The first Natural Selection was a mod for Half-Life. I never played it, so nothing more to say there.
Natural Selection 2 is available on Steam, and looked like an interesting game for multiplayer because it combines real time strategy and team-oriented shooters. There are two sides, marines and aliens. Most members of a team are grunts, and they move around the map killing enemies, destroying objectives, and building stuff. Their activity generates resources for the individual and for the team.
One member of each team is the commander. They view the map from above, and can use team resources to place buildings, research upgrades, drop health/ammo, et cetera.
Marines are regular shooters. Aliens have a few different forms, with more emphasis on speed, stealth, and maneuverability.
A short tutorial teaches the player how to be a grunt, which isn't too hard to do. Do what the commander says, and otherwise carry on as in any other shooter.
There is no tutorial for being a commander, and that's the downfall of the game. I tried playing with bots, and was forced to be the commander. I had no idea what I should build, or where, nor did I have any idea on how to respond to grunts asking for health and assistance. After five minutes of clicking on a bunch of stuff and being informed I didn't have the resources to do that, I gave up.
Maybe this game was meant for people that played the first and knew what they were doing. I didn't, and the lack of instruction on how to be a commander was asinine. As another downside to this one, loading a level (either the tutorial or my off-line, bots-only map) took at least two minutes.
This game looked like a great idea, and I'm annoyed that I can't play it. It would just take some more tutorials, or a short campaign, to fix this fundamental flaw. Not recommended as it is currently.