exist. In most films and TV series, the racial distribution seems to be identical to that of the country in which the thing is filmed. Thus in most US science fiction, the characters have the same racial distribution as the modern USA does: mostly white, with a few blacks, Asians and Hispanics, and the occasional Amerind.

Now why should this be? Imagine an SF show set in the 25th century, with humans who are descended from the humans of modern Earth. In the present day, most humans on this planet are non-white (although most humans in countries where most SF is filmed are white). So I would expect one of two things to have happened by the 25th century:

(1) Humans have largely interbred, so that most people are of an intermediate skin colour, with white or black skin being very rare. Almost all hair would be black or dark brown, and almost all eyes brown (since these are the characteristics of over 70% of the modern human race, and coded for by autosomal dominant genes). Characteristics like the epicanthic fold (slant eyes), flat versus straight noses, and the negroid hair type would be randomly distributed among the population, either not or only loosely linked to skin colour.

(2) Humans have maintained distinct races, and these are descended from the modern races. The majority of humans are non-white.

Future histories could be imagined in which either of these scenarios came about, or in which there was some middle course between them. How the future will develop in this regard hinges on a number of factors, of which these seem to me to be among the most important:
- Population movement. How quickly will human populations on Earth in the 21st century and beyond mix with each other?
- Isolation through expansion in space. To what extent will Earth send out colonies which will subsequently be cut off from an exchange of people, leading to genetic isolation?
- Taboos against racial interbreeding. To what extent will people keep separate racial genepools, even when the separate populations are mixed?

But either of the scenarios described above would not give rise to the human racial distributions seen in Star Trek, Blake's Seven, Babylon 5 or Doctor Who. (Who is particularly bad - I can't remember a single non-white character in it, either as an Earth-human or as a humanoid alien.)

Of course, it is possible to imagine future histories in which either mass racial extermination happened (leaving the human race 90% white) or in which non-whites were kept in economic suppression (so that even in the far future, few non-whites are going to be in privileged economic positions like starship officers). But if this is the case, this history should have left its mark on the people of the time in which the show is set. This is particularly true in the latter case, in which the majority of humanity is non-white but an apartheid system keeps whites in positions of economic privilege. If this is so, the whole society should be a mass of seething racial tension, like South Africa in the late 20th century.

But I don't believe that any writers have imagined such a history for their societies. In most cases, they haven't thought about histories for their societies at all. The question of racial distribution has never really occured to them at all, because in think about societies they never really look beyond their own.

In Star Trek, humans only interbreed with humans of the same race. Humans seem quite happy to cross the species barrier with Vulcans, Betazoids, Klingons and what knows what else, but with humanity, taboos still run strong. The only cross-racial breeding I can think of is between Chief O'Brien and Keiko, and he only married her because he accidentally got her up the spout. Geordi LaForge and Ben Sisko only ever seem to be attracted to black women. The same is true of B5's Stephen Franklin.