Blueprints and recipes

Greg Mayer has a post on preformationism and epigenesis on the Why Evolution Is True blog:“Development is epigenetic”.

He later quotes Richard Dawkins in a similar light, but I’m linking because of Mayer’s own useful synopsis of the blueprint analogy versus the recipe analogy for development.

Preformationism, though wrong, is frequently reinforced by the common (though badly mistaken) practice of referring to DNA or the genome as a blueprint for the organism. It is of course no such thing. A blueprint is a two dimensional representation of a three dimensional object. There is, in a blueprint, a scaled representation of all the parts of the object. We can tell, for example, that the window on the second floor is 4 m above and 2m to the left of the door. There is nothing like that in your DNA: there isnt a gene for your left eye, which is a scaled distance away from the gene for your right eye. Your DNA (and your development) is much more akin to a recipe. In a raisin cake recipe, there isnt a line in the recipe that says place a raisin 2 cm in from the upper left hand corner (there would be, if we had a blueprint for the cake). Rather, if you combine the right ingredients, in the right sequence, in the right environment, the result is a cake with raisins distributed through it at a certain density.

In the end both these analogies entail some mechanism. A blueprint needs some past mechanism capable of producing an iconic representation of the final object. A recipe needs some mechanism capable of recording a sequence of steps. Neither of those is impossible to evolve (Mayer briefly mentions the iconic nature of the arrangement of Hox genes), but it’s pretty clear that the blueprint analogy does not apply to most developmental processes.

I was thinking about this issue in light of the nativist and learning theoretic views of language development. In that problem, the question is about the locus of the recipe – did evolution lay down special instructions for language learning, or does the language environment contain most of the structure necessary for children to learn without special instructions beyond those used for learning many other kinds of behavior? Chomsky argued that language environments cannot in principle supply the necessary structure, so biology must have done so (“Language and spandrels”). But he was essentially preformationist in this position, even to the extent of denying that language could have evolved. He instead preferred to see language as a side-effect of other evolutionary processes, or emerging as a physical principle from humanlike brains.

Anyway, I’ll return to this later, I just wanted to register a note on preformationism and epigenesis in relation to the issue.