Notice nowhere in the outline you posted does it say that the system is not able to tolerate change. This solely is your own (incorrect) overly simplistic interpretation. Development is a complex phenomena. So what? Thanks for pointing this out to us, I would never have guessed it.
In the gradient model, a signaling molecule induces different fates at different threshold concentrations.
This sentence should clue you in to the flexibility that has thus far eluded you.
we examine the conserved pathway leading to cell death
P.S. here they are refering to evolution .
I have a simple idea, Gore. Why dont you write to the authors and see if they agree with your assertion that small changes would necessarily lead to disasterous results. I predict they will respond with an unequivocal NO, that they tinker with drosophila all the time with varied results. But dont take my word for it. Go ask them. And while you are at it, ask them if they believe in evolution.
It is more than complex, it is completely interrelated. You cannot shotgun your way with random mutations to create such a way of doing things. The authors call this whole developmental process a "program". As I think I said before, go around changing bits and bytes at random in any computer program, see if it works better.
In the gradient model, a signaling molecule induces different fates at different threshold concentrations.
Flexibility is always the result of design, not the result of deterministic forces. The program is making choices, natural forces do not make choices.