Ectomorph: Micro-gravity physiology (Asteroids and interplanetary space). Typically long brittle/porous bones and hyper-flexible joints. Very weak muscles and circulatory systems. Unable to function at all in medium or heavy gravity environments without technological support.
Endomorph: Light to medium-gravity physiology (Luna and Mars). Stronger and only slightly elongated bones, with slight hyper-flexibility in the joint. Weakened muscles . With hard physical training an Endomorph can function adequately in an Earth normal weight environment for a limited time. Circulatory systems are weakened as well, and prolonged exposure to heavy-grav situations can cause cardio distress.
Mesomorph: Heavy world physiology (Earth, the LEO Colonies, Galileo Station and the Armstrong). Heavy solid build with strong circulatory systems and limited flexibility. Mesomorphs are faced with many smaller challenges when adapting to light-grav/zero-grav environments and often find working in space a challenging situation (belching in microgravity is a socially disastrous situation for someone used to gravity, and some Mesos never master the technique when emigrating from Earth).
In the early days after the Great Collapse, humanity faced a great many challenges, not the least of which was how to adapt physically to the varied environments of the solar system. Unfortunately because the environment of Earth was spinning out of control faster than they could keep up, humans were left with no alternative but to adapt to the problems of living in minimal gravity situations, and deal with the consequences as they became obvious.
Basically there are three types of environments humans can occupy in the solar system; heavy gravity (earth and large spin-grav stations), medium gravity (the moon and mars), and micro-gravity (like most asteroids and interplanetary space). Within a few generations, human physiology had split into distinct physical branches depending on where people settled, and medicine and conditioning could only do a limited job of holding it together.
The invention of the Pressure Support Exosuit allowed those with an Ectomorph physiology to function in a high-grav environment for limited time frames, and this invention alone kept society from fragmenting entirely.
Although in the nearly 150 years since colonizing lighter gravity places there has been no appreciable genetic change between the physiologies, Ectomorphs are rapidly becoming isolated in the gene pool because interbreeding with Meso partners (and even to a lesser extent Endo partners), is often physically dangerous.
It is feared that this behavioral trend will move humanity toward a selective breeding situation, and a distinctive subspecies will emerge within a few dozen generations. WellCartel is actively researching ways to prevent this from happening.