Evolution Class 12 Notes

What Is Evolution?

Evolution is the study of the different forms of life, its characteristics and the changes over several generations. It is the scientific theory used by biologists to study the genetic variation in a population.

Note: As per the revised CBSE curriculum, this chapter has been removed from the syllabus for the 2020-21 academic session.

There are many theories about how life originated on the planet earth. The theory of evolution is based on the idea that all species are correlated and progressively varies with time.

See Also: Important Questions for Class 12 Chapter 7: Evolution

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Darwin’s Theory of Evolution

Charles Robert Darwin, an English naturalist, geologist, and biologist, is well known for his theory of evolution and the process of natural selection. He gave the theory of evolution, which is known as ‘Darwin’s theory of natural selection’ and also published a book, on “Origin of Species by Natural Selection”.

According to Darwin’s Theory, the main features of this theory are as follows,

  • All organisms reproduce and multiply enormously.
  • No two individuals are alike. They are different from each other either in their size, shape, behavior, etc.
  • Some traits are consistently passed on from their parent to the offspring.
  • The rate of reproduction varies in all living species. Some reproduce more and some minimum.

A Brief Account of Evolution

According to the several theories of Evolution:

  • The first cellular forms of life appeared on earth was about 2000 million years ago.
  • Later single-celled organisms, multicellular forms, and invertebrates were formed and became active.
  • Jawless fish evolved and different organisms started to invade from water to land. Later, amphibians, reptiles some of them to viviparous mammals came into existence.
  • About 23.03 to 5.333 million years ago, evolved primates that resembled to toady’s gorillas and chimpanzees. During ice age between 75,000-10,000 years ago, modern Homo sapiens arose.

Evidence For Evolution

  • It indicates about the paleontological evidence which denotes that life forms have arisen at various instances in the earth’s history which covers fossils, rocks from sediments etc.
  • Ernst Heckel proposed embryological support for evidence which was based upon the observation of some traits during the embryonic stages that persisted in all vertebrates that are not found in adults
  • The proposal was disapproved by Karl Ernst von Baer as he notes that embryos do not go through the adult phases of other animals
  • Divergent evolution is used to refer to the animals having the same structure developed along different directions as a result of adaptations to different needs. The structures are referred to as homologous, where homology represents common ancestry. Example – The tendrils and thorns of Cucurbita and Bougainvillea indicate homology
  • Convergent evolution refers to different structures evolving for the same function thereby having similarity. This is why analogous structures are as a result of convergent evolution. Examples of analogy – Flippers of Dolphins and Penguins
  • It was indicated that entities in a mixed population could adapt better, survive and cause an increase in the size of the population. But, none of the variants got wiped out completely
  • Examples of evolution by anthropogenic action is the appearance of resistant cells/entities in a time span of months or years as  opposed to centuries
  • Through these evidences it was also indicated that evolution is not a directed process in the context of determinism rather is a stochastic process basis the chance events in nature and chance mutations in entities

Adaptive Radiation

  • It is referred to the process of evolution of various species in particular geographical regions right from the point and radiating literally to other geographical areas. example – Australian marsupials, Darwin’s finches
  • Entities are better adapted to survive in an otherwise hostile environment as the adaptive ability is inherited, it being a genetic basis whereas fitness is the outcome of the ability to adapt and hence getting selected by nature
  • Two  key concepts of the Darwinian theory of evolution are branching descent and natural selection

Hardy-Weinberg principle

  • This principle states that allele frequencies in a population are stable and is constant from generation to generation, the gene pool remains constant. This is referred to as genetic equilibrium where the sum total of all allelic frequencies is 1.
  • In a diploid, let p and q represent frequency of allele A and a. The probability that an allele A with a frequency of p appears on both the chromosomes of a diploid individual is simply the product of the probabilities.
  • Five factors affecting Hardy-Weinberg principle are – gene flow, genetic drift, genetic recombination, mutation and natural selection
  • When the original drifted population becomes founders, the effect is known as founder effect

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Few Important Questions

  • Write a brief account of Evolution?
  • List out the 5 mechanisms of evolution?
  • What is Charles Darwin’s theory of evolution?
  • What Is Adaptive Radiation? Explain with examples.

Learn more about evolution and its theories from the topics given below:

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