The genetic code of the first generation was completely random. The fittest individuals (those in the top one fourth of the population) were allowed to breed, and all the others were killed off. This process was repeated until the simulation was manually terminated.
Each individual had a single "chromosome" which was meant to encode the desired image. Fitness was determined by comparing the individual's "color" values to those in the image -- the closer, the fitter.
The simulation took about eight hours to attain the result shown at the end of the video. Performance could be greatly improved with a few optimizations.
Python source code is available here: jake-hart.com
Music is Philip Glass' Metamorphosis I. This is the article that inspired this project: