Tweaks are both the most numerous phyle in the Terragen Sphere and the most poorly defined. Ever since the Interplanetary Age, the enormous variety of different human forms has hindered attempts at categorising these bionts for census purposes, and only a few general principles have been widely agreed upon. Any human-derived clade that has diverged from the human Baseline to the point that they can no longer be considered Nearbaseline can be considered Tweaks, but the term has traditionally been associated with humans adapted to extreme environments. Although rianths, neogens, provolves and xenosophonts may modify themselves drastically too without any qualifiers, the term Tweak is assumed to refer to a human-derived biont.
The origins of the Tweak phenomenon can be traced back to the challenges faced by the early space colonists in adapting to life beyond Earth. Although pressured habitats and closed-loop ecosystems could allow colonists to live wherever they wished, the technology of the time could not fully replicate the conditions they had evolved in. Low gravity, high radiation levels and limited resources made life in space difficult. Habitats such as Stanford Tori and O'Neill Cylinders were expensive, faced with these challenges many groups began researching ways to adapt their bodies to meet the demands of their new environments.
After many errors and minor successes these enhanced beings soon diverged radically from their Baseline ancestors; replacing their feet with extra hands, adopting prehensile tails and redesigning their circulatory systems for life in space. On the moons of the outer system they went even further by developing gills, altering their metabolism and fusing their legs into tails for exploring Europa and Titan's icy seas. Many Baselines found the appearance of these new humans strange and disturbing, subjecting the Tweaks to harassment and discrimination in some cultures.
It was around the moons of Jupiter that the Tweak movements of today began. At first the colonists banded together to form the Jovian League, later to become the Gengineer Republic, hoping to protect their economic and social interests against the Cis-Lunar powers, but cultural differences soon began to emerge that divided the polity. The Genomorpher faction wished to continue optimising their bodies for the Jovian environment, ultimately aiming to live on Jupiter itself, while the Mutationists considered their environmental adaptations just as limiting as the Baseline forms they had left behind, and preferred more radical experimentation with their body plans.
The Genomophers saw the Mutationists as endangering the colonies by encouraging pointless deviations from their ideal Jovian form, while the Mutationists in turn saw the Genomorphers as holding back the development of the Tweaks with their narrow-minded views. These two cultures later splintered into a wide range of competing philosophies during the Dark Ages, but the two philosophies would go on to have a profound influence on Tweak culture. When the Tweaks were finally contacted by the First Federation, they finally splintered into separate polities, unable to agree on whether to participate in this new government. The Genomorphers welcomed Federation membership, while the Mutationists left Solsys behind to settle the newly discovered worlds beyond its territory.
These developments would shape how Tweak culture developed over the next thousand years. The Mutationists settled on Frog's Head, Penglai, Pacifica, Dionysos and many other worlds, where they adopted many exotic new forms and actively encouraged other humans to do the same. These new beings called themselves Tweaks, as they had back in Solsys, and use of the term soon spread on these worlds to refer to those who followed the Mutationist philosophy. Meanwhile, the worlds of the Federation came to see Tweaks from the Genomorpher's perspective, with Tweaks becoming synonymous with extremophiles in most dialects of Old Anglish that were in use throughout the Federation.
Following the collapse of the Federation in 2193 AT, the colony of Frog's Head became increasingly important as a major centre of Tweak culture and political thought. The many strains of Genetekker philosophy that had been separated for the last millennium came to be absorbed by the influential House Genen, who held spheres of influence across the Inner Sphere as well as on Frog's Head. With the emergence of the Zoeific Biopolity and the other Sephirotic empires, the many Tweak clades came to see themselves as being united by their emphasis on biological enhancement, no matter what end it served. Becoming a Tweak was not just about rejecting the limitations of the human form, but also about embracing the endless possibilities of new forms made possible with genetic engineering.
House Genen alone could never have hoped to unify the incredible diversity of Tweak clades, and in the long term only a minority of the vast Tweak population embraced full House membership. By the dawn of the Age of Establishment, more new Tweak cultures had emerged than could be counted, mostly from Nearbaselines who had gradually shed their humanity over time. Some had religious reasons for changing their bodies, or wanted to experience having extra limbs, or simply wanted to keep up with the latest body fashions. The ideological battles of old have been long forgotten, and in modern times anyone can become a Tweak for any reason they choose. And out on the Terragen periphery, it is often a matter of survival, just as it was in ancient times.
- Amphisapiens - Text by Basu with additions by the AI Vin
Semiaquatic human-derived clade.
- Anakim - Text by Michael Walton
Common type of tweak that is adapted for higher than earth-normal gravity. There are many variants but all are similar in appearance and capabilities. They were originally designed to settle mineral-rich worlds with unusually deep atmospheres.
- Convocation, The - Text by Steve Bowers
A clan of vampire tweaks first encountered in Cableville in 8666 in the unlighted section known as the Dark.
- Daveth Tweaks - Text by Basu, based on original material by Anders Sandberg
A tweak species from the planet Daveth, extremely adaptable and militant that rose to prominence in the volume around the Coathanger Cluster, expanded with their feared habitat-raper ships at enormous speed in the period 5100-5340 before they were totally annihilated by the Dawn Mutual Protection Treaty Organisation.
- Deines - Text by M. Alan Kazlev
Wealthy social group or subclade of Radiation Nation that live on Hekufast.
