Molecular evolution and neutral theory - How do we test the neutral theory?
Testing between the two theories
The difference between the selective and neutral theories is the relative frequencies of neutral and selectively advantageous mutations.
The direct way to test between them would be to measure the fitnesses of many genetic variants at a locus, and count the numbers with negative, neutral, or positive selection coefficients under certain environmental conditions. It has proved impossible to settle the controversy in this way. It is a major research program to measure the fitness of even one common genetic variant; it would be completely impractical to measure the fitnesses of many rare variants.
Testing between the theories has been attempted by less direct means. There are four main kinds of test:
1. The absolute rate of molecular evolution and degree of polymorphism, both of which have been argued to be too high to be explained by natural selection.
2. The constancy of molecular evolution, which has been argued to be inconsistent with natural selection.
3. The observation that functionally less constrained parts of molecules evolve at a higher rate, which has been argued to be the opposite of what the theory of natural selection would predict.
4. The observation that the degree of polymorphism and the rate of evolution of a molecule are not correlated in exactly the way the neutral theory predicts.