Wednesday, October 24, 2007

Fitness landscapes: a way to visualize adaptation of complex systems

"Evolution is sometimes characterized by biologists as a metaphorical uphill struggle across a “fitness landscape” in which mountain peaks represent high “fitness,” or ability to survive, and valleys represent low fitness. As evolution proceeds, a population of organisms in effect takes an “adaptive walk” across such a landscape." (
"In biology, fitness landscapes are used to visualize the relationship between genotypes (or phenotypes) and reproductive success. It is assumed that every genotype has a well defined replication rate (i.e. fitness). This fitness is the "height" of the landscape. Genotypes which are very similar are said to be "close" to each other, while those that are very different are "far" from each other. The two concepts of height and distance are sufficient to form the concept of a "landscape". The set of all possible genotypes, their degree of similarity, and their related fitness values is then called a fitness landscape." (Wikipedia)
Fig: "Fitness is shown as a function of sequence: the dotted lines are mutational paths to higher fitness. a, Single smooth peak. All direct paths to the top are increasing in fitness. b, Rugged landscape with multiple peaks. The yellow path has a fitness that drastically lowers its evolutionary probability." (Poelwijk et al. 2007. Nature)
"Fitness landscapes are often conceived of as ranges of mountains. There exist local peaks (points from which all paths are downhill, i.e. to lower fitness) and valleys (regions from which most paths lead uphill). A fitness landscape with many local peaks surrounded by deep valleys is called rugged." (Wikipedia)
Effectively, rugged fitness landscapes have many local fitness optima (peaks), and the higher you are on one, the less likely “mutations,” whether real or metaphorical, will enable you to climb to a taller one. The reason is that the organism or system must decrease in fitness as it walks down one hill in order to start the adaptive climb up another. In general, the more connections (edges) between nodes in a complex system, the more rugged its fitness landscape.

Hopefully this biology-focused primer will help us apply the fitness landscape concept to human-environment complex adaptive systems, such as that modeled by Lansing & Kremer.



McKinsery Quarterly_Escaping the red queen effect:

Poelwijk et al. 2007. Empirical fitness landscapes reveal accessible evolutionary paths. Nature 445, 383-386.

Wikipedia, fitness landscape:

1 comment:

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