Proposed by Louis Dollo, a Belgian palaeontologist,
The rule says
“an organism never returns exactly to a former state, even if it finds itself placed in conditions of existence identical to those in which it has previously lived … it always keeps some trace of the intermediate stages through which it has passed
” Evolution is irreversible and irrevocable. A structure that changes it’s form in evolution doesn’t revert to it’s earlier form”.
This principle is classically applied to morphology, particularly of fossils, but may also be used to describe molecular events, such as individual mutations or gene losses.
It implies, once an animal lineage has passed through a number of different stages, a reversion, stage by stage to the original ancestral condition does not occur.
The statement is often misinterpreted as claiming that evolution is not reversible, or that lost structures and organs cannot reappear in the same form by any process of devolution.
According to Richard Dawkins, the law is “really just a statement about the statistical improbability of following exactly the same evolutionary trajectory twice (or, indeed, any particular trajectory), in either direction”.
Stephen Jay Gould suggested that irreversibility forecloses certain evolutionary pathways once broad forms have emerged:
- In dentition, once a tooth of particular series is lost, it does not recur again in the same series in the same form.
- The loss of gulonolactone oxidase, the final enzyme in the biosynthetic pathway of vitamin C, is responsible for the dietary requirement of vitamin C in humans, as well as many other animals.
- After extinction of flying reptiles, the combination of wings and reptile living did not come together.
- Similar structures or same adaptive patterns could occur second time in evolutionary record. Ex: a) After flying reptiles become extinct, wings are evolved independently in birds and bats.
- Cetaceans (whales,dolphins) returned to aquatic environment in which vertebrates first evolved, but they haven’t become fish, only developed similar structures (flippers) and shapes analogous to fish (fins).
- Reversed mutations occurred in some bacteria, but only single step backward and not the whole sequence justifying Dollo’s principle.
- Many taxa of gastropods have reduced shells, and some have lost coiling of their shell altogether.
- The wings of stick insects,
- the larval stages of salamanders,
- Lost toes in lizards,
- Lost lower teeth in frogs,
- Clavicles in non-avian theropod dinosaurs,
- Neck, pectoral region, and upper limb musculature in primates, including the lineage leading to humans.
Being a descriptive generalization, it could not be considered as a law of nature, but only a property of living organisms.