Natural Selection is a multiplayer modification of the original Half-Life game developed by Unknown Worlds Entertainment and released in October 31, 2002. The game attempts to blend real-time strategy elements to a first-person shooter with a two-team basis, marines and aliens. On any given level, the aliens have the ability to create Hives in specific places and in these Hives the aliens can spawn in and create a base camp of sorts, whereas the Marines are attempting to prevent the creation of Hives. Each team can build structures on resource nodes in order to get resources for team upgrades. The game is won when one side is completely eliminated with no possibility of respawning.
Classic -- The traditional game mode in which each team tries to completely eliminate the other team's bases or hives. In this mode, one marine will take the role of the Commander who issues orders to the team. Resources are gained by capturing resource nodes, or by killing a member of the opposite team. In this mode there are three hive locations in which the aliens can capture, with each hive capture the aliens will gain access to more abilities and upgrades. Strategies for this mode on the Frontiersmen is to 'lock down' one or two hive locations to prevent the aliens from capturing them.
Combat -- Combat mode is a quick alternative to the standard gameplay mode introduced in version 3.0 of Natural Selection. In this mode there are no RTS elements to speak of. The Commander position as well resource nodes no longer exist. Each player is awarded experience points by being involved in combat, and with those experience points they can get any upgrade of their choice. The marines have one command chair and the aliens have one Hive, the team to destroy their opposing main structure first is the winner. Unlike Classic, Combat usually has a timelimit tied to it, which in the case of time running out, the defending team usually marked as the aliens are the winners of the round, while the attacking team are considered the losers.
The RTS elements are most applicable to the Commander on the Marines team. As the Commander, he has a top-down view of the battlefield area while in the Command Chair. From here, he can assign squads, issue attack orders, issue build orders, and drop supplies for his team. No marine can drop any given structure, however marines are required as builders for the structures dropped onto the battlefield, without them the structures are unusable. Likewise, no upgrades or item drops can occur without the Commander issuing it. As such, the Commander is very much the leader of the marines and they are dependent on the Commander for victory. Likewise, without good marines following orders, victory is difficult to attain.
- Command Chair - The structure in which the Commander operates from.
- Infantry Portal - Spawns Marine players with a Light Machine Gun, Pistol, and a Knife (and a Grenade if they have been researched)
- Armory - Dispenses health and ammo, is required to drop shotguns, wielders and mines. Can be upgraded into an Advanced Armory which can drop Heavy Machine Guns and Grenade Launchers
- Arms Lab - Requires Armory. Upgrades Weapons and Armor to a maximum of three levels. Can research Catalyst Packs here.
- Resource Tower - Gathers resources for the team, can be electrified to inflict close range damage
- Turret Factory - Gives the ability to drop turrets so long it is in range of the Turret Factory. Can be upgraded to an Advanced Turret Factory which can drop Siege Turrets
- Sentry Turret - Requires Turret Factory. Standard turret which attacks non-cloaked alien lifeforms
- Siege Turrets - Requires Advanced Turret Factory. Only attacks alien structures and can shoot through walls as long as the structure is spotted first.
- Prototype Lab - Requires Advanced armory and Arms Lab. Allows for research of Heavy Armor and Jetpacks.
- Observatory - Requires Armory. Allows for research of Motion Tracking and Phase Tech. Can scan areas of the map to reveal any alien structures or lifeforms. Will also reveal any lifeforms and structures in a large radius from where it is built. Can use Distress Beacon, where all Marines (alive or dead) will instantly be brought back to Marine Start (as indicated on the Map)
- Phase Gate - Requires Observatory and Phase Tech upgrade. Will instantly transport a Marine to the next Phase Gate.
The basic marine has a standard-issue light machine gun, pistol, and knife. There are various upgrades that can be acquired once it has been researched by the Commander.
Instead of having a commander each alien is given their own amount of resources. Each alien can use these resources to evolve, build structures or hives.
There's several different types of aliens on the alien team:
The Skulk is a small fast moving alien, that can run on walls.
- Parasite (Marks the player it hits=
- Xenocide (Suicide bomb)
Cost: All players starts as skulks - Costs 2 resources to evolve back to skulk
The Gorge is a small, fat and slow alien. It's the builder for the alien team.
- Healspray (Heals teammates and buildings)
- Bilebomb (Massive damage to enemy buildings)
- Web (Marines can get stuck in it)
Cost: 10 resources
The Lerks is a larger flying version of the Skulk.
- Spores (Poison Gas)
- Umbra(Defensive gas for teammates)
- Primal Scream (Boosts teammates)
Cost: 30 resources
The Fade is a large up-standing alien with 2 claws as arms. It has a lot of armor and hp compared to the smaller aliens.
- Blink (Leaps forward)
- Metabolize (Self-heal)
- Acid Rocket
Cost: 50 resources
The Onos is a massive rhino-like alien. Think of it as an alien tank.
- Gore (Stap with tusk)
- Digest (Eat Marines and heals itself meanwhile)
- Stomp (Paralyzing Marines)
- Charge (A strong attack whick damages every marine who touches the onos - also an increase in speed
Cost: 75 resources