Neutral theory

The neutral theory of molecular evolution, proposed in the 1970s by Motō Kimura, is/was a controversial theory that suggests that most mutations in an organism are, on the whole, selectively neutral, making genetic drift a more powerful mechanism of evolution than natural selection. It downplays the power of selective pressures by asserting that in general evolutionary changes are too minor to result in much actual selective advantage or disadvantage, although it acknowledges that some changes may be overwhelmingly positive or negative. Part of the implication of this is that a species can change substantially through mutation before any major change is seen; some of this holds true with current theories of pseudogenes and junk DNA mutation.

Neutral theory is not generally considered to represent a complete model of evolution, and does not hold strictly true in all (arguably not even in most) cases, but it adds a great deal to our understanding and ability to model evolutionary changes on a genetic level: Namely, it marked the first major drift from purely selectionist evolutionary theory, arguing that most mutations confer a selective advantage or disadvantage. It also provides an effective null model for positive selection. It's vital to note that neutral theory is in no way "anti-selection" or "anti-Darwinian"; it simply represents another facet of understanding to the existing model of evolutionary theory.

Selectionist opposition
While neutral theory has become the consensus in population genetics, some studies have challenged it. One special example is studies of Homo sapiensgenome in comparison to Pan trogolodytes genome, in which the estimates of adaptive evolution (as the reason of genetic variation) is estimated from 0-45%. One study on Capsilla grandiflora found that 40% of amino acid substitutions could be non-neutral. A study on Drosophilia melanogaster found that adaptation plays a role in evolution of small populations too. Other studies found similar results by sequencing regions of the Drosophilia melanogaster genome.