Viability of the artificial selection of extra limbs from tetrapod animals?

Question

Based in the existance of mutations in different animals which make them get extra limbs, like the next image which show a chick with four legs, gonna call them griffin.

I dont' how common is this is in the animal breeding, but in nature animals with this kind of mutations bring a lot of problems for the survival of the animals which born with them, like the limbs aren't controlled by the animal turns a carry and are depredated easier, they die so they don't pass on their genes and the mutation is never polished to turn to a real useful adaptation.

But a controlled, guided and secured enviroment might allow the animals with this mutation to extend it without the predatory problems and probably getting health support.

The premise is next, an intelligent species specially selects individuals with this mutation to breed them and get the best features for generations until they get individuals which turned the mutation useful.

This maybe could work because persons born with strange mutations can live relatively normal lives with the benefits of technology an comfortable enviroments but probably in the past their life would not be long.

But the problem is that extra limbs eventually require a way to get connected with the skeleton, correctly developed muscles and with good nervous connections, and since currently this is not the case, I want to know the viability or possibilities of this. So a question included into of this might be, how could the mutated limbs eventually turn useful?

The objective is to get hexapods (or more limbs) like griffins or dragons from current Earth's animals.

Answer 1

how could the mutated limbs eventually turn useful?

Not without great difficulty.

Here's a few problems that spring to mind:

1. The basic tetrapod bodyplan (very loosely speaking arms/wings/forelimbs attached to top of chest, legs/hindlimbs attached to pelvis) doesn't have much scope for duplicating either the forelimbs or the hindlimbs, because there's not another "end" to stick them on to.

   It follows then that to have functional extra limbs that work well and don't interfere with the operation of pre-existing limbs you'll need some entirely novel way of attaching them and that is a major new engineering effort... design new joints, new mounting locations, new support structures, new bones, new fascia layout, then try and get all your good ideas to actually grow.

   For a fictional example of this, the Amarantin in Revelation Space radically re-engineered their upper body in order to restore the wings their ancient evolutionary forebears had, but retain the rather useful arms they'd evolved since.

2. You need to be able to operate those extra limbs. All those new muscles need nerves to make them go, and those nerves need to be added to the spinal cord (is there enough space? will it expand to fit?) and finally you need suitable chunks of the animal's brain to do the driving. Brains aren't grown with spare capacity... they're already expensive enough, so there's no other functionality you can cannibalise to run the new limbs instead. No... you need to engineer a bigger brain, and make sure the skull fits it nicely

   The heavily modified transhumans in Peter Watts' Blindsight sacrifice some parts of their body functioning in order to drive their new enhanced abilities or cybernetic prostheses... one character has an effectively paralysed face, using all those complex nerves for other purposes instead.

3. You've thrown off the centres of gravity and drag and so on, so a whole bunch of reflexes that evolved to assume that those hadn't been changed (probably around walking, flying, climbing, falling) will probably not work properly anymore. They might even be counterproductive or even downright dangerous with the new body layout. Furthermore, complex things like flying are going to impose a lot of constraints on how the new body is laid out... your gryphons might not be able to get off the ground if you try to copy the basic design of classical gryphons. Winged mammalian tetrapods as designed by an aerospace engineer are likely to look quite different from their fantastic forebears.

So no, I don't think you can do this by careful mutation or selective breeding. It needs to be a carefully planned engineering project by a group with significantly better grasp of biology than we have today.