Concept of Evolutionary Biology | Anthropology Optional for UPSC PDF Download

Doll’s Rule

• Dollo's Rule, proposed by Belgian paleontologist Louis Dollo in 1893, states that evolution is an irreversible and irrevocable process. This means that once a structure has evolved and changed over time, it cannot return to its previous form. This is because every species is shaped by its environment, and evolutionary changes cannot be undone.
• For example, various species, genera, and races that have developed over time cannot go back to their original form. Humans have also gained new evolutionary traits compared to their ancestors from the Pleistocene era, and it is impossible for us to return to our earlier state. Changes in aspects such as the lower jaw, dentition, erect posture, and brain size cannot be reversed. Similarly, fish have lost their gills over time, and these cannot reappear. However, the re-evolution of certain characteristics similar to our ancestors can still occur.
• When structural changes lead to functional specialization, a species becomes more adapted to a specific and narrow environment. This makes it impossible for the species to revert to its original, more primitive organization. As these evolving groups become more specialized, they acquire new, beneficial variations that further reinforce the irreversibility of evolution.

Features

• In the process of evolution, a structure that changes its form does not revert to its earlier form. Dollo's law of irreversibility (also known as Dollo's law and Dollo's principle) asserts that an organism never returns to its previous state, even if it encounters the same environmental conditions as before. 
• This is because the organism retains traces of the intermediate stages it has gone through during its evolution. This hypothesis was first proposed by historian Edgar Quinet.

Reasons for irreversibility

• Dollo's law aims to explain the path of evolution. It suggests that evolutionary changes cannot be reversed because each species is a product of its environment, and the same environment cannot be recreated.
• When structural changes lead to functional specialization, a species becomes confined to a specific and limited environment, making it impossible to revert to its primitive organization.
• Evolving groups tend to become more specialized, acquiring useful variations that result in the irreversibility of evolution.

Examples

• Races, species, and genera develop over time through changes in their genetic composition. These alterations cannot be reversed to their original form.
• During the Pleistocene epoch, humans adapted to changing environmental conditions with changes such as a smaller lower jaw, dentition, erect posture, and increased brain size, which cannot be reversed.
• Land vertebrates that have adapted to an aquatic environment do not revert to being fish, nor do they reacquire fish-specific characteristics, such as gills.
• If a premolar tooth is lost during evolution, it will not reappear as a premolar in future evolution.
• For instance, ichthyosaurs adapted to an aquatic environment but retained their reptilian structural features.
• Similarly, whales remained mammals with their own unique structural characteristics, even though they adapted to living in water.

Cope’s Rule

• Cope's Rule, proposed by American Paleontologist Edward de Cope, is a theory based on his extensive study of fossils. According to this rule, organisms tend to increase in size during the process of evolution.
• Examples of this phenomenon can be seen in the evolution of herbivorous species such as camels and horses, which have grown larger over time and thus exhibit gigantism. Other mammals, like crocodiles and tortoises, have also experienced this increase in size. Dinosaurs are perhaps the most well-known example of gigantism.
• However, it is important to note that Cope's Rule does not apply universally to all species. There are exceptions, as not all evolutionary lineages of mammals display a trend towards gigantism. For instance, flying bats cannot afford to become much larger, as this would greatly hinder their ability to fly. Additionally, there is evidence to suggest that certain mammalian species have actually decreased in size compared to their ancestral forms from the Pleistocene age.
• One possible explanation for the increase in size seen in some organisms during the ice ages is that their larger size helped them conserve heat in extremely cold environments, allowing them to survive in such harsh climates. In the case of egg-laying mammals, larger body size could be a means to increase fecundity, as a larger body would be able to accommodate more eggs and therefore produce more offspring. However, this theory is not universally applicable either.
• Cope's Rule suggests that organisms tend to increase in size during evolution. While this theory holds true in some cases, such as with dinosaurs and certain herbivorous species, it is not applicable to all species. Exceptions exist, and some mammals have even experienced a decrease in size throughout their evolutionary history.

Examples

• Throughout evolutionary history, there have been numerous instances of species increasing in size: mammals grew larger after the extinction of dinosaurs; reptiles expanded in size following the decline of therapsids; amphibians increased in size after adapting to land environments; and all animals experienced size growth after the Cambrian Explosion 540 million years ago, among others.
• The evolution of horses, camels, and other herbivores demonstrates this increase in size, leading to gigantism. Similarly, other mammals such as tortoises and crocodiles also exhibit this phenomenon.
• Dinosaurs provide one of the most fascinating examples of size increase in the history of evolution.

Exceptions

• However, there are several exceptions to this trend. Many mammalian lineages do not demonstrate a tendency towards gigantism throughout their evolution.
• Insectivorous mammals: Due to their burrowing habits, it would be physically challenging for moles to evolve into larger sizes.
• Flying bats: The ability to fly imposes a limitation on the body size of bats, preventing them from becoming too large.
• In some cases, mammalian lineages have experienced a decrease in size. For example, carnivores reached gigantic sizes during the Pliocene and Pleistocene epochs but have since decreased in size.
• A.J Arnold, who studied planktonic foraminifera, found that newer species tend to be smaller in size.
• Hooijer pointed out that many vertebrates have experienced a progressive size decrease during the Quaternary period, which is still ongoing today.
• While this theory attempts to provide a comprehensive explanation for organic evolution, it is not universally applicable across all species and situations.

