How does Endless Space 2 determine which population type to deduct?

There are several reasons why a system may lose population, such as using the "Chain Gang" improvement to convert population to manpower, using the "Draft" action when defending a system or simply not having enough food to sustain the current population.

If your system consists only of one population type, then simply that type is deducted from one planet (either the least desirable one or a random one).

However, when there are multiple different population types, how does the game decide which population to deduct?