When you have a creature that reproduces, and this reproduction is imperfect, then you have the necessary ingredients for evolution.
This simple simulation shows evolutionary mechanics occurring in a virtual petri dish.
Each "individual" has a single gene, that determines its color and its reproduction probability. When an individual reproduces, there is a small chance that its offspring will mutate, earning a smaller or higher value of the gene.
Over time, individuals with higher reproduction probability will slowly dominate the petri dish over their slower neighbors.
Inspired by Avida-ED
I still want to improve things a little bit. Give the creatures more visual variety, and add some more audio-visual juice. Suggestions / comments / hacks welcome!