Algodoo

[DO NOT DELETE THE RED BLOCK OR YOUR COMPUTER WILL CRASH SHORTLY AFTER]
Eight initial species exist, and at the end only a few remain.
Every creature has a growth rate, random element to the growth rate, color restitution, and friction, and when it reproduces, one of its children will be identical to itself, but the other will be slightly mutated in the aforementioned attributes. This allows for different creatures to exist and passively compete for space, leaving the best suited creatures left.
The environment seems to favor high growth rate, low randomness, low friction and low restitution.

**After a long while of evolution, the circles grow so quick that if unsupressed by the red ceiling, they will grow exponentially, doubling several times in one second. Because of this, to protect your own computer, DO NOT REMOVE THE RED BLOCK.**