Principles of Evolution for Competitive Exams

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Image of Principles of Evolution

Image of Principles of Evolution

Principles of Evolution: Irreversibility by Louis Dollo and Adaptive Radiation (Anthropology)

Principles of Evolution

  • Speciation

  • Irreversibility

  • Parallelism and Convergence

  • Adaptation Radiation

  • Extinction


Image of Irreversibility

Image of Irreversibility

  • Louis Dollo, a French born Belgian paleontologist proposed the principle of irreversibility in 1890. Also called the Dollo’s Law of Irreversibility. Dollo’s Laws states that “An organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors.”

  • He was of the view that once an animal has passed through a number of stages, a reversion, stage by stage, to the original ancestral condition does not occur.

Image of Biston betularia

Image of Biston Betularia

Biston betularia

  • Irreversibility is a descriptive generalization and not a property of living organism.

  • Dollo’s Law has been often refuted by evidence that evolutionary specialization can be undone. For instance, reversible evolution has been observed on a relatively short evolutionary timescale in the peppered moth (Biston betularia).

  • In the 19th century a dark morph of the moth emerged in response to air pollution during the Industrial Revolution and became the dominant color morph, almost completely replacing the light- colored form. By the late 20th century, however, the light morph was on the rise again, its increase coincident with the decline of air pollution in England.

Image of incisor, canine, premolar or molar

Image of Incisor, Canine, Premolar or Molar

Image of incisor, canine, premolar or molar

Image of Incisor, Canine, Premolar or Molar

  • In studies of primate evolution, irreversibility is an important principle. The dentition of a given form is often crucial evidence of its ancestral or descendant status with respect to another form.

  • Once a tooth of a particular series (incisor, canine, premolar or molar) is lost, it does not recur again in the same series in the same form. Change in dentition is irreversible.

Parallelism and Convergence

  • Parallelism is usually restricted to the development of similar adaptive features in animals that are related, such as animals belonging to the same order.

  • The parallel resemblances are, most likely, the realization of a genetic potential that is present in the entire group.

  • Convergence is defined as development of similarities in adaptive relationships or structures in two unrelated animal species or major groups that are not closely related.

  • Parallelism and Convergence imply that close phylogenetic relationship does not exist.,

Image of Old world and New World Monkeys

Image of Old World and New World Monkeys

Old world and New World Monkeys

  • It is difficult to classify all cases of similar evolution as convergent or parallel.

  • The old-world monkeys and new world monkeys provide excellent example of parallelism between groups living today, since they appear to have evolved in parallel from a prosimian ancestor that probably lived at least 35 million years ago.

  • An example of convergent evolution is the incisor tooth comb of flying lemur and tooth comb of various prosimian primates (formed by lower incisors and canines).

Adaptive Radiation

  • Adaptive Radiation is the process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, creates new challenges, or opens new environmental niches. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different morphological and physiological traits.

  • The prototypical example of adaptive radiation is finch speciation on the Galapagos (“Darwin’s finches”), but examples are known from around the world.

  • According to Simpson (1953), adaptive radiation is the rapid proliferation of new species from ancestral group.

Image of Adaptive Radiation

Image of Adaptive Radiation

Four Features Used to Identify an Adaptive Radiation

  • A Common ancestry of component species: specifically, a recent ancestry. Note that this is not the same as a monophyly in which all descendants of a common ancestor are included.

  • A phenotype- environment correlation: a significant association between environments and the morphological and physiological traits used to exploit those environments.

  • Trait utility: the performance or fitness advantage of trait values in their corresponding environments.

  • Rapid Speciation: presence of one or more bursts in the emergence of new species around the time that ecological and phenotypic divergence is underway.


Image of Extinction

Image of Extinction

  • Extinction is termination of an organism or of a group of organisms (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point.

  • Because a species’ potential range may be very large, determining this moment is difficult, and is usually done retrospectively. This difficulty leads to phenomena such as Lazarus taxa, where a species presumed extinct abruptly “reappears” (typically in the fossil record) after a period of apparent absence.

Causes of Extinction

  • Genetics and demographic phenomena

  • Genetic pollution

  • Habitat degradation

  • Predation, Competition and disease.

  • Co-extinction

  • Climate Change

# Principles of Evolution

#Irreversibility- Dollo’s Law

# Parallelism & Convergence

#Adaptive Radiation

#Four features used to identify an adaptive radiation


#Causes of Extinction

#Physical Anthropology


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