Evolution is a mathematical model of a simplified ecosystem of foxes and rabbits using the Lotka–Volterra equations. The logic was that too small a population of rabbits would provide insufficient food for foxes whose numbers would then decline, until too few foxes would result in the rabbit population again increasing, and the cycle would continue.
One nominated a number of rabbits and number of foxes to start, then watched the basic graph of the two populations rising and falling. At any point in time, one could pause the simulation and save to the audio cassette by pressing 'Record' on the connected remote tape recorder, then later resume the simulation on the Spectrum from that point.