Hi
It occured to me while thinking about inceasing enviromental diversity: what do you think about splitting the screen in terms of physical properties, like gravity? For example, the upper half of screen would be earth gravity, the lower no gravity, like a pond and its surface. Viscosity, light and such things could also be adjusted this way. I think that would force lineages to diverge, while allowing some slow transfer.
However, I would be suprised hadnt this been already suggested or considered.