The game was created by Charlie "Flayra" Cleveland, who later founded the company Unknown Worlds Entertainment. Natural Selection v1 was first publicly released on Halloween 2002, and is presently at version 3.2. Production of Natural Selection 2 is underway. The official Natural Selection forums and constellation money donation system were disabled for months due to repeated hacking attempts, but have since been brought back up with the announcement of Natural-Selection 2.
The game features two teams: Kharaa (alien species) and Frontiersmen (human space marines). The alien vs. human conflict is a common science fiction theme, notable examples being the StarCraft game and the 1986 film Aliens. It should be considered that the visible Kharaa "units" are actually simply the spawn of the real Kharaa (aliens) which are microscopic life-forms according to the NS story line.
As of 2008, the game is one of the ten most played Half-Life modifications in terms of players, according to GameSpy.
Natural Selection is played primarily in the first-person perspective, using atmospheric maps of spaceships or space stations, which have been invaded by the Kharaa.
There are two game modes in Natural Selection: Classic and Combat.
In this mode, one member of the marine team must enter the "command chair" to lead the team. From here, he can purchase upgrades, issue movement orders and drop supplies - all from an overhead perspective, as in many RTS games. The commander can also place buildings, although these are inactive until built by players in the field. The role is somewhat similar to the Intelligence Officer of Global Operations in that it is provided an overview of the map, though the ability to place objects and control is much more extensive. A similar 'commander' mode can also be seen in Battlefield 2 and the Empires modification for Half life 2.
The alien team has no defined leader, and so must communicate and co-operate to ensure different roles are fulfilled as needed. They too extend their control by building, to do this they must first gestate into the Gorge evolution. Kharaa "Hive Sight" reveals teammates through the walls and structures of the map, this was originally their answer to the commander with his top-down overview of the game. However in later versions both teams have gained substantial advantage from the addition of a map hotkey. This overlays the whole level, showing teammates and friendly structures; plus enemies and structures currently in view of comrades, and areas under attack.
The aliens start with one active hive, randomly chosen from the 3 hive spots of the map. Active hives heal damaged aliens and respawn dead players. Marines initially spawn in a set location on the map, and after that spawn from Infantry Portals, which can be built in a set radius of the Command Chair. Healing and ammunition can be dropped by the commander in 'packs' (at a cost) or obtained free from an Armoury.
The teams compete for territory, and critically for the resources ('res') it offers. The currency for both sides, resources are obtained by building resource towers to tap the nozzles sited around the map. This is another area where the teams have a key difference - while marines draw on a common pool of resources, each alien accumulates a personal store. This further increases the requirement for teamwork on the alien side, to achieve the right balance of hives, lifeforms, resource towers and chambers.
The game ends when one side has destroyed the other's ability to survive. For the Frontiersmen (marine) side, this usually entails destroying all alien hives, ensuring no further alien players may respawn, and then hunting the rest down. For the Kharaa (alien) side, winning requires destroying all marine "Infantry Portals", ensuring that they do not respawn, and then eliminating the rest of the marines. Other possibilities exist such as destroying the command chair, or destroying all finished hives, and killing the whole alien team before the remaining hive is fully grown.
Game duration has varied through the game's development. In v1, games were slower and often measured in hours. One of the stated aims of v2 was to address this, by introducing a broad range of changes to abilities, structures, etc. In current releases (v3), a typical game lasts 15-30 minutes, but can run over an hour, with both sides vying for control over strategically important Hive Rooms and Resource Nodes.
The stark asymmetry between the two sides makes "balancing" Natural Selection a difficult and perpetually contentious issue. Almost every release includes some balance changes, most via tweaking of numerical values - e.g. damage inflicted by a certain attack, or time taken to complete a certain building. The difficulty of balancing the game is complicated by the need to address varying levels of skill and numbers of players. Competitive games are normally played 6v6, whereas it is not uncommon for public Internet servers to have 20 slots (allowing 10v10) or more. Skill is more difficult to quantify, but one can regularly observe members of top clans achieving high kill:death ratios against experienced non-competitive players.
Neither team can build structures, the marines do not have a commander, and aliens have only one hive. Each player has an individual experience meter, increased by killing enemies; dealing damage to the enemy hive/command chair or healing/repairing their own hive/command chair. When the experience meter reaches maximum, the player gains a level and it resets. This level provides 1 point which can be spent on upgrades (which includes higher lifeforms for the alien team). The maximum level is 10, providing a total of 9 spendable points. However many servers have an additional modification (ExtraLevels 3 by White Panther) installed that allows players to gain experience past level 10 and in some server configurations, up to level 100. This modification also provides several more upgrade categories for both the aliens and marines.
Field experience can also be gained simply through proximity to teammates who are scoring kills. Players will sometimes position themselves near higher-level players, while avoiding immediate danger, to gain experience. This is commonly called 'leeching'.
There have been various changes to the spawn system in the various betas to try and make matches more interesting and more winnable but also without making it such that a team that suffers a major loss of players can still come back. An admin configurable time limit (default 15 minutes) was introduced in beta 5 which remains in the final version. With the time limit, Marines are usually on the attacking side and Kharaa are on the defending side. Failure of the attacking side to win within the time limit results in a victory for the defending side. This has somewhat reduced the phenomena of both sides "camping" in their respective bases, unwilling to attack the other side.
These customizations can be classified as single releases or "packs". Single releases usually involve changing of the ambient sounds, alien/marine commander HUD, crosshair, minimaps or player, map and weapon models, while the packs are collections of customization files of the same style or theme. The Special Ops pack and AvP pack are some examples of such packs.
Traditionally clan play in Natural Selection comprises 6v6 gameplay - Aliens versus Marines. The objective of clan matches is the same as in public matches - to eliminate the opposing team. After one clan is successful, they then switch sides. If the score is 1-1 after two rounds, they generally accept a tie if in group play or play another set of rounds when playing a playoff game of a tournament.
Though never reaching the popularity of other Half-Life Mods such as Counter-Strike, Natural Selection has had a competitive community since its inception - one which still exists today, since the original release in 2002.
Previously North American clans competed in the Natural Selection division of the Cyberathlete Amateur League before it was shut down due to several factors, those being a lack of participating teams, mostly attributed to a rise in player's cheating, and poor leadership of League Officials. The remaining League community created the American Natural Selection League, which remained active till early 2008. European players formerly had several Cups and Leagues to compete in such as UGL, Clanbase, and Gamingorb, but now they play exclusively in the European Natural Selection League, with occasional single-day tournaments. Australia and New Zealand formerly had semi-frequent online tournaments run by the now defunct AusNS, and then later NSPlayer.net, which also drew in teams from Asian countries such as Singapore and South Korea, but have since stopped due to lack of participating teams.
The development team has announced that a technology release will be made available prior to the game's release to allow members of the community to develop maps for the game. They have announced their intentions to purchase the best community developed maps and to include these maps in the game upon release.