I'm thinking of an alien alien world that has a vast biosphere. Now the creatures of this planet have evolved to be hexapods though there are a few creatures that are tetrapods (only have four limbs). Now I though it would be more locical to assume that the tetrapods evolved from the hexapods, but I'm not sure if that could really happen. So, as the title says, can a tetrapod evolve from a hexapod?
Edit: On what conditions would a hexapod creature need to be "exposed to" in order to lose the extra pair of limbs? What would be the most likely reason as for why they became tetrapods and why would only a handfull of them evolve like this?