Polymorphism and evolutionary change

The relation between the degree of polymorphism within species and the rate of evolutionary change between species is a fourth test of the neutral theory. Natural selection and the neutral theory make different predictions about their relation:

• Neutralists predict the two will be correlated. For them, it is the same factor - the neutral mutation rate - which explains both polymorphism and the rate of change. All polymorphism is 'transient', and both polymorphism and the substitution of one gene for another are manifestations of the same process. There should, therefore, be a positive correlation between the rate of evolutionary change and the heterozygosity of different regions of the DNA.

• If natural selection dominates molecular evolution a positive correlation is not expected. Natural selection can produce only transient polymorphism: it takes time for selection to substitute one allele for another. Permanent polymorphism is the result of different processes such as heterozygote advantage or frequency dependence. For a selectionist, any correlation between the rate of evolution and polymorphism can only be coincidence.