I've read about those fruit fly experiments.
Yes, but you didn't understand them.
The fruit fly experiments show that two populations of animals, once divided, evolve into more distinct types of animals.
Since there is NO EVIDENCE for something that would stop or limit this evolution, they WILL eventually become VERY DIFFERENT, i.e. what you think of as different species.
You can disprove this. You can go ahead and find this stop or limit, this reason for why they would stop changing. But that stop has never been found. Just claiming that something would stop is not enough. You'd have to show evidence for it.
And if you think that "species" is that limit, you clearly haven't understood the point of this. Species borders do not exist in this process. The differences between species that you perceive (and that do exist) are just a lack of surviving populations between the two different species. This is exactly what Darwin's theory predicts and hence Darwin's theory remains the thing to teach.
A scientific theory is not something that is very very true, it is something that is very very disprovable. That's why Creationism is NOT a scientific theory: it cannot be disproven.
In science we do not look for explanations that cannot be challenged, we look for explanations that can.
Everything else is philosophy or theology and should be taught in philosophy classes.
The misconception you fell victim to is the belief that something is more scientific if it is harder to disprove. But quite the opposite is true. Science is what is easier to disprove.
Here's how Darwin's theory can be disproven:
1. Show how one species "turns into" another consistently.
Darwin's theory claims that this doesn't happen, that instead one species branches to become two or more. By showing how "one species turns into another", you would disprove Darwin's theory.
2. Show how there is a physical real limit over which change cannot happen.
The first is very difficult to do for Creationists since they have so little knowledge of Darwin's theory that they don't even know that it doesn't claim that one species turns into another.
The second is something you simply take for granted without proving it.
Either way, these are but two ways evolution could be proven wrong. Since those possibilities exist, evolution is science.
Creationism, to qualify as a scientific theory, would have to allow ways to disprove it too. So how one disprove Creationism?
Still not getting it? Let me explain it using another example.
Things fall towards the floor.
I can come up with two explanations:
1. This is caused by gravity, which could be disproven, for example, by observing just one heavy item fall up with no other power influencing it.
2. This is caused by G-d deciding that it should fall down. This cannot be disproven since observing it falling up can be explained by the fact that G-d, making His own decisions, might have decided so in this one case.
The first is science (because it can easily be disproven). The second is religion (since it cannot be disproven).
The same applies to evolution and Creationism. The first can be disproven (by many, many theoretically possible observations which have yet to be made). The second cannot be disproven (since no observation cannot be explained by "G-d made it that way").
Creationism explains absolutely everything. It explains what we observe and it explains everything we might ever observe. That's why it isn't science.
Evolution explains only what we observe. It does not explain everything we might observe. That's why it isn't religion.
If a rabbit appeared in front me, out of thin air, right now, Creationism would explain it and evolution would not. That's the difference.
Creationism is too true to be science. Truth explains both the observed and everything else. Science explains only the observed. Science is about facts, not truth.