A population is group of people united by the bonds that matter most to them. At the start of the game, this means they are of the same people and share the same values. As the game progresses, populations may split into smaller groups, separating by gender or status or other reasons.
Perhaps more than anything else, a population is defined by its shared values. These are the things which the people of the population hold dear or disdain, which determines how they attempt to live in their world. A population's values does not exactly describe every member of the population, but rather its general culture. In fact, when a population splits, the values of the two new populations diverge due to the differences of the individuals in each group.
Populations also control resources. As with their values, the resources of a population are not necessarily equally divided amongst all its members. Instead, the resources of a population reflect the resources that the population can put to use and which other populations can't take without violence.
Lastly, populations may learn advances. These are broad categories of understanding which can be developed and utilized over the course of the game. Advances range from Bronzeworking to Ritual to Boating, and are ranked. Each level of an advance allows the population to do something new, whether it is construct a new building, practice a new value, or enjoy a bonus to their other actions.
Each season, each population works together at a work site, eats what food they are able to access, and interacts with their neighbors. They might also practice religious rites, conduct experiments, go on raids, or any number of other activities. What a population chooses to do is dependent on its present circumstances (such as if they have enough food to feed themselves) and the population's values.
Populations may lose members to starvation or violence as the game progresses. Each winter, populations lose their eldest and gain new members as children come of age. If a population loses all of its members, it is removed from the game.