Fisher's geometric model

A model in which a phenotype is a point in a multidimensional trait space and genotype fitness declines with displacement from an optimum. With displacement $\mathbf{z}$ and mutational step $\mathbf{a}$, improvement in squared distance requires $2\mathbf{z}\cdot \mathbf{a}+\Vert \mathbf{a}{\Vert }^2<0$.