Crittersim is a simulation program that runs a section of a virtual world. There are plants and animals and some semblance of natural selection going on.
This includes the most fundamental pieces for a simulation. This interactive simulation will hopefully entertain the curious.
In it you can control various characteristics such as the rate of plant seeding, apply herbicide and pesticide, and observe how various species compete for limited resources. Using an advanced editing mode you can even create your own species or modify existing ones in one fell swoop.