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Ontogeny and phylogeny

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

Table of contents
1 Observed connections
2 Explanation
3 See Also
4 External links, references, and resources

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:

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:


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.

See Also

External links, references, and resources