This theory, in Kimura’s own words, states that “the great majority of evolutionary mutant substitutions are not caused by positive Darwinian selection but by random fixation of selectively neutral or nearly neutral mutants.” (It is important to underline that the adjective neutral does not mean without function. It only means that a mutation is adaptatively indifferent, i.e. it is neither better or worse than the previous one in respect to the organism’s adaptation to the environment).
Kimura’s theory has not been universally accepted, and the debate between selectionism and neutralism is still going on, but the experimental data have changed for good our view of molecular evolution. Today, all biologists are aware that neutral mutations are a reality of life, and that genetic drift is, at the molecular level, at least as important, if not much more important, than natural selection. It must also be noticed that this does not diminish in the least the key role of natural selection in phenotypic evolution, and Kimura himself explicitely acknowledged that “the basic mechanism of adaptive evolution is without doubt natural selection”.
He added however that “underneath the remarkable procession of life and indeed deep down at the level of the genetic material, an enormous amount of evolutionary change has occurred, and is still occurring. What is remarkable, I think, is that the overwhelming majority of such change are not caused by natural selection but by random fixation of selectively neutral or nearly neutral mutants. Although such random processes are slow and insignificant for our ephemeral existence, in the span of geological times they become colossal. In this way, the footprints of time are evident in all the genomes on the earth. This adds still more to the grandeur of our view of biological evolution”.

Macroevolution

In 1972, Stephen Jay Gould and Niles Eldredge published in Models in Paleobiology a paper whose title sounded like a war declaration: “Punctuated Equilibria: an alternative to Phyletic Gradualism”. As we have seen, ‘phyletic gradualism’ is the name that was given to Darwin’s classical gradualism by the proponents of the Modern Synthesis, and an alternative to this concept appeared to deliver a direct challenge to Darwinism, to natural selection and to the entire Synthesis. In reality, Gould and Eldredge had nothing of the kind in mind, and their paper was simply an attempt to show that macroevolution is a much more complex phenomenon than people had thought.