From punctuated equilibrium to heredity to the Big Bang theory, this is simply untrue.......because scientists HAVE had to change it plenty to keep it viable.
This is a pretty inaccurate (or at least misleading) characterization, IMO. The discussion of PE, for example, revolves around the
mode and
tempo of evolution, not the basic facts of evolution: descent with modification, non-constancy of species, lack of discontinuities, gradualism, and natural selection.
On heredity, Darwin recognized the fact that he didn't know how it was done - he had an erroneous idea that variation was unlimited but had no good idea how such variation arose in the first place. It was left to Hugo de Vries, one of the "rediscoverers" of Mendel's works around the turn of the century to show the particulate nature of heredity. In fact, de Vries corrected
Mendel, who thought that heritable variation was discrete and limited because, either by design or accident, Mendel picked species and traits that WERE discrete - de Vries reproduced Mendel's experiments using different species and found radical new traits arising
de novo; he called these traits "mutations". Ultimately Fisher, Wright and Haldane showed how a synthesis of Darwinian selection and Mendelian genetics explained the diversity of life. Up to now, this has been the last great conflict in the theory - Darwin's basic views have been borne out, not thrown out or replaced as you suggest.
All of the current arguments within biology revolve around details. Which is more important: ns or drift? How fast is "gradual"? Does the fossil record show more stasis than continuous transition? How important are regulatory genes in modifying phenotypes? etc etc. Not one single practicing biologist or ecologist to my knowledge denies the underlying fact of evolution - exactly as originally described by Darwin.