NEET Biology Notes Concept of Evolution
Concept of Evolution
Concept of Evolution
The change in the inherited characteristics of biological populations over successive generation is called evolution. Evolutionary processes give rise to diversity at every level of biological organisation including species, individual organisms and molecules such as DNA and proteins.
The Origin of Life
According to Big Bang theory proposed by Abbe Lemaitre (1931), our solar system was created from gaseous cloud called solar nebula. The first indications of primitive life occurred just over 3 x 10 9 years ago. Eubacterium isolatum is the oldest fossil (i.e. 3.2 billion years ago), which belongs to cyanobacteria, its fossils were found in sedimentary rocks of South Africa
The Origin of Earth
The earth is about 4.6 billion years ago, while the oldest rock on earth are 3.3-3.6 billion years ago and sedimentary rocks are 2.7-3.2 billion years ago. Sedimentary rocks are of particular evolutionary interest because their formation requires liquid water. Fossils are mainly found in sedimentary rocks.
Primitive Atmosphere on the Earth
Primitive atmosphere of earth was reducing, containing methane (CH4), ammonia (NH3), hydrogen (H2) and water vapour (H20). There was no free oxygen. Temperature, rain fall and electric storms were high and Ultra Violet rays (UV rays) and cosmic rays could reach directly to earth surface because ozone layer in stratosphere was absent. In contrary to primitive atmosphere, present atmosphere of earth is oxidising, containing nitrogen (N2), oxygen (02) and carbon dioxide (COz) gases. There is no free hydrogen and UV rays cannot reach directly to earth surface due to the presence of ozone layer.The water is super heated containing hydrogen sulphide (H2S), methane (CH4), iron (Fe) and sulphide ions
Theories of Origin of Life
Several theories have been put forward to explain the origin of life (i.e. biopoiesis), such as
- Theory of Special Creation
This theory was given by Father Saurez and supported by Milton. As per this theory, life was created by some super natural power (God). - Spontaneous Generation
According to theory of spontaneous generation (abiogenesis or autobiogenesis), life was originated from non-living (life less) materials automatically. It, was given by Anaximander, Thales Aristotle, Xenophanes, Plato and von Helmont in 1942. - Theory of Biogenesis
According to theory of biogenesis, life originated from pre-existing life (Omnis vivum ex ovo or vivo). This theory was supported by Francesco Reddi, Lazzaro Spallanzani, Louis Pasteur, Harvey and Huxley. Francisco Reddi was an Italian scientist.
- Theory of Cosmozoic/Panspermic/ Interplanatery
According to theory of cosmozoic origin or extraterrestrial theory, both living and non-living objects were formed simultaneously and protoplasm reached to earth from other parts of universe in the form of spores, seeds, sperms, etc. - Theory of Catastrophism
The theory of catastrophism was supported by Cuvier, a French Palaeontologist, who believed that world has passed through many ages with catastroph at the end of each age. - Modern Theory or Oparin-Haldane Theory
As per this theory and proposed that, origin of life is the result of a long series of physico chemical changes, which brought about first by chemical evolution and then biological evolution.
Chemical Evolution
Evolution up to formation of coacervates is termed as chemical evolution, in which complex organic compounds were foraled that are essential for formation of cellular structure.
- Formation of inorganic molecules and compounds
- Formation of simple organic compounds
- Formation of complex organic compounds
The chemical evolution occurred in following steps:
Biological Evolution
Evolution from coacervates to simple cell structure is known as biological evolution. JBS Haldane (1920) used the term pre-biotic soup or hot dilute soup of organic substances for oceanic water containing mixture of simple organic compounds. Darwin used the term warm little pond for early hot sea rich in biomolecules.The transformation from inorganic compounds to organic compounds is not possible in the present oxidising atmosphere as in primitive atmosphere because oxygen or microorganisms will decompose or destroy the living particle that may arise by ever chance.
The main steps in the origin of life are as follows:
Miller and Urey’s Experiment
The first experimental support to Oparin-Haldane’s theory of origin of life came from Urey and Miller’s experiment in 1953. In those days, Stanley Miller was a graduate student of Harold C Urey (1893-1981) at the university of Chicago.
Urey (an astronomer) asked his student Stanley L Miller (a biochemist) to replicate the primordial atmosphere as propounded by Oparin and Haldane.
Miller (1953) made first experimental apparatus to access the validity of the claim for origin of organic molecules in primitive earth condition.
In 1953, Miller built an apparatus of glass tubes and flasks in the laboratory. He created an atmosphere containing hydrogen (H2), ammonia (NH3), methane (CH4) and water vapour (H20) in one large flask and allowed condensed liquids to accumulate in another small flask. The ratio of methane, ammonia and hydrogen in large flask was 2:1:2.
