[csense]

Yes, so you observe and see whether or not it does. If it causes the organism to be less likely to pass on it's genes, but the percentage of the population that possesses that gene increases, then natural selection is falsified.

This is yet another logical contradiction. No offense, but I've had enough....

[Sofa King]

"This is yet another logical contradiction."

The logical contradiction here is that you're asking for an experiment that could prove natural selection to be false even if it was true.

The real problem here is it's so painfully obvious to you that natural selection is an accurate description of a natural process that any description of case where it doesn't happen seems absurd to you, and you are trying to use this very obvious truthfulness as grounds against it.