Von Baer's Law

Early developmental stages of a characteristic tend to be more similar among related species than later stages. This means that most characteristics that differentiate taxa , and which are commonly used to distinguish among species, represent later modifications to a fundamentally similar developmental plan. Von Baer's Law states that structures that form early in development are more widely distributed among groups of organisms than structures that arise later in development.

This law of embryology is named after the nineteenth-century German biologist Karl Ernst von Baer, who first articulated it. Although von Baer was not an evolutionist, his concept of increasing differentiation between species during ontogeny , or embryonic development, fits well with an evolutionary view of embryology. His law comes from the first two of four generalizations he made in 1828:

1. General features common to a large group of animals appear earlier in the embryo than do specialized features.
2. The development of particular embryonic characters progresses from general to specialized during their ontogeny.
3. Each embryo of a given species, instead of passing through the adult stages of other animals, departs more and more from them as ontogeny progresses (in direct contrast to the invalid biogenetic law of Ernst Haeckel, discussed below).
4. Therefore, the early embryo of a higher animal is never like the adult of a lower animal, only similar to its early embryo.

Von Baer's Law dictates that a generalized state is present in the early embryo. This generalized state gives way to more specialized states as the embryo develop, as can be seen by direct observation of embryos. In other words, an early stage of a chick embryo might be recognizable as a member of the phylum Vertebrata, but not as any particular subtaxon. Later, as structural specialization continues, it is recognizable as a member of the class Aves, and finally as a member of a particular species, Gallus domesticus.

Charles Darwin recognized that von Baer's Law provides a connection between embryology and biological evolution, namely that primitive features tend to be generalized and derived features tend to be specialized. Darwin realized that evolutionary change could be inferred from changes in development.

An example of the connection between embryonic development and evolutionary history is the development of the vertebrate limb. Structures of the limb develop in a proximodistal sequence, or from nearest to most distant from the body: shoulder bones develop first, and bones of the digits develop last. Lungfish and coelacanths, distant living relatives of the ancestral tetrapod, have fins with well-developed "shoulders," but their appendages do not contain bones homologous to the digits of tetrapods (their similarity in various taxa derives from their common origin in a shared ancestor). Digits appear as specialized, derived features in early tetrapods. Limb morphology, or form, proceeds from generalized to specialized as we climb the phylogenetic , or evolutionary, tree, and limb development in the embryos of advanced tetrapods parallels the evolutionary history of these tetrapods.

An additional example of von Baer's Law is seen in the development of the notocord . All vertebrates develop a dorsal cartilaginous rod called the notochord and gill pouches in the pharyngeal, or throat, region. Both are early developmental traits and are common to many species. Later in development, however, these traits are lost and replaced by traits that are distinctive to different groups of vertebrates.

Thus, early embryos of different vertebrate species are remarkably similar. For example, the early embryos of fishes, salamanders, tortoises, chickens, pigs, cows, rabbits, and humans appear similar, yet each species follows its own embryonic developmental trajectory (sequence of events during development) to develop distinctive traits.

Mutations that alter early development are usually lethal because they can introduce drastic changes to subsequent development. Because of these developmental constraints, early developmental stages are less likely to change through evolutionary time. These early stages are likely to be similar among taxa and differences among them are more likely to arise later in ontogeny, or embryonic development.

Another nineteenth-century German biologist, Ernst Haeckel, proposed what he thought was a law of evolution. Although Haeckel's law is invalid, it is widely known and resulted in the familiar phrase "ontogeny recapitulates phylogeny." Haeckel's biogenic law thus claims that embryonic development repeats evolutionary history. Embryos, he was saying, repeat the adult stages of ancestral species in chronological order from primitive to most recent and changes are added only during final stages, called terminal addition.

In some cases, the ontogeny of a trait does go through stages much like the adult forms of ancestral species. However, although Haeckel's biogenic law describes a possible evolutionary pattern, it provides no causal explanation for the pattern. The biogenetic law eventually lost credibility with the rise of experimental embryology and Mendelian genetics . Embryological studies showed that many types of change in developmental timing (heterochrony) are possible and that different parts of the organism may differ in rates of development. Genetic studies have demonstrated that genes could effect changes at any stage of development so that terminal addition was not the only possibility.

see also Embryonic Development.

Andrew G. Gluesenkamp

Bibliography

Futuyma, Douglas J. Evolutionary Biology. Sunderland, MA: Sinauer Associates, Inc., 1986.

Gould, Stephan J. Ontogeny and Phylogeny. Cambridge, MA: Belknap Press, 1977.

Raff, Rudolf A. The Shape of Life. Chicago: University of Chicago Press, 1996.

Raff, Rudolf A., and Thomas C. Kaufman. Embryos, Genes, and Evolution: The Developmental-Genetic Basis of Evolutionary Change. New York: Macmillan, 1983.