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Evolution- CBSE Notes for Class 12 Biology

Origin of Life

RNA World has been the prevailing theory for the origin of life since the 1980s. The emergence of a self-replicating catalytic molecule accounts for signature capabilities of living systems, but it doesn't explain how the protobiological molecule itself arose.

Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre-existing non-living organic molecules (e.g. RNA, protein, etc.) and that formation of life was preceded by chemical evolution, i.e., formation of diverse organic molecules from inorganic constituents.

The conditions on earth were – high temperature, volcanic storms, reducing atmosphere containing CH4, NH3, etc. In 1953, S.L. Miller, an American scientist created similar conditions in a laboratory scale. He created electric discharge in a closed flask containing CH4, H2, NH3 and water vapour at 8000C.

He observed formation of amino acids. In similar experiments others observed, formation of sugars, nitrogen bases, pigment and fats. Analysis of meteorite content also revealed similar compounds indicating that similar processes are occurring elsewhere in space. With this limited evidence, the first part of the conjectured story, i.e., chemical evolution was more or less accepted.

Class 12 Biology Notes

Evolution of Life Forms – A Theory

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.

What are the Evidences for Evolution?

Paleontological evidence denotes that life forms have arisen at various instances in the earth’s history which covers fossils, rocks from sediments etc.

Embryological support for evolution was also proposed by Ernst Heckel based upon the observation of certain features during embryonic stage common to all vertebrates that are absent in adult.

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

What is Adaptive Radiation?

In evolutionary biology, adaptive radiation is a 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, alters biotic interactions or opens new environmental niches.

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.

Biological Evolution

Evolution by natural selection, in a true sense would have started when cellular forms of life with differences in metabolic capability originated on earth.

Biological evolution encompasses three issues: (1) the fact of evolution; that is, that organisms are related by common descent with modification; (2) evolutionary history; that is, when lineages split from one another and the changes that occur in each lineage; and (3) the mechanisms or processes by which evolutionary change occurs.

The fact of evolution is the most fundamental issue and the one established with utmost certainty. During the nineteenth century, Charles Darwin (1809–1882) gathered much evidence in its support, but the evidence has accumulated continuously ever since, derived from all biological disciplines.

Mechanism of Evolution

Populations evolve, but individual organisms do not. A population is an interbreeding group of individuals of one species in a given geographic area at the same time. A population evolves because the population contains the collection of genes called the gene pool. As changes in the gene pool occur, a population evolves.

Hardy-Weinberg principle

In population genetics, the Hardy–Weinberg principle, also known as the Hardy–Weinberg equilibrium, model, theorem, or law, states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. These influences include genetic drift, mate choice, assortative mating, natural selection, sexual selection, mutation, gene flow, meiotic drive, genetic hitchhiking, population bottleneck, founder effect and inbreeding.

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.

A Brief Account of Evolution

About 2000 million years ago (mya) the first cellular forms of life appeared on earth. Some cellular forms had the ability to release O2 and slowly single cell organisms became multicellular organisms.

By the time of 500 mya, invertebrates were formed and active. Jawless fish probably evolved around 350 mya. Sea weeds and few plants existed probably around 320 mya.

First organisms that invaded land were plants. Fish with stout and strong fins could move on land and go back to water was about 350 mya.

In 1938, a lobe finned fish caught in South Africa happened to be a Coelacanth which evolved into first amphibians that lived on both land and water and these were the ancestors of modern day frogs and salamanders.

The amphibians evolved into reptiles which lay thick shelled eggs which do not dry up in sun. The modern day descendants of reptiles are the turtles, tortoises and crocodiles.

In the next 200 million years or so, reptiles of different shapes and sizes dominated on earth. Giant ferns (pteridophytes) were present but they all fell to form coal deposits slowly.

Some of the reptiles went back into water to evolve into fish like reptiles around 200 mya. The land reptiles were the dinosaurs and the biggest of them is Tyrannosaurus rex was about 20 feet in height and had huge fearsome dagger like teeth.

Origin and Evolution of Man

The stages of evolution of man are-

  1. Dryopithecus and Ramapithecus - About 15 mya, primates called Dryopithecus and Ramapithecus were existing. They were hairy and walked like gorillas and chimpanzees. Ramapithecus was more man-like while Dryopithecus was more ape-like. Few fossils of man-like bones have been discovered in Ethiopia and Tanzania. These revealed hominid features leading to the belief that about 3-4 mya, man-like primates walked in eastern Africa.
  2. Australopithecus- About two mya, Australopithecines probably lived in East African grasslands. They hunted with stone weapons but essentially ate fruit. Some of the bones among the bones discovered were different. They were intermediate between apes and man.
  3. Homo habilis- The brain capacities were between 650-800cc. They probably did not eat meat. They were the makers of stone tools.
  4. Homo erectus- About 1.5 mya, Homo erectus arose. Homo erectus had a large brain around 900cc. Homo erectus probably ate meat.
  5. Neanderthal man- The Neanderthal man with a brain size of 1400cc lived in near east and central Asia between 1, 00,000-40,000 years back. They used hides to protect their body and buried their dead.
  6. Homo sapiens or modern man- Arose in Africa and moved across continents. During ice age between 75,000-10,000 years ago modern Homo sapiens arose. Pre-historic cave art developed about 18,000 years ago. Agriculture came around 10,000 years back and human settlements started.

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