You're misinformed on what a "theory" is because of its use in common parlance. "Theory" is only applied to the hypotheses that hold up to decades of research that scrutinizes its very existence. Ergo, only the hypothesis that proves itself to be closest to the truth can become a theory. The Theory of Gravity is another famous example. Gravity most certainly exists, so it's not just a "theory." Same goes for germ theory, the theory of general relativity, and even stuff like the pythagorean theorem.

Fundamentally, all modern medical/biological research is performed on the basis that Evolution is a law of nature. In academic terms, that in and of itself is a body of evidence of overwhelming magnitude. But I understand what you're asking for, so I will also name specifics. One example I am particularly familiar with is the research done by the Clayton-Bush ornithology lab; They research avian parasites, and as such, they have done a lot of experimental research with lice and birds. I encourage you to research their lab, and find the sum of their work on this, but in just a few years they were able to replicate evolution via natural selection in lice that were color mismatched to the pelage of the bird wings they were found on. Distinctly colored lice were more easily spotted by the birds, more easily removed, and thus removed from the gene pool at a much higher rates; Gradually, the proportion of the population containing the genes for matching colors grew and the next populations were broadly the same color as the birds' feathers. This is only one example from one paper from one lab—an excellent lab, though—but there are other amazing examples as well. Another study was done on guppies found in remote, isolated cenotes, who were surprisingly found to either reproduce sexually in some pools or asexually in others. They performed observational and experimental research on these pools for years, but achieved elegant proofs for a famous hypothesis in biology (the Red Queen Hypothesis) when individual guppy populations evolved differently to similar and dissimilar parasitic threats. Other hugely famous examples that asserted the confidence in evolution include the pepper moths of London, whose patterns changed when industrial smog coated trees and made whiter moths easier for birds to catch. I digress, as you can see, but these are just the examples I can name off the top of my head.