The Bak-Sneppen model is a simple model of co-evolution

Co-evolution

In biology, coevolution is "the change of a biological object triggered by the change of a related object." Coevolution can occur at many biological levels: it can be as microscopic as correlated mutations between amino acids in a protein, or as macroscopic as covarying traits between different...

 between interacting species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

. It was developed to show how self-organized criticality

Self-organized criticality

In physics, self-organized criticality is a property of dynamical systems which have a critical point as an attractor. Their macroscopic behaviour thus displays the spatial and/or temporal scale-invariance characteristic of the critical point of a phase transition, but without the need to tune...

 may explain key features of the fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 record, such as the distribution of sizes of extinction events and the phenomenon of punctuated equilibrium

Punctuated equilibrium

Punctuated equilibrium is a theory in evolutionary biology which proposes that most species will exhibit little net evolutionary change for most of their geological history, remaining in an extended state called stasis...

. It is named after Per Bak

Per Bak

Per Bak was a Danish theoretical physicist who coauthored the 1987 academic paper that coined the term "self-organized criticality."- Life and work :...

 and Kim Sneppen.

The model dynamics repeatedly eliminates the least adapted species and mutates it and its neighbors to recreate the interaction between species. A comprehensive study of the details of this model can be found in Phys. Rev. E 53, 414-443 (1996). A solvable version of the model has been proposed in Phys. Rev. Lett. 76, 348–351 (1996) , which shows that the dynamics evolves sub-diffusively, driven by a long-range memory.

An evolutionary local search

Local search (optimization)

In computer science, local search is a metaheuristic method for solving computationally hard optimization problems. Local search can be used on problems that can be formulated as finding a solution maximizing a criterion among a number of candidate solutions...

 heuristic based on the Bak-Sneppen model, called extremal optimization

Extremal optimization

Extremal Optimization is an optimization heuristic inspired by the Bak-Sneppen model of self-organized criticality from the field of statistical physics...

, has been introduced in Artificial Intelligence 119, 275-286 (2000).

Description

We consider N species, which are associated with a fitness factor f(i). They are indexed by integers i around a ring. The algorithm consists in choosing the least fit species, and then replace it and its two closest neighbors (previuos and next integer) by new species, with a new random fitness. After a long run there will be a minimum required fitness, below which species don't survive.

External links

• Bak-Sneppen Evolution Model as an interactive java applet.