Why is it inevitable that evolution by mutations alone should be a common cause of evolutionary change in most natural populations? And do you expect mutation-driven evolution to be more common in typical protists or typical prokaryotes? Why?
Your question, as I understand it, concerns the relative role (compare to natural selection and drift probably) of mutation in evolution. But evolution is not one process. Mutation and natural selection are processes that influence populations in very a different manners. At first sight, it seems to me that it does not make sense to weight the important of mutation versus other processes such as selection. But again, in what term would weight these processes?
Your question doesn't make a lot of sense.
Mutation is the process whereby new alleles are created, by the DNA chemically changing its sequence. Natural selection is the process where in a population, alleles change frequency over time due to differing survival rates. (And there are lots of other kinds of selection, and allele frequencies can change just due to chance, too)
So I don't think "evolution by mutation alone" makes much sense. In sexually reproducing eukaryotes, variety can be created by mixing and matching different alleles in the population in different combinations, which isn't something that prokaryotes really do, so I guess that's a difference in the importance of mutations between sexually reproducing eukaryotes and prokaryotes.
A good post about "mixing", which means that a population generally harbors multitude of different versions of a gene, called alleles. So one organism, like a human, has a unique combination of which two alleles out of thousands possible represent each gene in its body. Besides this, major changes are often made not so much by mutations as by changes in the level of gene expression. For example, a mutation in one gene that produces a protein important for regulation of the expression of 50 other genes can cause a lot of change in body development (morphology) and physiology by simply affecting the expression level (amount of protein produced) by these 50 genes. In turn, changed levels of these 50 proteins can make substantial changes in the whole protein interaction network in cells. If you think why sharks look different than elephants, for instance, it's much more because of differential gene expression (including the timing of the expression during body development) than differences brought about by mutations causing functional changes in the coding regions of multiple genes. And yes, mutations only provide raw material for evolution - diversity to select from. Mutations are a supermarket for evolution, selection is what evolution tends to buy more frequently in this supermarket or what's in demand currently. It is thus selection and drift that work to increase the frequency of certain mutant alleles that actually result in evolution. The mechanism of selection is simple. The functional diversity in populations created by mutations in particular creates different survival rates for particular phenotypes/genotypes based on how well the combination of the alleles representing its genome can create a phenotype that matches environment. For example, in a bacterial population adapted to neutral media there's still a random diversity in the range of pH tolerance. If the medium becomes more acidic, cells that happen to have better tolerance to low pH gradually increase in frequency over time. The difference between prokaryotes, like bacteria, and eukaryotes (like humans) mostly lies in that due to meiosis eukaryotes randomly shuffle alleles, so that they "intermix" in a population creating more possible phenotypes to select from. Bacteria do not have this mechanism and can only diverge by cell division, creating diversity only by mutations. However, it is not completely true, as now it is well known that bacteria share a lot of genes via horizontal gene transfer - direct exchange of genes between bacterial cells. This process is often aided by viruses. Horizontal gene transfer, just like meiosis, works to shuffle genes in the population, creating more diversity.