Can Natural Selection explain cell differentiation?

Darwin used the phrase "Descent with modification" to summarize his theory of evolution. Despite many particulars that more recent science has amended to Darwin's theory, the basic idea of descent with modification remains. The descendant modifications are taken to be driven by varying degrees of "fitness" to environments. Even according to today's NeoDarwinian (or Modern Synthesis) model, the modifications observed in descendants are taken to be the result primarily of adaptations to environmental contingencies, operating under the mechanism of variation + selection, which runs without the benefit of any plan or program that might provide direction. And this "blind" process has produced all the phenotypes that were or are or will be. (An emerging Extended Synthesis model relies less on exogenous factors to explain phenotypes. Its influence on the discipline of evolutionary biology is yet to be known.)

However, another case of biological descent with modification apparently does benefit, or is assumed to benefit, from a guiding plan, or program. That is the descent of various tissue types from an undifferentiated zygote during ontogeny. This poses a paradox.

If natural selection is so powerful a causal agent that it can generate all the phenotypes that make up an ecosystem, then why is it necessary to suppose that there occurs in a zygote some sort of genetic plan or program that guides development of the organism? Why not just chalk it up to natural selection -- competition and cooperation among the cells in the organism? What evidence is there of a developmental program?

All the cell types that make up the body of a complex organism share the same genotype but differ as to which genes are active and which not. And that info must be heritable, hence a source of variation ("copying errors"). But any variation among cells in an embryo might provide an advantage to some cells and/or disadvantage to others. So, the stage is set for natural selection.

The tissues that make up a complex body and their symbiotic interdependencies are just the happenstance of competition among the cells -- is that a defensible proposition? The fit survive and go on to take their place in the somatic ecosystem of the body. The unfit go extinct. A clear case of unguided evolution. No need for a developmental program.

I am NOT proposing that this is what happens. I am only asking the question: What OBSERVATION could disprove this argument -- that the cells descend with modification from their common ancestor, a zygote, through a process of variation + selection?

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