A new model of developmental constraints as applied to the Drosophila system.

Von Baer's laws of development observe that an embryo, in the course of its ontogeny, progresses through a series of forms which diverge increasingly from the embryonic forms of related species, and in an evolutionary interpretation, from those of its phylogenetic ancestors. This observation on the relation of phylogeny to ontogeny is explained by Wimsatt's (1986) "Developmental Lock" model of complex generative systems, which proposes that evolution is constrained to alter developmental programs in a manner that usually modifies or adds new complexity to pre-existent developmental functions at positions relatively "downstream" in the causal structure. If the Developmental Lock model is correct, (1) evolution should have resulted in hierarchically ordered developmental programs, and (2) the most important developmental functions in the hierarchy should be ancient. Wimsatt also suggests that developmental functions be analyzed according to a degree property called "generative entrenchment", which replaces the temporal analysis in the traditional formulation of von Baer's laws. Herein, a substantial body of data on Drosophila ontogeny is analyzed according to generative entrenchment, in order to try the effectiveness of this form of analysis, and also to empirically test these two main predictions of the Developmental Lock model. The novel analytic approach proves to be fruitful, both in generating experimental hypotheses and in ordering existing data. Moreover, data concerning the developmental functions discussed here indicate that the order of the Drosophila developmental program conforms to the predictions of Wimsatt's model with few deviations. Explanations of the anomalies are offered, along with proposals for experiments to test some of those explanations.