@FBM,
RE: 'You and your stomach are better when working together then being separated'
... and you and your stomach will be much better if you have one more stomach in reserve, for example. Oh, no it is too spacious and you will start looking like a panther with those two types of body metabolism ... and may start running at 195 km/h ... but wait a minute - this is not a 'positive mutation', at least not enough positive to attract the attention of the evolution.
O.K., if not another stomach at least one more pancreas in reserve. We have two kidneys, two lungs, two hemispheres of the brain (dealing with different functions which has no evolutionary logic) - why don't we have two thyroid glands, to remain with something when one of them goes into the dimension X. Oh, no, this mutation is too positive to become evolutionary. Actually there are 'positive mutations' like fish with two heads, but the biologists consider them as deviations (freaks in plain English) and get rid of them ... in one way or another.
What about the teeth - how many rows of teeth by birth would be a 'positive mutation'? The Dinosaurs have had 6 rows, and we have only 2 by birth ... which is considered evolutionary survival of the fittest ... and how exactly it is evolutionary and how exactly it shapes the fittest? Why is this competitive advantage not preserved with the other species - in which case they be able to have healthy teeth 99% throughout their life - if the evolution is preserving, as it claims, only the good features and gets rid of the bad ones.