Ontogeny and phylogeny
Learning about phylogeny from ontogeny
This idea is an extreme one. If it were strictly true, it would predict, for example, that in the course of a chick's development, it would go through the following stages: a single celled organism, a multi-celled invertebrate ancestor, a fish, a lizard-like reptile, an ancestral bird, and then finally, a baby chick.
This is clearly not the case a fact recognized by many scientists even when the idea of ontogeny recapitulating phylogeny was introduced. If you observe a chick's development, you will find that the chick embryo may resemble the embryos of reptiles and fish at points in its development, but it doesn't recapitulate the forms of its adult ancestors.
Even on a smaller scale, ORP is often untrue. For example, the axolotl evolved from a salamander ancestor that had internal gills in the adult stage. However, the axolotl never develops through a stage with internal gills; its gills remain external in flagrant violation of ORP.
If ORP were completely true, it would certainly make constructing phylogenies a lot easier. We could study an organism's development and read its history directly. Unfortunately, phylogeneticists are out of luck here.
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Salamander image (Pseudotriton ruber ruber) © 2002 John White; Axolotl (Ambystoma mexicanum) image © 2003 Jessica Miller.
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