You prove my point.
Gravity is an observable phenomenon - a fact - which we cannot yet satisfactorily explain in detail, and there are therefore different theories regarding its nature - the particle/wave debates, string theory, etc.
Biological diversity is an observable phenomenon - a fact - which we cannot yet satisfactorily explain in detail, and there are therefore different theories regarding its nature - evolutionary theory, design theory, etc.
I think maybe you should check your facts.
The theory of gravity is very easy to prove. Gravity can be predicted with great accuracy, where it will exist, and how much of it will exist there. It can easily be tested, has been tested, and it works every single time.
The Theory of Evolution, is not the equivilant to the Theory of Gravity. It is equivilant to the Theory of What Makes Gravity Work. And in that regard, schience has no bloody idea.