Darwin’s theory for evolution mechanisms was based on inheritance, variation and selection. Of these, only selection works approximately in the way Darwin thought. Some researchers tried to see evolution as controlled just by selection, but this was not very successful, and Darwin himself protested against such a way of thinking. But at the turn of the century both inheritance and variation got their solution. The solution for inheritance was based on the observations that Mendel had made 35 years earlier, but which had not been fully understood before Hugo de Vries analyzed them. He also found the only source of variation that we know about today, mutations. For 30 years there was a theory based on Darwin’s principles: inheritance, variation, selection. But many scientists were not convinced that mutations could produce useful novelties. Nearly all mutations that were analyzed were deleterious. They were often sources of diseases.
A common misconception about mutations was in this period allowed to take form. In medicine the term was used to denote any inheritable disease, i.e. it was assumed that in some ancestor of the person with the disease there had been a deleterious mutation.
From 1930-1940 there was increasing interest in adaptation of allele frequencies in populations. Working with such a system they believed that they did not have to depend on creation of new variation, because it was assumed that enough variants were already present in all populations. A lot of mathematical simulations of the progression of allele frequencies were developed. Theories for the progression of alleles and changing of allele frequencies as a result of changes in the environments were the primary for people who wanted a theory for evolution. But the limitations to the system that they studied made it useful just for adaptations, not for evolution.
They claimed that their findings could be directly transferred to a larger system. That claim has been accepted as truth by a lot of readers of books from the following years, but they have never given any proof. There is now a problem with terminology, because they used the word evolution for what takes place their limited system. It is therefore a great confusion, because the word was defined by Darwin to denote the common descent of all organisms, not the adaptation in one species.
Consistent and terminology that everybody can agree about is important to avoid misconceptions. The confusion that I have described above is therefore rathercreating miconceptions in evolution theories than avoiding them. You can find more details about this at this blog.