Simple model: Each generation, one randomly selected male and female produce one random-gender offspring. Individuals are immortal. There's probably lots of incest, especially if the population started with just an Adam and Eve.
Draw the family "tree". It will be a messy graph.
Consider restricting mating so that the family tree must remain planar. This could be turned into a puzzle.
Many possible additional constraints (that probably make planarity more difficult): A child must be drawn on the tree below, as defined by comparing vertical coordinates, at least one of the parents, or below both parents, or below all other members of the population born earlier. Siblings must be all on the same horizontal level (what about half-siblings?). Siblings must be ordered left to right by age.
Add an optimization criterion: minimize the size of the drawn graph somehow, maybe minimizing the greater of the width and height, while requiring a minimum node separation.
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