This economic structure is one of two that is effected by location (the other being the mine), as the growth rate of wheat is dependent on soil fertility and and biome. Unlike 1.4, version 2.0 seems to completely ditch the idea of "X soil fertility within range" and just apply the fertility bonus if you place a farm over the same tile.
As of 2.0, your citizens need to eat to naturally increase the settlement's population (check bottom left of city pop-up); it's defined as "starving: X citizens", "fed", or "well fed". These, of course, correlate to X citizens will die of starvation if food deficit is not recovered soon, enough to feed current population yet will grow slowly, and all citizens fed and natural increase rate in drastically increased.
As of 2.0, farming is the only method of obtaining food; it is planed that pigs can be eaten and citizens can fish along the rivers and shorelines for additional food. (Have not yet confirmed if pigs provide food for 2.0; take that with a grain of salt until someone finds out and erases this...)