Fisher's geometric model
= Fisher's geometric model
{c}
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 $2\mathbf z\mathbin{\cdot}\mathbf a+\|\mathbf a\|^2<0$.