• The idea was formulated by Belgian Palaeontologist Louis Dollo in 1893 in which he stated that "Evolution is irreversible & irrevocable". A structure that has been altered in the course of evolution cannot return to its previous shape. Changes in evolution cannot be reversed since every species is a result of the environment in which it lives.
  • For instance, different genera, species and races that have evolved in the course of time can never return to their origins. Humans also have developed new traits of evolution when we examine them against their earlier counterparts in the Pleistocene period. Obviously humans are not able to return to their original form. The changes that affect the lower jaw and dentition, erect posture as well as brain size can't be reversed. Fish are missing their gills, which can't be restored. Re-evolution of specific characters that are similar to our ancestors may occur for anthropology-optional
  • If structural changes lead to particularization of functions, the process binds the species to a small and restricted space; there is no chance of returning to the basic organization that it originally formed. Since these evolving groups exhibit tendency to become more specialized and develop beneficial and progressive changes that result in irreversible evolutionary change.

Features: EDEN IAS

  • A structure that alters its shape in the course of change does not return to its previous shape.
  • Dollo's irreversibility law (also called Dollo's law as well as Dollo's principal) states that "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."
  • The theory was first proposed by a historian named Edgar Quinet.

The reasons for irreversibility: EDEN IAS

  • Doll's rule attempts to provide a rational explanation for evolution. The rule suggests that evolutionary changes can't be reversed since every species is a product of its the environment, and the same environment can't be replicated.
  • If structural changes cause particularization of functions, the process restricts the species to a small and limited environment. There is no chance to return to the basic organization from which it originated.
  • Because these evolving groups have the tendency to become more skilled and to acquire more advanced beneficial variations that lead to irreversibility of the process.


  • Genera, races, species are created over a period of time by mutations in genetic composition. These changes cannot be reversed to their original form.
  • In the Pleistocene humans adapted to changing conditions in the environment with modifications to the lower jaw and dentition, erect position as well as brain size. which are unchangeable.
  • Land vertebrates who undergo the process in secondary adaptations to living in water do not turn fish once more neither do they acquire traits that are characteristic of fish (for instance the gills).
  • If a premolar gets lost during evolution, it won't recur as a premolar during subsequent evolution.
  • The reason for this is that ichthyosaurs who adapting to the aqueous environment, retained their structural characteristics typical of reptiles.
  • Similar to whales, whales were mammals, each with its own physical characteristics.

Cope's Rule

  • A Palaeontologist from America, Edward de Cope gave this rule after studying fossils thoroughly. This rule says that living organisms tend to grow in size as they evolve.
  • Illustrations include the evolution of herbivore species such as horses and camels that have experienced an increase in size and thus have achieved the status of gigantism. Other mammals, like the crocodile and tortoise have also experienced same phenomenon. Dinosaurs provide a wonderful illustration of gigantism.
  • Exclusions: But this rule is not universally applicable to all species. There are exceptions and we are aware that the mammalian evolution lineages do not show a tendency to gigantism. Bats flying in the air can't afford to grow into massive because if this happens their flight will be greatly affected. Additionally, there are proofs to show that certain mammalian species have seen an enormous decrease in size compared to their ancestral counterparts from Pleistocene age.
  • During the Ice Ages in the past, the massive dimensions of the organisms assisted in preserving heat in extremely cold conditions to be able to live in these conditions. In the case of egg-laying mammals, this could be an attempt to boost the fecundity of their eggs (a greaternumber of offspring are created due to the being able to lay many eggs in a larger body). The theory isn't all-encompassing.


  • There are numerous instances throughout the evolution of time of mammals growing in size following the demise of dinosaurs; reptiles expanding in size following the death of the therapsids, amphibians growing in size following being introduced to the earth; all animals increasing in size following the Cambrian Explosion about 540 million years ago, and more.
  • The evolution of camel, horse and other herbivores shows the growth of their size and Ethics Integrity and Aptitudehave reached megalomania.
  • Other mammals, such as tortoises and crocodiles show the same behaviour
  • Dinosaurs are the most fascinating illustration of the growth in size.


  • There are several exceptions to this rule. The evolution of several mammalian lineages does not exhibit a any tendency toward gigantism.
  • Mole that are insectivorous: It is physically impossible for Mole to be large since that would be a problem when borrowing
  • Bats flying: The habit of flying limits the size of the body of the bat.

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