Emergence of avalanches in fitness in the evolution of a thermodynamic model of rodent huddling.

<p>The thermodynamic huddling model was evolved through 10000 generations, at ambient temperature <i>T</i>_<i>A</i> = 10°C for this representative example. The fitness and metabolic rate of the least fit individual is essentially uniformly randomly distributed, and is therefore not shown in this figure for clarity. <b>A</b> The fitness of the remaining <i>N</i> − 1 = 11 individuals is shown at each of 10000 generations. A baseline fitness of <i>T</i>_<i>A</i> − <i>T</i>_<i>P</i> = −27 is shown as a dashed white line. Fitnesses below the baseline are due to high metabolic rates, <i>G</i>, and fitnesses above the baseline indicate thermoregulatory savings that are due to huddling interactions. <b>B</b> The onsets of avalanches in fitness occur simultaneously with increases in the metabolic rate of the second least fit. <b>C</b> Generations 5000–6000 are shown in greater detail, revealing that following the onset of an avalanche the composition of thermal physiologies in the evolving group takes several generations to recover back to a stable distribution of low metabolic rates. <b>D</b> The distribution of metabolic rates in the second least fit individual conforms to a power law model, as evidenced by a straight line in a log-log plot derived from the histogram <i>H</i>(<i>G</i>_min). The power law distribution in <i>G</i>_min is evidence for the emergence of self-organised criticality.