Evidences of Organic Evolution
The fact that evolution has taken place can be established by taking several kinds of evidences including direct evidences, which are provided by fossil record and indirect evidences, which are available from the study of organisms that are existing today.
The significant convencing evidences for occurrence of biological evolution come from
- Palaeontology
- Geographical distribution
- Taxonomy
- Morphology and comparative anatomy
- Embryology
- Connecting link
- Biochemistry and physiology
- Cytology
- Genetics
Evidences from Palaeontology
Palaeontology is the study of fossils of past life. Palaeobotany deals with the study of plant fossils including study of fossil pollens (i.e. Palynology), whereas, Palaeozoology deals with the study of animal fossils.
Birbal Sahni is called Father of Indian Palaeobotany. By the initiative of Prof. Birbal Sahni, Birbal Sahni Institute of Palaeobotany was established at Lucknow in 1946.
Fossils are the material remains (bones, teeth and shells) or traces of ancient organisms.
Some techniques used in dating of rocks are :
- Radiocarbon dating
- Potassium-argon (40K/40Ar) and Argon-argon (40Ar/39Ar) dating
- Uranium series dating
- Fission track dating
- Luminescence dating
- Electron-Spin Resonance (ESR) dating
- Geological Time Scale
The complete-life span of earth (i.e. 4600 million years) is known as geological time, which have been divided into eras, eras into periods and further periods into epochs.
The arrangement of eras, periods and epochs on the time scale according to their age is called geological time scale.
An Italian scientist Giovanni Ardulna, developed first geological time scale in 1760.
- Evidences from Geographical Distribution
The differences between the distribution of plants and animals provide biogeographical evidence in support of evolution.
According to continental drift or plate-tectonics theory, proposed by Alfred L Wegener (1929), during Carboniferous period, all the present day continents were in the form of a single giant landmass called Pangaea. In Triassic period, Pangaea had splitted into two parts, i.e. Laurasia (i.e. Northern half land mass consisting of North America, Europe and Asia) and Gondwana (i.e. Southern half landmass consisting qf South America, Australia, Africa, India and Antarctica).
- Evidences from Taxonomy
Euglena like unicellular organisms were common ancestors of animals and plants. During classification, organisms are grouped according to their resemblance and placed from simpler organisms towards complexity.
- Evidences from Morphology and Comparative Anatomy
One of the indirect evidences for evolution comes from the study of homologous organs in organisms that exist today. Homologous organs are those, which have the same basic structure, but different functions.
e.g. forelimbs of vertebrates, legs of insects, phyllode or phylloclade of Opuntia, Ruscus and cladode of Asparagus.
The organs that are different in basic structure and origin but perform similar functions are called analogous organs, e.g. Insect and bird’s wings are different in basic structure and origin because insect wing is formed from integument, while the bird wing is a modified forelimb but functionally both are adapted for flight. Homologous organs show divergent evolution while, analogous organs show convergent evolution.
The non-functional organs in an organism, which are functional in related animals and were functional in the ancestors also, called vestigial organs. Coccyx (tail bone), nictitating membrane (3rd eyelid), caecum, vermiform appendix, canines, wisdom teeth, body hair, external ear muscles, mammary glands in males, etc., are vestigial organs in human body. The sudden reappearance or refunctioning of some ancestral organs, which have either completely disappeared or are present as vestigial organs is called Atavism or Reversion.
- Evidences from Embryology
Embryology provides some indirect evidences for evolution, von Baer (1792-1867) proposed Baer’s law in 1828, which states ‘that during development general characters appear before specialized characters, an animal departs progressively from the form of other animals and organisms of different groups resemble with their embryos.
Ernst Haeckel (1866), an embryologist, proposed Biogeiietic law, which states that Ontogeny recapitulates phylogeny. It means evolutionary history (phylogeny) of a species is indicated by the development stages (ontogeny) that it passes through.
- Evidences from Connecting Links
Connecting link is that organism, which exhibits characteristics of more than one groups.
- Evidences from Biochemistry and PhysiologyMetabolic reactions are present in all living beings from bacteria to man. Trypsin and amylase enzymes are same in all animals. Cytochrome-c is a respiratory pigment found in all eukaryotic cells. A, B, AB and O types of blood groups are found in humans, while, apes has only A and B type and monkeys lack such type of blood groups. This indicates that humans are more closely related to apes than monkeys.
Ecogeographical Rules-Size
Shapes and Colour
- Bergman’s Law warm blooded animals are larger in size in the colder regions as compared to warm region.
- Allen’s Law Extremities like tail, ears becoms smaller in colder area.
- Gloger’s Rule Warm blooded animals have more melanin in hot wet areas but develop yellow-red pigment in hot dry areas.
- Jordan’s Law Fishes of colder water have larger size with more vertebrae than the fishes of warmer waters.
- Cope’s Law In the course of evolution there is tendency in animals to increase in size, e.g. dinosaurs.