Ontogeny, Phylogeny, and Recapitulation

Ontogeny, Phylogeny, and Recapitulation
Ontogeny is the history of the development of an organism through its entire life. Early developmental and embryological features contribute greatly to our knowledge of homology and common descent. Comparative studies of ontogeny show how the evolutionary alteration of developmental timing generates new characteristics, thereby producing evolutionary divergence among lineages.

The German zoologist Ernst Haeckel, a contemporary of Darwin, believed that each successive stage in the development of an individual represented one of the adult forms that appeared in its evolutionary history. The human embryo with gill depressions in the neck was believed, for example, to resemble the adult appearance of a fishlike ancestor. On this basis Haeckel gave his generalization: ontogeny (individual development) recapitulates (repeats) phylogeny (evolutionary descent). This notion later became known simply as recapitulation or the biogenetic law. Haeckel based his biogenetic law on the flawed premise that evolutionary change occurs by successively adding new features onto the end of an unaltered ancestral ontogeny while condensing the ancestral ontogeny into earlier developmental stages. This notion was based on Lamarck’s concept of the inheritance of acquired characteristics.

The nineteenth-century embryologist, K. E. von Baer, gave a more satisfactory explanation of the relationship between ontogeny and phylogeny. He argued that early developmental features were simply more widely shared among different animal groups than later ones. Figure 6-16 shows, for example, the early embryological similarities of organisms whose adult forms are very different (see Figure 8-19) The adults of animals with relatively short and simple ontogenies often resemble pre-adult stages of other animals whose ontogeny is more elaborate, but embryos of descendants do not necessarily resemble the adults of their ancestors. Even early development undergoes evolutionary divergence among lineages, however, and it is not quite as stable as von Baer believed.

We now know that there are many parallels between ontogeny and phylogeny, but features of an ancestral ontogeny can be shifted either to earlier or later stages in descendant ontogenies. Evolutionary change in timing of development is called heterochrony, a term initially used by Haeckel to denote exceptions to recapitulation. If a descendant’s ontogeny extends beyond its ancestral one, new characteristics can be added late in development, beyond the point at which development would have terminated in the evolutionary ancestor. Features observed in the ancestor often are moved to earlier stages of development in this process, and ontogeny therefore does recapitulate phylogeny to some degree. Ontogeny also can be shortened during evolution, however. Terminal stages of the ancestor’s ontogeny may be deleted, causing adults of descendants to resemble pre-adult stages of their ancestors (Figure 6-17). This outcome reverses the parallel between ontogeny and phylogeny (reverse recapitulation) producing paedomorphosis (the retention of ancestral juvenile characters by descendant adults). Because lengthening or shortening of ontogeny can change different parts of the body independently, we often see a mosaic of different kinds of developmental evolutionary change in a single lineage. Therefore, cases in which an entire ontogeny recapitulates phylogeny are rare.