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Evolution of life can be defined as the gradual changes in the genetic material and appearance of living organisms.


Evolutionary biology is the study of history of life forms on earth.

Evolution of life can be defined as the gradual changes in the genetic material and appearance of living organisms.


Origin of life - Origin of the universe

  • The universe is very old almost 20 billion years old comprise of huge clusters of galaxies.

  • Galaxies contain stars and the clouds of gases and dust.

  • The origin of universe is explained by the Big Bang Theory. According to this theory, a huge explosion occurred, the universe expanded, temperature came down and hydrogen and helium were formed later. The galaxies were then formed due to condensation of gases under gravitation.


Origin of earth

  • In the solar system of the Milky Way galaxy, earth was supposed to have been formed about 4.5 billion years back.

  • Initially, the earth surface was covered with water vapour, methane (CH4), carbon dioxide (CO2) and ammonia (NH3).

  • The UV rays of the sun broke water into hydrogen and oxygen.

  • Hydrogen escaped and oxygen combined with NH3 and CH4 to form water, CO2 and other gases, also forming the ozone layer.

  • Cooling of water vapour led to rain which filled the depressions on earth's surface, forming water bodies like oceans.

Evo Introduction

Theories of origin of life

  • Life appeared of 500 million years after the formation of the earth.

  • Different theories were given to explain the origin of life.

  1. Theory of special creation- According to this theory, God created life by his divine act of creation.

  2. Theory of panspermia/cosmozoic theory- According to early Greek thinkers, units of life called spores or panspermia came from outer space and developed into living forms. it is still favourite idea for some astronomers.

  3. Theory of spontaneous generation- According to this theory, life originated from decaying and rotting matter like straw and mud etc... Louis Pasture disproved the theory of spontaneous generation and demonstrated that life came from pre-existing life. He plays the killed yeast in pre-sterilized flask and another flash open to air. He showed that life did not originate in the former but new living organisms arose in the latter flask. 

  4. The theory of chemical evolution or Oparin- Haldane theory- this theory was given by Oparin and Haldane and stated that life originate from pre-existing non-living organic molecules. i.e., RNA, protein etc... And this also states formation of diverse organic molecules from inorganic constituent. The condition on the earth favouring chemical evolution were high temperature, volcanic storms, reducing atmosphere containing CH4, NH3 etc...


Experimental evidence of chemical evolution or Stanley Miller's experiment

  • S L Miller and H C Urey performed an experiment in 1953 for chemical evolution.

  • He created electric discharge in a closed flask containing CH4, H2, NH3 and water vapours at 800 oC.

  • After few weeks he observed formation of amino acids and complex molecules like sugars, nitrogen bases, pigments and fats in the flask.

  • Conclusions-

  1. It provides the experimental evidence for the theory of chemical origin.

  2. It showed that the first non-cellular form of life was created about 3 billion years ago.

  3. It showed that non-cellular biomolecules exist in the form of DNA, RNA, polysaccharides and proteins. Formation of first cell first non-cellular.


Formation of First Cell

  • Life-forms originated 3 billion years ago.

  • These could be giant molecules like RNA, protein and polysaccharides, which might have reproduced themselves.

  • First cellular form of life originated about 2000 million years ago.

  • These might have been single-cells formed in aquatic environment.

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Evidences for evolution

Paleontological evidences:

  • The study of fossils is called paleontology.

  • Fossils are the remains or Impressions of dead bodies of pre-historic plants and animals preserved in sedimentary rocks over a long period of time.

  • Different-aged rock sediments contain fossils of different life-forms who probably died during the formation of the particular sediment.

  • Variety of fossils ranging from the modern organisms to extinct organisms can be observed.

  • A study of fossils in different sedimentary layers indicate the geological period in which they existed.

  • The study showed that the life-forms varied over time and certain life forms are restricted to certain geological time-span.


Embryological evidence:

  • It was proposed by Ernst Haeckel based upon the observation of certain features during embryonic stage common to all vertebrates that are absent in adult. For example, the embryos of all vertebrates including human develop a narrow of vestigial gill slit just behind the head, but it is a functional organ only in fishes and it not found in any other adult vertebrates.

  • But it was disproved by Karl Ernst von Baer. He noted that embryos never pass through the adult stages for the other animals.


Comparative anatomy and morphological evidences:

  • The phylogenetic history can be revealed by comparative study of external and internal structures.


Homologous Organs:

  • The organs which are similar in their anatomical structure and origin [have common ancestry] but performs different functions are called homologous organs.

  • For example, the forelimbs of some animals like Whales, bats Cheetah and humans have similar anatomical structure i.e., humerus, radius, ulna, carpals, metacarpals and phalanges.

  • Some structures developed along different directions due to adaptations to different needs. This is called divergent evolution.

  • Other examples include vertebrate hearts or brains. In plants thorns and tendrils of bougainvillea and Cucurbita.


Analogous Organs:

  • The organs which are different in anatomical structure and have different origin but perform similar functions are called analogous organs.

  • Different structures evolving for the same function and hence having similarity. This is called convergent evolution. For example, Wings of butterflies and birds.

  • Other examples include eye of octopus and mammals’, flippers of penguins and dolphins, sweet potato (root modification) and potato (stem modification).


Biochemical evidence:

  • The similarities in proteins and genes performing a common given function among diverse organisms give clues to common ancestry.

  • The metabolic processes in in organisms are also similar with the same new materials and end products.


Biogeographical evidence:

species restricted two regions develop unique features. Also, species present in widely separated regions show similarity of ancestry.


