If I take ten thousand healthy adult humans, strap them into a colony ship, and fire them off on a three-million year journey to a distant star, what will the creatures climbing out of the colony ship look like at the end of the journey?
The colony ship is built using near-future technology, and has a full range of exercise equipment and medical equipment required for humans to survive and give birth to reasonably healthy children in a zero-g environment. The exercise section of the ship rotates, generating centrifugal force to make activities like running and swimming possible, but the majority (98%) of the ship has no form of artificial gravity.
The ship is an oblate spheroid with a 2 km long axis and a 1 km short axis. Engines and technical areas run through the middle of the vessel, with commercial and habitation layers wrapping around in cylindrical layers. The outermost layers of the ship are designated farm/nature areas, with a functioning self-regulating biosphere that keeps the atmosphere stable and the colonists fed. Major layers on the ship are fixed and cannot be altered, but the ship has been designed with smaller bulkheads that can be moved around internally according to the wishes of the occupants. The ship travels via a large array of solar sails, while internal power comes from a large array of hydrogen fusion generators, which are restocked via small transport craft that harvest fuel from gas giants and interstellar gas clouds en route.
No effort has been spared in selecting and equipping the colonists to survive their journey. The inhabitants of the ship have a wide range of expert knowledge required for running and maintaining the ship, and the initial generation has been selected to be as psychologically compatible as ten thousand people can be. They've been selected from groups of people around the world to keep the genetic pool as diverse as possible, and all individuals have been screened against genetic dispositions for disease and disability. They are not, however, obeying any sort of selective breeding schedule to guide their evolution, which will be driven by the fitness of individuals to survive in space. How will evolution shape the colonists?