For those scientists who take it seriously, Darwinian evolution has functioned more as a philosophical belief system than as a testable scientific hypothesis. This quasi-religious function of the theory is, I think, what lies behind many of the extreme statements that you have doubtless encountered from some scientists opposing any critical analysis of neo-Darwinism in the classroom. It is also why many scientists make public statements about the theory that they would not defend privately to other scientists like me.