The wiki article linked below is VERY WELL footnoted…
Conclusion quoted below -
Takahata  describes it thus:
“Unlike neo-Darwinism, which regards mutation as merely raw material and natural selection as the creative power, Nei’s mutationism assumes that the most fundamental process for adaptive evolution is the production of functionally more efficient genotypes by mutation (especially birth and death of duplicated genes) and by recombination.”
Stoltzfus argues that the historical mutationists embraced ideas about evolutionary causes that neo-Darwinists rejected and claimed to have refuted, but which have been shown to be theoretically valid implications of population genetics, supported by evidence from comparative data and evolution experiments.
One of these is that biases in variation are a cause of direction in evolution— the heresy of “orthogenesis”—, which can be argued from theoretical models  and empirical results.
Another is that the variations used in evolution are not always infinitesimal or slight. A third, argued by Masatoshi Nei and Stoltzfus (see, is that evolution often occurs in a mutation-limited regime of evolutionary genetics rejected by the Modern Synthesis, which was based on the “shifting gene frequencies” view of polygenic, quantitative change.
This position is elaborated in a review of the mathematics, history, and implications of origin-fixation models, a widely used class of models in which evolutionary change is a 2-step process of (1) the introduction of a new mutation, and (2) its subsequent fixation or loss.
Such models represent a theory of change distinct from the Modern Synthesis that was not developed until 1969, based on an emerging view of molecular evolution. If these arguments are accepted, it would mean that a variety of theoretical and empirical results, some of them well known (e.g., Lenski’s experimental evolution studies ), already provide support for mutationism.
Not just Nei, but authors such as Eugene Koonin argue that gene duplications and fusions, genome doublings, gene transfers, and other events common in molecular evolution are discontinuous changes whose importance supports a saltationism view and counts against Darwinism.
There is little evidence on the reception of mutationism among evolutionary biologists, as it rarely mentioned. Reviews of Nei’s book in mainstream journals were more positive than negative: Wright, writing for the conservative journal Evolution, rejects Nei’s thinking as mistaken, while Galtier, Weiss, Stoltzfus, and Wagner, while not necessarily agreeing with Nei’s position, treat it as an alternative view relevant to reforming or improving evolutionary thinking on the role of mutation in evolution."
 A. Stoltzfus (2014). In search of mutation-driven evolution. Evolution & Development 16 (Publisher). pp. 57–59. doi:10.1111/ede.12062.
 Takahata, N (2007). “Molecular Clock: An Anti-neo-Darwinian Legacy”. Genetics 176 (1): 1–6. doi:10.1534/genetics.104.75135. PMC 1893057. PMID 17513888.
 Yampolsky, L.Y. and Stoltzfus, A (2001). “Bias in the introduction of variation as an orienting factor in evolution”. Evol Dev 3 (2): 73–83. doi:10.1046/j.1525-142x.2001.003002073.x. PMID 11341676.
 A. Stoltzfus (2006). Mutation-Biased Adaptation in a Protein NK Model. Mol Biol Evol 23. pp. 1852–1862.
 Stoltzfus, A and Yampolsky, L.Y. (2009). “Climbing Mount Probable: Mutation as a Cause of Nonrandomness in Evolution”. J Hered 100 (5): 637–647. doi:10.1093/jhered/esp048. PMID 19625453.
 David M. McCandlish and Arlin Stoltzfus (2014). “Modeling Evolution Using the Probability of Fixation: History and Implications”. Quarterly Review of Biology 89 (3): 225–252. doi:10.1086/677571.
 Elena; et al. (1996). “Punctuated Evolution Caused by Selection of Rare Beneficial Mutations”. Science 272 (1): 1802–4. Bibcode:1996Sci…272.1802E. doi:10.1126/science.272.5269.1802.
 Koonin, Eugene (2011). The Logic of Chance: the nature and origin of biological evolution. Financial Times Press Science, Upper Saddle River, NJ.
 Koonin, Eugene. (2009). “The Origin at 150: is a new evolutionary synthesis in sight?” Trends Genet 25: 473-475.
 S. I. Wright (2014). Mutationism 2.0: Viewing Evolution through Mutation’s Lens. Evolution (Publisher). doi:10.1111/evo.12369.
I have never seen an explanation of how genetic variation originated. The variation seems to have been assumed by Darwin. Mutationism concentrates on the variation, but amounts to a refinement of Darwin, not a refutation of his theory.
Generally speaking, genetic variation comes from the inability of living cells to replicate their chromosomes perfectly and/or from the perfect replication of chromosomes that have been damaged by outside forces.
If chromosomes were always perfectly replicated, mutations would only be caused by external forces.
Below is a good example of how 2 primate chromosomes became ONE (1) Human chromosome!!!