[ejk314]

That hypothesis isn't falsifiable. We know "X effects Y" from experimental result. You're asking for proof that "X and only X effects Y", which is the same as asking for "There does not exist some X' that effects Y" where the set of X' is unbounded. No, that's not testable and doesn't need to be.

Just like we didn't need to know about the X' of Einstein's equations to know that the X gravity was the primary force holding you to the ground.

There very well may be an X' that effects observable biological features other than evolution, but that X' needs to be tested on an individual basis. So while it's productive to hypothesize an X' and test it, there's not much we can do to rule out all X' as a class.

[EGreg]

So if you can't rule them out, your theory shouldn't postulate that they don't exist. It's one thing to do 100,000 experiments and have the results COMPLETELY predicted by Einstein's equations beforehand. It's quite another to invent just-so stories in terms of the theory itself after each observation!