  Fisher developed his geometric model to support the micro-mutationalism
hypothesis which claims that small mutations are more likely to be beneficial
and therefore to contribute to evolution and adaptation. While others have
provided a general solution to the model using geometric approaches, we derive
an equivalent general solution using a probabilistic approach. Our approach to
Fisher's geometric model provides alternative intuition and interpretation of
the solution in terms of the model's parameters: for mutation to improve a
phenotype, its relative beneficial effect must be larger than the ratio of its
total effect and twice the difference between the current phenotype and the
optimal one. Our approach provides new insight into this classical model of
adaptive evolution.