- Dotties - Text by M. Alan Kazlev
Gerontomorphic beamrider / Deeper clade that originated on Yang during the dark age following the Technocalypse. The loss of some important technologies resulted in the development of a gerontomorphic clade around Yang, not unlike the newrobisonite geronotomorphs
- Europans - Text by Anders Sandberg, some additional content by M. Alan Kazlev
Highly derived aquatic tweak race, Sol System, 1st millennium A.T. to present.
- Extremophile - Text by M. Alan Kazlev
A rather chauvinistic term for any biological organism, whether terragen or xenobiont, natural or tweaked, that requires extreme (non-Earthlike) environments for growth or metabolism.
- Eyre, Kingdom of - Text by Anders Sandberg and M. Alan Kazlev
Tau Ceti II (Nova Terra) aquatic tweak kingdom.
- Genen, House Genen - Text by Anders Sandberg
Bioaugmented traditionalist superclade and House.
- Genetekkers, The - Text by M. Alan Kazlev
Early superior/tweak biocorporation culture and later superpower of the middle and late Interplanetary and Nanotech Age, internalized to the Jupiter moons and the outer Sol System, developed into a number of clades. A number of nearby star systems were colonized by Genetekkers. Some First Federation age Genetekkers became the Genen.
- Gigantes - Text by Omega Tyrant
Gigantes- A sub clade of highly cyborged and bioborged Nephilim designed to be able to engage in "riskier" activities.
- Grays - Text by Todd Drashner
Subculture/Clade that modified their form to duplicate the alleged appearance of the so-call 'grays', hypothetical alien beings.
- Highbrows - Text by AI Vin
Early tweak-su human clade with increased cranial capacity.
- Homo cosmoi / Space Adapted Human - Text by M. Alan Kazlev and Steve Bowers
Early microgravity-adapted tweaks.
- Human Low Gravity Tweaks - Text by Arik
Human clades geneered to thrive on low gravity worlds
- Humans - Text by M. Alan Kazlev; additions by Stephen Inniss
The term "human", or hu, is generally applied to Terragen near-baseline modosophont biont hominids, specifically Homo sapiens and closely related or derived species, who bear moderate bodymods, or none at all, and are not radically different from the original human stock in overall form and behaviour.
- Jötunn - Text by Michael Miller
Giant, vacuum-adapted human-derived clade.
- Keymales - Text by Steve Bowers
First developed by the GeneTEK megacorp in the Jovian League golden age, the Cryptokey geneline was additional to the male and female genome involved in ordinary sexual reproduction.
- Kobolds - Text by Steve Bowers
A clade of high gravity tweaks gengineered for life on the planet Sisyphos. A metre tall and the same wide. Despite their short stature, they are strong and hardy and capable of great feats of strength in Earth-like gravity regimes.
- Lazarus Longidae - Text by Steve Bowers and Michael Capriola
Tweak clade known to live for thousands of years. Named after a fictional character created by the Information Age novelist Robert A. Heinlein, who told Long's story as a series of memoirs. The Lazarus Longidae clade write all their long term memories down in very small thin leaved notebooks, use tiny shorthand ideogrammatic writing, and keep the notebooks in hundreds of small pockets in their overalls, moving all the notebooks around sequentially each time they fill one and throwing the oldest one away.
- Martian Tweaks - Text by Felipe Brando Cadavid, Arik and Steve Bowers
Humans adapted to the Martian environment.
- Megaptera - Text by Steve Bowers and A.J. Garth
Homo megaptera; whale sized merpeople from Zennor system in Libra. These water breathing adapts have powerful arms, a whale-like tail but no legs.
- Merpeople - Text by M. Alan Kazlev and Steve Bowers. Updated by Ryan_b 2016
Early aquatic tweak human clade.
- Methanoids - Text by Steve Bowers
Methane-respiring cold-adapted extremophile humanoids created by House Genen in the Zoeific Biopolity; very widespread and often actively involved in opposition to terraforming projects. Important faction in the Epp War.
- Nephilim, The - Text by AI Vin
Giant Human clade, Homo gigantopithecus maximus.
- Nisse, Clade - Text by AI Vin
A miniaturised human clade, with several associated subclades of various kinds.
- Pantropy - Text by Steve Bowers
The practice of adapting humans (and other bionts) to live comfortably in a planetary environment rather than attempting to change that environment to resemble the home world.
- Radiation Nation - Text by Anders Sandberg
Synclade of radiation-resistant human-derived bionts.
- Sailors of the Ebon Sea - Text by John Snead
Advanced vacuum-adapted clade.
- Soft Radnad - Text by M. Alan Kazlev
Members of the tweak synclade Radiation Nation who use highly enhanced DNA repair systems, often based on Deinococcus radiodurans and on Xenobacter mirabilis from Mykropht III.
- Space People - Text by M. Alan Kazlev
 (archaic) A common early term for Space Adapted Humans, still used by some descendants of the original clade.
 (Second Federation Era) A term for vacuum adapted tweaks, now rarely used.
- Sritht - Text by Liam Jones
Heavily derived eusocial hominid tweak/bioborg clade.
- Tro'el, Clade - Text by AI Vin
A clade of cold adapted humans living on ice moons.
- Vacc-Suckers - Text by Elliot Schutjer
A failed clade of early Interplanetary Era tweaks that were designed to operate in open space without protection.
- Vacuum Adapted Humans - Text by Anders Sandberg, with modifications by M. Alan Kazlev
- Vampires - Text by M. Alan Kazlev (with additions by Pran Mukherjee)
A human-derived clade based on human legends.
- Virds and Vats - Text by John Snead
Flying clades from Mazukata's Holiday.