Darwin's Nightmare, aka 'The Strain' was my one and only OpenGL-ES 2 top down shooter.
In this game enemy ships evolved following Darwinian evolution - as they came down the bottom of the screen they got rated (according to how close to hit the PC) and their genes recombined.
NPCs had 16 boolean genes, each controlling a different ability (such as being fast, or shooting side-ways, or bonus motion towards the PC unit).
The game was 99% shader art. Each gene expression also controlled a parameter in the shader that drew enemy sprites.
Since Apple pulled the game from their store (for lack of updates on my part) the best way to enjoy this is probably Echoen's 20 minutes stand against the endless evolving wave (typical plays lasted 3; eventually I pushed that slightly over 30 minutes, then a Japanese player beat that and I gave up).
For a geekier approach, I also wrote a fairly detailed article on Gamasutra.