If you've ever engaged in debate with a creationist, you'll know that they can be extremely slippery. One of their favorite tactics is to claim that evolution is "just a theory" because it cannot be scientifically demonstrated. (Of course, it's not as if any of the stuff they believe can be scientifically demonstrated, but let's forget that minor detail.)
The question of what constitutes scientific proof is what's really at stake. We can prove that evolution is scientifically valid by looking at the fossil record, DNA evidence, and lab data from genetics experiments--but Creationists will claim that evidence like that is insufficient, or even invalid. They will claim that to demonstrate the proof of a theory, it must be possible to demonstrate the entire theory, right there before their very eyes.
This is clearly an unreasonable demand. There's no serious doubt that dinosaurs once existed, even though I can't actually show you a diplodocus grazing quietly in a field. For that matter, there's no serious intellectual debate about whether the Second World War actually happened, even though I can't actually show you Germany invading Poland, or let you ask Hitler why he decided to do so.
Nevertheless, there's something vaguely dissatisfying about deducing how animals evolved by looking at the imprints their skeletons left in a rock. It would be much more exciting if we could demonstrate new species appearing from nowhere; or if we could demonstrate self-reproducing entities appearing by pure random chance.
Well, now we can. The increase in the power of computers over the past thirty years has brought us to the point where a cheap desktop computer can perform over a million floating point calculations every second. Complicated simulations that were unthinkable ten years ago can now be performed by the computer you're staring at right now. In fact, we can go beyond simulation--we can show evolutionary principles applied to real situations, in new fields such as genetic programming and artificial life. Creationists may scoff that simulations prove nothing--but their protests are wide of the mark, as this article will attempt to explain.