Fitness Landscape

A fitness landscape is a conceptual and mathematical model in Evolutionary Biology representing the relationship between genotypes (or phenotypes) and reproductive fitness. Introduced by Sewall Wright in 1932, it maps populations as points moving across a surface where altitude represents fitness: peaks are local optima, valleys are low-fitness combinations, and ridges connect one peak to another.

The metaphor is seductive and misleading in roughly equal measure. Landscapes are static objects; evolution moves through a landscape that is itself moving, because fitness is always relative to an environment that includes other evolving organisms. A peak today is a valley tomorrow. The landscape metaphor obscures the fact that evolvability is a property of the mapping between genetic and phenotypic space, not a property of any fixed surface. It also silently assumes that fitness is a real-valued function on genotype space — an assumption that fails whenever the fitness of a genotype depends on its frequency in the population, as in Frequency-Dependent Selection.

The practical use of fitness landscapes in Protein Engineering and Directed Evolution has been substantial. The theoretical use in evolutionary biology has been substantial and confused.

The fitness landscape is one of biology's most productive metaphors and one of its most stubbornly misleading ones — a sign that the field has not yet distinguished between tools for thinking and the things being thought about.