Parasites

Heres a cool flash-based animation/simulation:
"http://www.footloosemoose.com/michael_wp/2007/04/12/experiment-parasites/"

The simulation starts with all these bugs of different random colors and sizes. The bugs move around in a random direction and when they encounter each other one of them takes a bite off the other and grows. After reaching a certain size, the bug splits into two smaller ones. The most surprising part of it is that eventtually there is only one type of bug left in the whole screen.