The original game by Unknown Worlds, which was a mod for Half Life.
Natural Selection features two opposing teams of players, the Marines and the Kharaa, seeking to destroy the other's respective base. While the two teams have the same essential goals, gameplay for each team varies drastically. Marines largely rely on guns and other pieces of tech to annihilate the alien presence. Aliens, however, rely primarily on melee attacks. Certain alien lifeforms can walk on walls, blink great distances, and even fly. Each team can also build structures to aid in their efforts.
The primary feature that differentiates Natural Selection from others in the FPS genre is its strategy component. The Marines may have one player act as a commander, purchasing and distributing items and abilities using collective currency. This player is given a top-down view of the map and may plan and direct combat actions accordingly.
The Kharaa have to combat this by coordinating attacks with good communication skills.
Due to the strategical and tactical depth, the game is also extremly good for Competitive Play.