Natural Selection 2, a standalone version of the popular Half-Life mod, is now available for pre order. Natural Selection 2 is currently under development by Unknown Worlds, which consists of four team members.

Natural Selection 2 is currently slated for a Q4 2009 release and can be pre ordered at http://www.naturalselection2.com/buy for $19.99. You can also find more information after the jump.

From the press release:

Natural Selection 2 is a standalone immersive, online FPS that pits aliens against humans in a strategic and action-packed struggle for survival. Two teams of up to 16 players face off with unique abilities in dynamic, evolving environments that change as you play. Watch spreading alien infestation deform hallways, and then use a flamethrower to clear it. Cut the power to disable marine structures or weld a loading door shut for defense. No matter what side you choose, you’ll never play the same game twice.

The original Natural Selection was downloaded more than 2 million times and grew to be the most popular independent Half-Life mod. Natural Selection 2 is being created by an independent core team of four people and is scheduled for release on the PC this fall. The hallway and alien footage shown in this movie was filmed entirely in-engine and showcases one of the playable alien classes – the “Onos.”