[3][1] In his model, Fisher argues that the functioning of the microscope is analogous to the fitness of an organism.
The performance of the microscope depends on the state of various knobs that can be manipulated, corresponding to distances and orientations of various lenses, whereas the fitness of an organism depends on the state of various phenotypic character such as body size and beak length and depth.
The more independent dimensions of variation the phenotype has, the more difficult is improvement resulting from random changes.
Fisher noted that the smaller the effect, the higher the chance that a change is beneficial.
This argument led to the widely held position that evolution proceeds by small mutations.