I considered changing evolution in the article to natural selection but after reading Evolution I think that would be inappropriate When scientists speak of the theory of evolution they mean the general idea that all species on the planet descend from a common ancestor and arise through descent with modification They do indeed disagree on the source of the modifications and the mechanisms of selection but the general idea itself is sufficently precise and falsifiable that it can be considered a theory in its own right Any of the following would cause great trouble for evolution   Different species having completely different biochemistry  Different species having completely different genetic mechanisms  Different species sharing no genetic code  The fossil record showing that there has been no trend from lesser to greater complexity over time  But as you know the evidence in all these areas has consistently shown the opposite and no other theory has come remotely close to giving an alternative This is why so many scientists believe that evolution is in the same league as say heliocentric theory