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This is an old ALife program I wrote years ago just for fun.
Each coloured square on the grid is a single creature. Each creature can examine the colour of grid cells directly in contact with it (up, down, left, right, and the diagonals).
The program starts off with a bunch of randomly-generated creatures. Most of those die out very quickly, but a few manage to reproduce.
Some parts of this video are showing the program running at regular speed. Others show a 'time-lapse' mode that runs through simulation time much faster. There's no visible marker to indicate which mode is active, but you can usually tell by the amount of change from moment to moment. This video is not a single run, but a combination of several independent runs.
The creatures' genetic code contains a list of rules. Translated to English, the rules would read something like "if there's a green square above you and a food square on the right, move to the right". There are more conditions, more options, and more actions available than just that example - but that's the general idea.
There's a kind of 'gravity' that pulls everything down in the grid. Creatures can climb up and around each other to overcome gravity, but if unsupported, they will fall.
Food pellets (dark grey squares) fall into the grid at the top.
Creatures can reproduce either sexually or asexually. Their rules include actions like moving, eating, reproducing asexually, trying to reproduce sexually with a neighbour, sharing food with a neighbour, and things like that. Some other genes help determine who is willing to mate with whom, who will win in fighting encounters, and so on. The winner in a fight eats the loser.
Since food falls from above, 'species' that climb up high or build colony structures that choke off the top of the grid are rewarded well (just by getting the food, I mean - I there's no programmed reward for any particular behavior). Those stuck below them often have to get food by attacking others.
Predators can appear and wipe out all of their prey, and then go extinct themselves. All kinds of interactions like that can occur in the grid.
Creatures of the same type (or closely related) have the same colour (this doesn't mean two creatures of the same colour are necessarily closely related - but usually it's true).
Creatures can die off just from lack of food. When creatures die like this, they are 'recycled' as food pellets that fall from above. The total 'energy' budget (or food-pellet-budget) remains constant.