Gause’s Rule

• This rule was given by a Russian biologist, “Gause”. According to this rule, competition may occur between populations within an ecosystem for any available resources such as food, space, light & shelter. Consequently, one arrives & other gets eliminated because species is also an ecological unit.
• The reason why this happens is because, if one or more species occupy the same trophic level in a food chain, they will try to eliminate one another for better access to food resources.  Consequently, they start occupying separate niche within the same trophic level, thus minimizing the extent of competition.
• If the competitors occupy the same trophic levels in a strongly overlapping niche & equilibrium situation may have been reached where neither succeeds, then one of the competitors will decline in number to the point of extinction. It is difficult to witness this phenomenon in the wild. Gause witnessed these phenomena in the lab conditions.
• Paramecium Aurelia & P. Caudatumexist at the same trophic level & compete with each other. When cultured separately, both prosper. When cultured together, caudatum is eliminated & Aurelia survives. This principle strongly advocates the theory of “the survival of the fittest” & hence has been universally accepted.

The competitive exclusion principle, sometimes referred to as Gause’s law, is a proposition named for Georgy Gause that two species competing for the same limited resource cannot coexist at constant population values.

• When one species has even the slightest advantage over another, the one with the advantage will dominate in the long term.
• This leads either to the extinction of the weaker competitor or to an evolutionary or behavioral shift toward a different ecological niche.
• The principle has been paraphrased in the maxim “complete competitors cannot coexist”.
• His rule says that competition may occur between populations within an ecosystem for any available resources such as food, space, light or shelter.
• Consequently, one survives and other gets eliminated because species is also an ecological unit.

Application to Humans

• Evidence supporting the competitive exclusion principle's role in human groups has been examined and incorporated into regality theory to help understand the dynamics of warlike and peaceful societies.
• This theory suggests that when hunter-gatherer groups coexist in the same ecological niche, they are more likely to engage in conflict, at least occasionally. In contrast, hunter-gatherer groups living alongside groups with different subsistence strategies can coexist more peacefully.

This principle is universally accepted. It tries to explain organic evolution through the principles of struggle for existence & survival of the fittest.

Hardy-Weinberg Equilibrium

• The Hardy-Weinberg Equilibrium is a concept in genetics and evolution, proposed by G. H. Hardy and W. Weinberg. It refers to a population of sexually reproducing organisms where genes combine randomly due to non-selective mating, known as a panmictic population. In other words, panmictic populations are those where mating partners are not chosen based on specific genetic traits. For instance, humans typically do not select partners based on blood type, so human populations are panmictic in this regard.
• The Hardy-Weinberg Principle asserts that in a panmictic population, if there is no influence from mutation, selection, genetic drift, and other factors, the relative frequency of any pair of genes remains constant across generations. For example, if a gene has two alleles (p and q) in a population and no mutations or other factors are occurring, the frequency of these alleles will stay the same from one generation to the next. This can be represented mathematically as: (p + q)² = 1, or p² + 2pq + q² = 1.

Conclusion

The discussed rules and principles provide insight into various aspects of evolution and genetics. Dollo's Rule asserts that evolution is irreversible, meaning species cannot return to their previous forms. Cope's Rule suggests that organisms tend to increase in size during evolution, but exceptions exist. Gause's Rule explains competition within an ecosystem and the survival of the fittest, stating that two species competing for the same resources cannot coexist. Lastly, the Hardy-Weinberg Equilibrium explores the concept of panmictic populations, where genes combine randomly due to non-selective mating, and predicts that allele frequencies will remain constant across generations in the absence of external factors. Overall, these principles help to understand the complex processes governing evolutionary change and the dynamics of species within ecosystems.

Frequently Asked Questions (FAQs) for Concept of Evolutionary Biology

Is Dollo's Rule universally applicable to all species?

Dollo's Rule, which states that evolution is irreversible and irrevocable, is generally applicable to most species. However, there can be instances where certain characteristics similar to ancestral forms may re-evolve, but the exact previous form is not likely to be restored.

Does Cope's Rule apply to all organisms?

While Cope's Rule suggests that organisms tend to increase in size during evolution, it does not apply universally to all species. There are exceptions, and some mammals have even experienced a decrease in size compared to their ancestral forms from the Pleistocene age.

What is the main idea behind Gause's Rule?

Gause's Rule, also known as the competitive exclusion principle, states that two species competing for the same limited resource cannot coexist at constant population values. One species will eventually dominate and either lead to the extinction of the weaker competitor or force it to shift towards a different ecological niche.

Can the Hardy-Weinberg Equilibrium be applied to all populations?

The Hardy-Weinberg Equilibrium can be applied to panmictic populations, where mating partners are not chosen based on specific genetic traits. It assumes that there is no influence from mutation, selection, genetic drift, and other factors. If these conditions are met, the relative frequency of any pair of genes remains constant across generations in such populations. However, in real-world populations, these ideal conditions are rarely met, making the Hardy-Weinberg Equilibrium more of a theoretical concept.