Directional variation in evolution: consequences for the fitness landscape metaphor

The concept of a fitness landscape, which describes the relation between genotype (or phenotype) and fitness as a surface on which the population climbs uphill towards local peaks, is a central conceptual tool in evolutionary biology. Inherent in this metaphor is the assumption that the distance between any two points can be defined in the same way as for Euclidean space. However, many of the processes which generate genetic variation, such as gene duplication and lateral gene transfer, are not symmetric by nature, but occur more readily in one direction than the other. This asymmetry is also found in phenotypes, for reasons associated with the genotype-phenotype map and developmental constraints. This article provides an overview of of these processes and suggest how existing methods can be used for incorporating these asymmetric processes when visualising fitness landscapes.