In Genetic Diversity I mention that the bulk of the colonist on board Iliad-07 were selected from random lottery. However, certain specialties require a lifetime of study in a university setting, involve field studies, and require extensive review by established experts to certify that new experts really do understand the subject matter.

While they will undoubtedly stock the ship with a few experts, how do they replace the Ship's Doctor, or the Chief Engineer when they retire?

Odds are the most talented person in the next generation of colonists will train under the old expert. And that expert will also bring along an extensive library of material for his or her understudy to draw on when they finally stop working.

Or, they could have some sort of freak science that involves Test tube babies and brain implants.. Though I would bet that would best be left for emergency vacancies to fill, and the good old fashioned Mentor/Apprentice model would be preferred.