Adaptive Radiation

            It is the evolutionary process in which different species starting from a common point in a geographical area radiate to other geographical areas. Examples:

1. Darwin's finches

  • Darwin observed many varieties of finches in the same island.

  • All varieties had evolved from original seed eating finches.

  • With alteration in beaks some became insectivorous and some vegetarian.


2. Marsupials of Australia

  • Within the Australian continent, many different marsupials or pouched animals are seen.

  • These have evolved from a common ancestral stock.


3. Placental animals in Australia

  • A variety of placental mammals have evolved which appear similar to a corresponding marsupial.

  • When more than one adaptive radiation appears to have occurred in an isolated geographical area (representing different habitats), and two or more groups of unrelated animals come to resemble each other for similar mode of life or habitat, it is called Convergent Evolution.

  • For example, Australian marsupials, placental mammals.

Evolution Theories

Theories of Evolution (Biological evolution)

1. Lamarck’s theory of evolution or Lamarckism:

  • According to Lamarck, evolution of life forms occurred due to use and disuse of organs.

  • Example, giraffes initially did not have long necks. But to access leaves on tall trees, they had to adapt by elongating their necks. By passing this acquired character to succeeding generation, they acquired long necks.

  • This theory is no more accepted.


2. Darwinian theory of evolution or Darwinism:

  • Charles Darwin, based on his observations during a sea voyage around the world in the ship H.M.S Beagle, concluded the following:

  • Varying degrees of similarities can be observed between existing life forms and those that existed millions of years ago.

  • There has been gradual evolution of lite forms with new forms arising at different periods of history.

  • Any population has built-in variations in characteristics which adapt it better to environment.

  • The characteristics which enable some populations or individuals to survive better in natural conditions (climate, food. physical factors) would out-breed others (Survival of the fittest).

  • Those populations which are better fit (reproductively fit) in an environment will be selected by nature and will survive more (Natural selection).

  • Adaptability is inherited and fitness is the end result of ability to adapt and get selected by nature.


Natural selection is based on following factual observations:

  • Limited natural resources.

  • Stable population size except seasonal fluctuation.

  • Varying characteristics of members of a population.

  • Most of the variations are inherited.

The two key concepts of Darwinian theory are branching descent (adaptive radiation) and natural selection.


Examples of natural selection-

Industrial melanism:

  • In England, before industrialisation, white-winged moths were more in number than dark- winged moths.

  • But after industrialisation, dark-winged moths became more in number than white-winged moths.

  • This is because during industrialisation, the tree trunks covered by white lichens became dark due to deposition of dust and coal particles.

  • As a result, white-winged moths could be easily picked up by the predators from the dark background and dark-winged moths survived.

Chemical resistance:

  • Excessive use of herbicides and pesticides has resulted in evolution of resistant varieties of microbes in much lesser time scale.

  • As a result, pathogenic bacteria are appearing in very short period.


3. Mutation theory of evolution:

  • This was put forth by Hugo de Vries based on his work on evening-primrose.

  • According to him, evolution is caused by sudden large differences in the population, i.e., mutation and not the minor variations as per Darwin.

  • He believed that mutation caused speciation and called it saltation or single step large mutation.

  • Mutations are random and directionless in contrast to small directional variations as per Darwin.

Hardy-Weinberg Principle

  • This principle states that allelic frequencies in a population are stable and remains constant from generation to generation, i.e., gene pool (total number of genes and their alleles in a population) is constant. This is called genetic equilibrium or Hardy-Weinberg equilibrium.

  • It can be expressed as p2 + 2pq + q2 = 1 where p and q are frequencies of different alleles.

  • Disturbances in genetic equilibrium results in evolution.


Factors Affecting Hardy-Weinberg Equilibrium:

  1. Gene migration or gene flow: When individuals migrate to another place or population, new genes or alleles are added to new population and are lost from old population, in turn changing the frequencies. When gene migration occurs many times, it is called gene flow.

  2. Genetic drift: Changes occurring in frequencies by chance is called genetic drift. Sometimes, due to change in allele frequency in a new population, some form a different species. This effect is called founder effect and the original drifted population is called founder.

  3. Mutation: Advantageous mutation lead to new phenotypes and over few generations, result in speciation.

  4. Genetic recombination: During gametogenesis, variations due to recombination result in new phenotypes.

  5. Natural selection: Heritable variations that enable survival of the fittest will have greater number of progenies. Natural selection can have following three effects:

  • Stabilisation: Larger number of individuals acquire mean character value so peak gets high and narrower.

  • Directional change: Large number of individuals acquire value other than mean character value so peak shifts in one direction.

  • Disruption: Large number of individuals acquire peripheral character values at both ends of the distribution curve and hence 2 peaks are formed.


A Brief Account of Evolution

First cellular forms of life appeared around 2000 million years ago (mya).

Some of these cells are said to release O2 by splitting water with the help of solar energy captured by light harvesting pigments.


Evolution of plants:

  • Plants invaded land before animals.

  • Bryophytes originated earlier than the vascular plants like pteridophytes and gymnosperms.

  • The first vascular plants developed in Silurian period.

  • Sea weeds and few plants existed around 320 mya.


Evolution of animals:

  • Around 500 mya, invertebrates originated and were active.

  • Around 350 mya, jawless fish and amphibious fish with stout and strong fins originated.

  • Lobefins were the first amphibians and ancestors of modern-day frogs.

  • Around 200 mya, reptiles dominated the earth.

  • The land reptiles were dinosaurs, of which Tyrannosaurus rex was biggest.

  • Dinosaurs suddenly disappeared around 65 mya.

  • The first mammals that evolved in Jurassic period were like shrews.

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