Imagine life evolving on a planet. First creatures crawl out of the ocean and start colonizing the land. Then one of the species gets into such conditions where it needs to specialize in two different tasks simultaneously and that specialization cannot be covered with sexual dimorphism alone.
In other situations this species would've become a "jack of two trades", find a symbiote or just go extinct, but a lucky mutation happens and this species gains an additional dimorphism capability based on a different pair of chromosomes. So now in addition to XX/XY variety they have, say, AA/AB variety while remaining a single species.
Problem solved. AA-type evolves into one task, AB-type evolves into another and the diverse, yet single, species gains an advantage over their rivals. So in a few million years all tetrapods on the planet have this non-sexual dimorphism feature.
So the questions are:
- What realistic circumstances could cause this dimorphism to appear?
- To stabilize and consolidate this feature the evolution has made it so that only cross-type couples can breed successfully, i.e. only XXAA/XYAB or XXAB/XYAA couples can have offspring. How to make this more plausible and what are possible evolutionary dangers here?
- What else should I take into consideration to make this scenario more scientifically plausible?