Compare and contrast the neutral theory of evolution versus the Darwinian (i.e., selectionist) theory of evolution. Explain why the neutral theory of evolution is sometimes called non-Darwinian evolution.

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Both theories propose that random mutations occur in populations. The primary difference between the neutral theory and the selectionist theory is the relative contributions of neutral mutations and nonneutral mutations for explaining present-day variation. Both sides agree that natural selection has been an important force in shaping the phenotypes of all species. It is natural selection that favors the traits that allow organisms to survive in their environments. Within each species, however, there is still a great deal of variation (i.e., not all members of the same species are genetically identical). The neutral theory argues that most of this variation is neutral. Random mutations occur that have no effect on the phenotype of the individual, and genetic drift may cause the allele frequency to rise to significant levels. The neutral theory suggests that most (but certainly not all) variation can be explained in this manner. In contrast, the selectionist theory suggests that most of the variation seen in natural populations is due to natural selection. The neutral theory of evolution is sometimes called non-Darwinian evolution because it isn’t based on natural selection, which was a central tenet in Darwin’s theory.