Other than possibly small scale genetic changes observed by researchers, little if any hard, physical evidence exists for evolution on a global scale.
Most of the evidence is inference (ex. using assumptions to fill in the gaps); there isn't for example any physical evidence of apes evolving from microscopic organisms.
So why is the theory of evolution so often taken as absolute truth despite there being relatively little hard, physical evidence to support such as massive conclusion?
First of all, you are not well informed about the theories of evolution.
Secondly, inference is a perfectly valid method in scientific study. Without it we would not have the theories of relativity, quantum mechanics, atomic theory, or germ theory, just to name a few.
Lastly, the theories of evolution are explanations, descriptions, and predictive tools of hard data of the fact of evolution. Organisms evolve.
No biologist thinks differently.
Inferring small genetic changes between descendants of a species to mean that the entire diversity of life originated solely from natural selection is a huge inference.
Biologists do not infer commom ancestry
only from small genetic changes between descendants of a species. There is, among other lines of evidence, morphology and especially genetics. See retrovirus DNA signatures.
To infer that there is some arbitrary boundary where evolution stops so that one species,
an arbitrary category used only by people, doesn't evolve into a new one is without logical basis. Also, natural selection is not the only theory of evolution; see sexual selection, genetic drift, etc.
Why is it only this one scientific theory with which religious fundamentalists have an issue? Relativity is currently incompatible with quantum mechanics. Why don't religious fundamentalists take issue with that, being such scientifically critical thinkers?