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Ontogeny and phylogeny

In biology, ontogeny refers to the embryonal development process of a certain species, while phylogeny refers to a species' evolutionary history. Various connections between phylogeny and ontogeny are observed, explained by evolutionary theory and taken as supporting evidence for that theory.

Observed connections

It is generally observed that if a structure is evolutionary older than another, then it also appears earlier than the other in the embryo. Species which are evolutionary related typically share the early stages of embryonal development and differ in later stages. Examples include:

  • The backbone, the common structure among all vertebrates such as fish, reptiles and mammals, is one of the earliest structures laid out in all vertebrate embryos.
  • The cerebrum in humans, the most sophisticated part of the brain, is also the one to develop last.

If a structure was lost in an evolutionary sequence, then it is often observed that said structure is first created in the embryo, only to be discarded or modified in a later embryonal stage. Examples include:

  • Whales, which are thought to have evolved from land mammals, don't have legs, but tiny remnant leg bones buried deep in their body. During embryonal development, leg extremities first occur, then recede. Similarly, whale embryos (like all mammal embryos) have hair at one stage, but lose most of it later.
  • All vertebrates are thought to have evolved from fish, and all of them show gill pouches at one stage of their embryonal development.
  • The common ancestor of humans and monkeys had a tail, and human embryos also have a tail at one point; it later recedes to form the coccyx.

Explanation

Connections between phylogeny and ontogeny can be explained if one assumes that a species changes into another by a sequence of small modifications to its developmental program (which is specified by the genome). Modifications that affect early steps of this program will usually require modifications in all later steps and are therefore less likely to succeed. Most of the successful changes will thus affect the latest stages of the program and will retain earlier steps. Occasionally however, a modification of an earlier step in the program does succeed and that is why a strict correspondence between ontogeny and phylogeny, as expressed in Haeckel's discredited recapitulation law, is not observed.

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