Key Applications of Dollo’s Law in Modern Biology
Dollos law is also known as the dollos principle. This law was proposed in the year 1893. It is also known as Dollo’s law of irreversibility. This law states that an organism never gets back to its original state. Even if the conditions are favorable to its previous conditions, still it does not return back to its original state. There are some of the steps in between through which the organism has passed in its lifetime and thus is never able to return back to its original state. Dollos law was stated by Louis Dollo. He was a Belgian paleontologist. Thus, the organism goes through an irreversible type of evolution. It should not be interpreted that evolution is not reversible in nature. This Dollos law is applied to the field of morphology and especially for the study of fossils. This law can also be used to describe the molecular events that must have taken place during the course of the evolution of that organism. This can thus be used to study the individual mutations and also to understand gene losses. This is a definition of the Dollo’s law of irreversibility. Furthermore, we will understand the different fields in which this law finds its uses.
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Devolution
Devolution, as the name implies, means backward evolution. Dollo’s law of irreversibility states that the organism cannot come back to its original form because there are some irreversible changes or steps that have taken place in the course of evolution. Devolution states that the organisms can get back to their original forms over the course of time if it is necessary according to the needs of the organism. This type of evolution is known as devolution. Evolution is due to the fact that the organisms start adapting to their environment and then some changes are brought into their bodies that can be natural in nature or induced changes such as by the help of mutation. But in the field of evolutionary biology, no such types of changes are feasible. Dollo’s law denies the possibility of devolution. Orthogenesis is the basis on which the idea of devolution is based. When we get a closer look at the theories of evolution then we can see that evolution brings about changes that are complex in nature and they are to ease the adaptability of the organism towards the environment.
Dollo's Law in Phylogenetics
In brief, we can say that according to Dollo’s law, the organism acquires certain characteristics during the course of evolution and these factors are irreversible in nature. So the characteristics that are lost during the course of evolution can be regained in the lifetime. For example, in vertebrates, the loss of teeth can be well modeled under this law. The teeth though evolved multiple times in the course of the evolution of animals and were also lost in some organisms like that in birds, turtles, and seahorses. So this Dollo’s law can also be applied to molecular genetics that the individual organism inactivates the genes by itself. This can be seen by the loss of gluconolactone oxidase. This enzyme is the final enzyme required in the final step of the synthesis of vitamin C. This enzyme got lost and thus cannot be traced back and so we need to take vitamin C in our dietary supplements.
Molecular Example
There was a study proposed in 2009 for the evolution of protein structure and this proposed a new mechanism for Dollo’s law. There was a hormone receptor that was examined and it evolved from an ancestral protein. This protein had the ability to bind the two hormones to a new protein and this new protein was specific in nature. Amino acid substitutions helped in bringing out these changes. Also, these changes helped in preventing the binding of the second hormone. Also apart from these some changes also took place that was neutral in nature and did not affect the hormone binding. When artificially these changes were tried to bring back, they seemed to have destabilized the ancestral structure of the proteins. So, in the end, it was concluded that if these changes had to be brought back then the proteins need to evolve in the reverse direction and they must regain their ability to get bound to the two hormones and along with these several other neutral mutations need to take place.
FAQs on Understanding Dollo’s Law in Evolution
1. What is Dollo's Law of Irreversibility in evolution?
Dollo's Law, also known as the Law of Irreversibility, is an evolutionary principle stating that an organism is unable to return, even partially, to a previous ancestral stage. Once a complex structure or organ has been lost during evolution, it cannot be re-evolved in the same form by that line of descendants.
2. Can you provide an example of Dollo's Law from the fossil record?
A classic example is the evolution of the horse's foot. Ancestral horses had multiple toes, which gradually evolved into the single hoof of the modern horse, Equus. According to Dollo's Law, it is highly improbable that the descendants of the modern horse will re-evolve the multi-toed feet of their ancestors, as the genetic and developmental pathways for those extra digits have been lost over millions of years.
3. Why is it considered so unlikely for a complex trait to re-evolve according to Dollo's Law?
The improbability stems from genetics. When a complex trait is no longer needed, the genes and developmental pathways that control it are no longer under positive selective pressure. These genes start accumulating random mutations and can become non-functional (pseudogenes) or be repurposed for other functions. The probability of reversing these numerous, specific mutations to perfectly restore the original genetic blueprint is statistically close to zero.
4. Are there any known exceptions or challenges to Dollo's Law?
Yes, while Dollo's Law is a strong general principle, some apparent exceptions have been observed, which challenge its absolute interpretation. These include:
The re-evolution of wings in some lineages of stick insects that descended from wingless ancestors.
The regaining of lower teeth in some species of frogs.
The re-appearance of larval stages in certain salamander species.
These cases often involve the reactivation of dormant genetic pathways rather than a complete re-evolution from scratch.
5. How does Dollo's Law of Irreversibility differ from Cope's Rule?
These two principles describe different evolutionary trends. Dollo's Law focuses on the irreversibility of evolution, stating that complex traits, once lost, do not reappear. In contrast, Cope's Rule describes a different trend: the general tendency for species in many evolutionary lineages to increase in body size over geological time. One rule is about the directionality of trait loss, while the other is about a trend in size.
6. Does the occurrence of atavism, like a human born with a tail, violate Dollo's Law?
No, atavism does not violate Dollo's Law. Atavism is the rare reappearance of an ancestral trait resulting from the chance re-expression of dormant genes that have not yet been completely lost from the genome. Dollo's Law, however, applies to the complete re-evolution of a complex structure after its underlying genetic architecture has been significantly degraded over a long evolutionary time. Atavism shows that a genetic blueprint is still partially present, whereas Dollo's Law implies this blueprint is eventually erased, making re-evolution impossible.
7. What is the significance of Dollo's Law in understanding evolutionary pathways?
The primary significance of Dollo's Law is that it establishes that evolution is a historically constrained, non-repeatable process. It suggests a 'one-way street' for major evolutionary changes. This is crucial for biologists when reconstructing phylogenetic trees and understanding ancestral relationships, as it implies that once a lineage commits to a certain path (like snakes losing their limbs), it cannot simply reverse its course. It underscores the unique and contingent nature of each species' evolutionary history.
