Repetitive, I know, but it's important to hammer this point a little: "Only a theory" is not a valid objection to any scientific idea. Everything that's interesting in science is a "theory". If you want, you can get all technical about it and only call something a "Theory" with a capital "T" when it is supported by overwhelming evidence - but even that high hurdle is one that the Theory of Evolution clears with ease. Theories are what makes science different from stamp collecting. "Look! There's frog! Look, there's another frog! Look, there's a third frog! Oh look, I saw another frog!" Boring stamp collecting. "All frogs grow from tadpoles". Now you're talking about a theory, and you're suggesting a kind of regularity about nature. That's what science does. "Look! There's a gene. Ooh, another gene! Wow, it's a third gene!" Boring stamp collecting. "Genes determine what an animal looks like and how it acts." Now you have a theory. What use is the theory of evolution to biology and genetics? No use, except that without it neither field of study makes any sense at all. It's like asking what the theory of gravity has to do with astrophysics.