Evolution

Every creature starts identical. Any difference you see later, the simulation invented.

Each generation, a creature that finds no food dies, one food survives, and two or more survives and reproduces with a small mutation. Bigger and faster creatures reach more food but burn energy far quicker — speed costs roughly v², size roughly s³ — so there is no strictly best creature, only one that fits this environment. Change the environment and the population will chase it, but only as fast as mutation and death allow.