Fisher equation states royghly that the rate of evolution of a trait is proportional to the variance of that trait. Fisher equation was often misunderstood even by biologist since evolution is often stated implicitly in the sense that higher fitness drives evolution; it is differences in fitness that drive evolution. In a wonderful study of Darwin Finches, that measured actual mortality in relationship with evolution of bill size in the course of a few decades, it appeared that fitness was overall very low (mortality was huge), but that mintue differneces in moratlity lead to important change in bill size.
I want to emphasize that Fisher formula is closer to Wallace's formulation of evolution than to Darwin's. The title of the essay of Wallace was "on the tendency of varieties to diverge indefinitely from the type" instead than the "survival of the fittest" which is obiously equally correct but less precise and perhaps miselading as I have tried to show.
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