What are the different ways researchers have tried to relate ontogeny to phylogeny?
Some of the people involved: Meckel, Haeckel, von Baer, Gould.
The von Baer's law is a concept in biology introduced by Karl Ernst von Baer to explain the details of embryo development. He specifically aimed at rebutting the recapitulation theory introduced by Johann Friedrich Meckel in 1808. According to Meckel's theory, embryos pass through successive stages that represent the adult forms of less complex organisms in the course of development, and that ultimately reflects scala naturae (the great chain of being). von Baer believed that such linear development is impossible. He posited that instead of linear progression, embryos started from one, or a few, basic forms that are similar for different animals, and then developed in a branching pattern into increasingly different looking organisms. Defending his ideas, he was also opposed to the theory of common ancestry and descent with modification as proposed by Charles Darwin in 1859, and particularly the revised recapitulation theory ("ontogeny recapitulates phylogeny") of Ernst Haeckel, a supporter of Darwin's theory in Germany.
Gould's hope was to show that the relationship between ontogeny and phylogeny is fundamental to evolution, and at its heart is a simple premise—that variations in the timing and rate of development provide the raw material upon which natural selection can operate."
embryos pass through successive stages that represent the adult forms of less complex organisms in the course of development, and that ultimately reflects scala naturae (the great chain of being)
embryos started from one, or a few, basic forms that are similar for different animals, and then developed in a branching pattern into increasingly different looking organisms.
ontogeny recapitulates phylogeny
Ernst Haeckel's theory of recapitulation, had an evolutionary perspective. Evolutionary recapitulation differed from other forms of recapitulation as it integrates the theory of common ancestry for all organisms.
variations in the timing and rate of development provide the raw material upon which natural selection can operate
Ontogeny is the developmental history of an organism within its own lifetime, as distinct from phylogeny, which refers to the evolutionary history of a species. In practice, writers on evolution often speak of species as "developing" traits or characteristics. This can be misleading. While developmental (i.e., ontogenetic) processes can influence subsequent evolutionary (e.g., phylogenetic) processes (see evolutionary developmental biology), individual organisms develop (ontogeny), while species evolve (phylogeny).
Relationship to feather development:
In general, the polarities of developmental novelties in the model are congruent with von Baer’s rule—the hypothesis that stages that occur earlier in development are phylogenetically more broadly distributed and historically plesiomorphic (e.g., Gould, ’77). However, the model does not rely solely on relative timing of events in ontogeny to justify these polarities. The stages of the model are inferred from the hierarchical nature of the developmental mechanisms of the follicle rather than from an analysis of the ontogenetic progression of plumages grown within the follicles of birds. Thus, plumulaceous feathers (stage II) are not primitive to pennaceous feathers (stage IIIa and beyond) because the first plumage of extant birds is usually downy, but because the simplest differentiated follicle collar would have produced plumulaceous feathers.
One detail, however, of feather development appears to violate von Baer’s rule. During the development of the first feather papillae in the embryo (before day 12 in the chick, Gallus gallus), the barb ridge primordia appear as longitudinal condensations within the feather papillae before the follicle and collar are fully formed (Lucas and Stettenheim, ’72). However, this developmental event—the origin of the feather before the follicle and collar—is clearly derived because barb ridges would be unable to grow without the spatial organization provided by the collar.
The most important supporter of von Baer's law was Charles Darwin, who wrote in his Origin of Species:
[The] adult [animal] differs from its embryo, owing to variations supervening at a not early age, and being inherited at a corresponding age. This process, whilst it leaves the embryo almost unaltered, continually adds, in the course of successive generations, more and more difference to the adult. Thus the embryo comes to be left as a sort of picture, preserved by nature, of the ancient and less modified condition of each animal. This view may be true, and yet it may never be capable of full proof.
In terms of taxonomic hierarchy, characters in the embryo will be formed in the order, first from those of phylum, then class, order, family, genus, and finally species.
Von Baer's second law states that embryos develop from a uniform and noncomplex structure into an increasingly complicated and diverse organism. For example, a defining and general characteristic of vertebrates is the vertebral column. This feature appears early in the embryonic development of vertebrates. However, other features that are more specific to groups within vertebrates, such as fur on mammals or scales on reptiles, form in a later developmental stage. Von Baer argued that this evidence supporting epigenetic development rather than development from preformed structures. He concluded from the first two laws that development occurs through epigenesis, when the complex form of an animal arises gradually from unformed material during development.
It is important to realize that the feather stages up to developmental Stage IIIa are ALREADY present in the actinofibrils of the pterosaur. In other words, there is no need to evolve those stages (in the transition to basal Paraves) because they are already present in the pterosaur ancestor.