This allowed Simpson to conclude that macroevolution (the origin of taxa above the species level) can be explained by microevolution (the origin of species).
Phyletic gradualism (transformation and speciation) became in this way the one and only mechanism of evolution in the framework of the Modern Synthesis, and Simpson’s contribution was welcome as the long awaited conciliation of natural selection with paleontology.

Molecular evolution

The laws of population genetics are usually expressed with technical terms such as mutation frequency, nucleotide-substitution rate, amino-acid-turnover rate and so on, but until the 1960s it was impossible to make direct measurements of these parameters. The only way to estimate them was by deducing their values from their visible effects on real organisms, i.e. by transfering at the molecular level what is observed in the phenotypic world.
A mutation, on the other hand, can not only be positive or negative for a given organism, but can also be neutral, in the sense that could have no adaptive value. In this case natural selection would not work on it, and its destiny would be determined by the only other existing mechanism, i.e. by genetic drift. In order to understand evolution at the molecular level, therefore, it was necessary to estimate how many neutral mutations occur in nature, on average, in respect to adaptive ones. Since direct measurements were impossible, it seemed logical to resort to indirect observations and to say that the ratio between neutral and adaptive mutations must be fairly close to the ratio between neutral and adaptive phenotypic characters. And since neutral characters are a tiny minority, it was concluded that neutral mutations must be a very small percentage of the total. This is why Ernst Mayr concluded, in 1963, that “it is highly unlikely that really neutral genes do exist, or that a gene could remain neutral for a long time”.
Another way of obtaining indirect information on molecular evolution was offered by the study of phylogenetic trees. A typical example, in this field, is the comparison between amphibians and mammals.