Ontogeny

Ontogeny describes the entire life history of an organism from fertilization to death. It includes not only embryonic and prenatal development but also postnatal growth and development.

That patterns of ontogeny can provide fascinating insights into evolutionary history has long been recognized. Possibly the most famous concept linking the two areas is the "biogenetic law" of German biologist Ernst Haeckel (1834-1919), which states that "ontogeny recapitulates phylogeny." Haeckel suggested that, in the course of its development, each organism recapitulates (repeats the stages of) its entire phylogenetic history by taking on the morphologies of all of its ancestors sequentially, from the most primitive ancestor to the most advanced. Haeckel favored a fairly stringent interpretation of the biogenetic law, and spent considerable time identifying the ancestors represented by different developmental stages. For example, he viewed the gastrula stage (an early embryonic stage during which the three embryonic tissue layers are formed) of vertebrate embryos as representing the morphology of their invertebrate ancestors. Later developmental stages were interpreted as representing "higher" ancestors. For example, all avian and mammalian embryos go through a developmental stage in which gill slits are highly prominent. Haeckel interpreted this stage as a recapitulation of the "fish stage."

Haeckel's biogenetic theory fell into disfavor in the early twentieth century, when increasing evidence on the ontogenetic patterns and phylogenetic histories of different species indicated that there was not, in fact, a direct correspondence between the two. However, Haeckel's ideas are important because they stimulated considerable interest in embryological studies and because they emphasized the importance of links between ontogeny and evolution, an area that is still being actively studied today.

Current ideas regarding ontogeny and phylogeny rely on the concepts of another nineteenth-century scientist, Prussian-Estonian embryologist Karl Ernst von Baer. Von Baer noted that earlier developmental stages are simpler, with complexity increasing over time. He also emphasized development as a process in which related species diverge over time. That is, all species resemble each other fairly closely during the earliest stages of development, and gradually diverge in form over the course of ontogeny. The fertilized egg, the earliest stage of ontogeny, represents the time when different species are most similar.

Von Baer also stated that the "general" characteristics of a species appear before the "specific" ones, so that traits that characterize more inclusive groups, such as the phylum to which an individual belongs, appear earlier than those that characterize more restricted groups, such as the genus or species. This is apparent in human development in multiple ways. For example, the development of the neural tube, a trait possessed by all chordates, is a fairly early ontogenetic event, whereas the development of such species-specific features as the characteristic human facial or limb morphology appear much later.

There was a resurgence of interest in ontogeny as it relates to evolution in the late twentieth century. Stephen Jay Gould, an American pale-ontologist and evolutionary biologist, brought considerable attention to the field with the publication of his 1977 book, Ontogeny and Phylogeny. Of particular interest to evolutionary biologists and those who study morphology is the idea that the patterns and processes of development can channel or constrain the way in which evolution occurs. Several evolutionary biologists have attempted to explain morphological evolution as the product not only of natural selection but also of developmental constraints.