Wow you dont even understand evolution. The turtles need for nutrition didnt kick off an intirnsic system to build more elasticated cells in the neck. Those that had the longer cells had an evolutionary advantage over those that didnt. Evolution selected those with naturally occuring longer necks to survive, breed and pass on those genes to their offsrping. Its that simple.
Once again you are completely clueless as to what evolutionary theory states. Those characteristics that enhance an animals survival in his environment are passed on to his offspring.
And you cant even begin to fathom the amount of time it takes for speciation to really occur from say ancestral land dwelling mammal to its current whale and dolphin descendants. Think MILLIONS of years. At every step along the way micro evolution is occurring.