Ultrametricity and Memory in a Solvable Model of Self-Organized Criticality

TL;DR

This paper introduces an exactly solvable model of self-organized criticality in biological evolution, revealing how ultrametric memory structures influence avalanche dynamics and activity spreading.

Contribution

It provides an exact analytical solution for a self-organized critical model with ultrametric memory, linking hierarchical structures to avalanche behavior.

Findings

Avalanche propagation follows a diffusion equation with a nonlocal, history-dependent potential.

The activity spread exhibits a fat-tailed distribution with specific decay properties.

Hierarchical ultrametric structures generate a hierarchy of time scales in the model.

Abstract

Slowly driven dissipative systems may evolve to a critical state where long periods of apparent equilibrium are punctuated by intermittent avalanches of activity. We present a self-organized critical model of punctuated equilibrium behavior in the context of biological evolution, and solve it in the limit that the number of independent traits for each species diverges. We derive an exact equation of motion for the avalanche dynamics from the microscopic rules. In the continuum limit, avalanches propagate via a diffusion equation with a nonlocal, history-dependent potential representing memory. This nonlocal potential gives rise to a non-Gaussian (fat) tail for the subdiffusive spreading of activity. The probability for the activity to spread beyond a distance $r$ in time $s$ decays as $\sqrt{24\over\pi}s^{-3/2}x^{1/3} \exp{[-{3\over 4}x^{1/3}]}$ for $x={r^4\over s} \gg 1$. The potential represents a hierarchy of time scales that is dynamically generated by the ultrametric structure of avalanches, which can be quantified in terms of ``backward'' avalanches. In addition, a number of other correlation functions characterizing the punctuated equilibrium dynamics are determined exactly.