A creature looks like (a,b) where a and b are each digits. 5 randomly chosen creatures make up the first generation. Each generation, each pair of creatures reproduces, forming 10 new creatures. Each creature passes one of their two numbers on, each with 4/9 chance, and there's a 1/9 chance for a number to "randomly mutate" into any digit. Afterwards, the ten new creatures are sorted by greatest to least sum. Each creature has a greater chance of dying with lower sum, and five of these ten creatures will remain.
