Bak-Sneppen Co-evolution

Bak & Sneppen (1993): each species has a fitness f ∈ [0,1]. At each step, the least-fit species and its neighbors mutate (get new random fitnesses). The system self-organizes to a critical state where the minimum fitness threshold converges to f* ≈ 1 - 1/N → 2/3. Below f*: avalanches sweep through the ecosystem. Power-law avalanche sizes P(s) ~ s^(-τ) with τ ≈ 1 — same exponent as fossil record (Raup 1986). No fine-tuning needed: SOC is generic.