Recapitulation theory

Recapitulation theory is the rejected claim that an organism’s development repeats the evolutionary history of its species. Its famous shorthand, “ontogeny recapitulates phylogeny,” overstates the relationship between embryonic development and evolution.

Ernst Haeckel: Ernst Haeckel was a German biologist who popularized evolutionary ideas and proposed a strong form of recapitulation. He gave the theory its best-known formulation and used embryo drawings to support it.
Heterochrony: Heterochrony is evolutionary change in the timing or rate of developmental events. Changes in developmental timing undermine any fixed rule that embryos repeat ancestral stages.
Von Baer's laws of embryology: Von Baer’s laws describe embryos developing from shared general features toward increasingly specialized forms. They replace a ladder of ancestral adult stages with branching, diverging development.
Haeckel's embryos: Haeckel’s embryos are comparative illustrations of vertebrate embryos published to support his evolutionary arguments. The drawings made recapitulation persuasive, but their distortions became central to controversy.
Ontogeny and phylogeny: Ontogeny is an organism’s development; phylogeny is the evolutionary history of a species or lineage. The theory’s slogan equated these two distinct histories.
Paedomorphosis: Paedomorphosis is the retention of juvenile characteristics in an adult descendant relative to its ancestor. It shows evolution can preserve developmental features rather than move through an ancestral adult form.
Evolutionary developmental biology: Evolutionary developmental biology studies how changes in development generate evolutionary diversity. It connects development and evolution through mechanisms rather than literal recapitulation.
Embryo illustration controversy: The embryo illustration controversy concerns inaccuracies in Haeckel’s comparative drawings and their historical interpretation. It reveals how visual evidence helped popularize a claim stronger than the underlying observations warranted.
Biogenetic law: The biogenetic law is Haeckel’s claim that ontogeny recapitulates phylogeny. This is the formal name commonly given to recapitulation’s central claim.
Peramorphosis: Peramorphosis is an evolutionary pattern in which development extends beyond the ancestral adult condition. It illustrates how altered development can produce forms that do not retrace ancestral stages.

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