Quasispecies equation (Eigen 1971): ẋᵢ = Σⱼ Qᵢⱼfⱼxⱼ − φ(t)xᵢ, where Qᵢⱼ is the mutation matrix and fᵢ the fitness. Error threshold: the master sequence (fittest) is maintained only if μ < μ_c = ln(f₀/f̄)/L. Above μ_c, the quasispecies delocalizes — Eigen's catastrophe. The mutation matrix for per-site error rate μ: Q₀₀ = (1−μ)^L, Q_{ij} = μ^{d(i,j)}(1−μ)^{L−d(i,j)} where d is Hamming distance. Neutral networks shift and broaden the transition.