Note: This is part of the "Old stuff I wrote when I was a kid" series.
A simple natural selection simulator/demonstration I made in VB6 for a school science project in 2001 when I was 12.
A number of creatures are generated when you start the simulator. They have a single fitness property represented by their size on the screen. They are all small to start off with. You click through the phases (or let it play through them automatically) where the creatures randomly pair up for fighting (where the bigger one wins and kills off the smaller) and then randomly pair up for mating (where the offspring is the average fitness of the parents with some random deviation). This process continues.
The user observes that the creatures get bigger and bigger by this process which aimed to help illustrate how Natural Selection works.
Being VB6, you may not be able to get it to run on modern Windows. If you have a Windows XP machine handy or you can be bothered resolving the dependencies you'll be able to check it out. This downloadable from Microsoft might do the trick.