Could a tetrapod evolve from a hexapod?
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 logical 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 handful of them evolve like this?
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1 answer
Yes, what you are looking for has already happened (as @Christmas Snow and other mentioned, snakes are the brightest example). We humans have undergone a similar process when we lost the tail (you may want to look for vestigial organs, like the said tail-bone).
There are examples of creatures emerging from the sea, developing legs, only to get back in the sea and lose those legs (yea, I'm talking whales, dolphins and other sea-mammals).
Another example of limb loss (as pointed out by jamesqf) would be the flightless birds, like the ostrich, emu, and especially the kiwi: wiki. Though they still have vestigal wings, they are so small as to be practically invisible.
What you should be really asking yourself is why your creature have lost their pair of limbs. As is the case with whales, they lost the limbs because they were detrimental in the sea, or again, we humans have no much use for a tail once evolved the erect position.
So, your tetrapods should have a pretty good reason to have evolved four limbs when your enviroment clearly favours six.
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