Pay attention to the puzzle described in slides 14-21. Here’s a brief outline of the problem:
1. To establish cellular differentiation in a metazoan (i.e., an animal), instructions must be provided to the starting cell.
2. Natural selection is one possible process by which this occurred, when the metazoan in question first appeared.
3. A necessary condition for natural selection is reproductive capability.
4. But reproductive capability (in an animal) requires cellular differentiation.
5. Thus, a necessary condition for natural selection lies causally downstream from the phenotypic outcome — namely, cellular differentiation — natural selection was supposed to explain.
Those who have thought hard about this problem (e.g., Wolpert 1994, Schlichting 2003) have appealed to the Baldwin effect to generate the instructions at step (1).
The key to all development is the generation of differences between the cells, that is, making them non-equivalent (Lewis and Wolpert, 1976). Only if the cells are different can the organism be patterned so that there are organized changes in shape, and cells at specific sites differentiate into different cell types. How could this have evolved? …
Whatever the advantage, an environmental signal brings about a localized change in the organism which becomes elaborated with time. It could even lead to suppression of growth of adjacent cells and so the restriction of reproduction to cells at the opposite end of the embryo. An embryonic axis could be specified. Evolution of patterning has occurred….
Invoking again, the Baldwin effect, we can see how this process could be constitutive, that is the cells at the contact site could be genetically specified. This illustrates the advantages and economy for evolution of development that the Baldwin effect provides. In its absence it would be necessary to first genetically specify one group of cells as being different but without there being any selective advantage. Only then could there be changes in these cells similar to those described above. This sequence of events is unlikely in the extreme. The reason for invoking the Baldwin effect is that an environmental signal provides the initial basis for a developmental alteration, which could have a selective advantage.
(L. Wolpert, “The evolutionary origin of development: cycles, patterning, privilege and continuity,” in M. Akam et al., eds., The Evolution of Developmental Mechanisms [Cambridge, Company of Biologists Ltd., 1994], pp. 79-84; pp. 80-81, emphasis added.)
See also Carl D. Schlichting, “Origins of differentiation via phenotypic plasticity,” Evolution & Development 5 (2003):98-105.
Although he does not appeal to the Baldwin effect, Dan Brooks appears to agree that natural selection does not explain the origin of metazoan ontogenetic networks:
Nelson also claimed that natural selection is not capable of creating nematodes, which is true but irrelevant. In Darwinism, natural selection is not called upon to create, but to select. And yes, that does beg the question of where the entities acted upon by selection come from. Darwin suggested that the major mechanism was the then-unknown laws of inheritance, coupled with what he called the mysterious laws of growth, the mysterious law of the correlation of parts, and the cohesion of homologies. Today, those phenomena are less mysterious, but still fascinating and incompletely understood.
I look forward to further clarification from Dan.