"Science fiction is that branch of literature that deals with human responses to changes in the level of science and technology" - Isaac Asimov.
When looking at Science Fiction I believe as a genre, it most certainly has changed over time. The genre, like most others, is heavily influenced by contemporary affairs. The film makers tend to base their films upon the issues that concern people at the time. For example, in the 1950s films like "The Day the Earth Stood Still" and "Invasion of the Body Snatchers" focused on the alien invasion. This was probably due to the Cold War which created a general paranoia of anything not American, as well as the infamous Roswell spacecraft crash in 1947, which heightened an already large fascination with UFOs. As technology in society progressed so did the topic of Science Fiction films, which began to focus on cyborgs, clones and post-modern implosions in films such as "Blade Runner", "Planet of the Apes" and "The Terminator".
Most Science Fiction films of the 1950s portrayed the human race as victims and at the mercy of hostile and unfriendly forces (aliens). These included such classics as "The Day The Earth Stood Still", "Invasion Of The Body Snatchers", "It Came From Outer Space", "The War Of The Worlds" as well as many more. I believe the influx of the alien invasion film was induced by an interest created by the apparent discovery of a crashed alien space craft in Roswell, New Mexico. However, such subject matter was undoubtedly heavily influenced by Cold War politics. The war of words between America and Russia contributed to suspicion and paranoia of anything "un-American". Such Science Fiction films reflected upon the threatening political powers that surrounded America at the time (the aliens were perhaps a metaphor for Communism).
Science Fiction as a genre has changed in turn with current affairs of the time. The prospect of nuclear war continued to haunt America as the Cold War continued to drag on into the 1970s.