Why does every avidian end up with both extra abilities, even where those tasks pay nothing? The four quadrants are the four resources. Blue avidians are ancestors; green ones have unlocked AND, NOR or both.
Try this Set mutation-rate to 0 and press Reset. Nothing evolves and mean-merit stays at 1. Then set it to 40, press Reset, and compare how fast evolved-count climbs.
About this model Where it ends up
Avida, started by Charles Ofria, C. Titus Brown and Christoph Adami at Caltech in 1993, evolves self-replicating programs on virtual CPUs. Programs that compute logic functions of their inputs, from NOT up to EQU, earn extra CPU time; Lenski, Ofria, Pennock and Adami (2003) used it to watch complex functions evolve from simpler ones. This model borrows the reward scheme and leaves the programs out.
The world is split into four quadrants that supply NOT, NAND, AND and NOR resources. Every avidian cycles through an 8-step program: steps 2 and 3 do NOT and NAND, and steps 4 and 5 do AND and NOR only once a mutation has unlocked them. A step pays energy (4, 5, 8 or 12) only if it matches the quadrant underfoot and that patch's pool isn't empty. At 30 energy an avidian divides. When the population is at max-population, it can divide only by overwriting a neighbour with lower merit, where merit is 1, plus 4 for AND, plus 8 for NOR.
Mutations only ever add the two abilities, so what you see is a selective sweep rather than open-ended evolution. Merit counts abilities wherever an avidian lives, so evolved avidians win space even in quadrants where AND and NOR earn nothing.
Idea Charles Ofria · Christoph Adami · C. Titus Brown
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