CHAPTER 6 - EVOLUTION

 

Evolutionary Biology is the biology in which we study about history of life forms on earth. Various theory has been proposed about origin of life forms.

Theories about origin of life forms

  • Various theories has been given in support how first life appear on earth. They are as below -
    1. Some scientists believe that it came from outside. Early Greek thinkers thought units of life called spores were transferred to different planets including earth. ‘Panspermia’ is still a favourite idea for some astronomers.
    2. For a long time it was also believed that life came out of decaying and rotting matter like straw, mud, etc. This was the theory of spontaneous generation.
    3. Louis Pasteur by careful experimentation demonstrated that life comes only from pre-existing life. He showed that in pre-sterilised flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose from ‘killed yeast’.
    4. 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.
    5. Miller’s experiment - 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 800°C. 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.
    6. On above conclusion, theories about origin of life may be categorised as
      1. Biogenesis - Life originated from pre-existing life forms.
      2. Abiogenesis - Life originated from inorganic matters.

Evolution of Life forms

  • Various life forms evolved form similar and share common ancestors present at different periods in the history of earth (epochs, periods and eras).
  • This is firstly supported by Charles Darwin.

Charles Darwin

  • Charles Darwin (after a sea voyage in a sail ship called H.M.S. Beagle) concluded based on his observations made his journey.
    1. Existing living forms share similarities to varying degrees not only among themselves but also with life forms that existed millions of years ago.
    2. Many such life forms do not exist any more.
    3. There had been extinctions of different life forms in the years gone by just as new forms of life arose at different periods of history of earth.
    4. There has been gradual evolution of life forms.
    5. Any population has built in variation in characteristics.
      • Those characteristics which enable some to survive better in natural conditions (climate, food, physical factors, etc.) would outbreed others that are less-endowed to survive under such natural conditions.
    6. Another word used is fitness of the individual or population.
      • The fitness, according to Darwin, refers ultimately and only to reproductive fitness.
      • Hence, those who are better fit in an environment, leave more progeny than others.
      • These, therefore, will survive more and hence are selected by nature.
      • He called it natural selection and implied it as a mechanism of evolution.

Evidences in Support of Evolution

  • Evidence that evolution of life forms has indeed taken place on earth has come from many quarters.

Paleontological Evidence

  • Fossils are remains of hard parts of life-forms found in rocks.
    1. Rocks form sediments and a cross-section of earth's crust indicates the arrangement of sediments one over the other during the long history of earth.
    2. Different-aged rock sediments contain fossils of different life-forms who probably died during the formation of the particular sediment.
    3. Some of them appear similar to modern organisms.
    4. They represent extinct organisms (e.g., Dinosaurs).
    5. A study of fossils in different sedimentary layers indicates the geological period in which they existed.
    6. The study showed that life-forms varied over time and certain life forms are restricted to certain geological time-spans.
    7. Hence, new forms of life have arisen at different times in the history of earth.
    8. All this is called paleontological evidence.

Embryological Support

  1. This was also proposed by Ernst Heckel.
    • This was based on the observation of certain features during embryonic stage common to all vertebrates that are absent in adult.
  2. Karl Ernst von Baer noted that embryos never pass through the adult stages of other animals.
  3. Comparative anatomy and morphology shows similarities and differences among organisms of today and those that existed years ago.
  4. Such similarities can be interpreted to understand whether common ancestors were shared or not.

Types of Evolution

  • On the basis evidences in support of evolution, evolution can broadly be categoried into following -
    • Divergent evolution
    • Convergent evolution

Divergent Evolution

  • When the same structure developed along different directions due to adaptations to different needs. This is called divergent evolution.
    1. e.g. whales, bats, Cheetah and human (all mammals) share similarities in the pattern of bones of forelimbs. Though these forelimbs perform different functions in these animals, they have similar anatomical structure – all of them have humerus, radius, ulna, carpals, metacarpals and phalanges in their forelimbs. Hence, in these animals, these structures are homologous.
    2. Other examples are vertebrate hearts or brains.
    3. In plants also, the thorn and tendrils of Bougainvillea and Cucurbita represent homology.
  • Homology indicates common ancestry.

Convergent Evolution

  1. When the different structure developed along different directions due to adaptations to same needs. This is called convergent evolution.
    • e.g. Wings of butterfly and of birds look alike.
  2. They are not anatomically similar structures though they perform similar functions.
  3. These structures are called analogous organs.
  4. Hence, analogous structures are a result of convergent evolution - different structures evolving for the same function and hence having similarity.
  5. Other examples of analogy are the eye of the octopus and of mammals or the flippers of Penguins and Dolphins.
  6. It is the similar habitat that has resulted in selection of similar adaptive features in different groups of organisms but toward the same function:
    • Sweet potato (root modification) and potato (stem modification) is another example for analogy.

Adaptive Radiation

  1. The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation.
  2. Examples are Darwin’s finches, Australian marsupials and Placental mammals in Australia.

Darwin’s Finches

  1. During his journey Darwin went to Galapagos Islands.
  2. There he observed an amazing diversity of creatures.
  3. Of particular interest, small black birds later called Darwin’s Finches amazed him.
  4. He realised that there were many varieties of finches in the same island.
  5. All the varieties, he conjectured(guessed), evolved on the island itself.
  6. From the original seed-eating features, many other forms with altered beaks arose, enabling them to become insectivorous and vegetarian finches.

Australian Marsupials and Placental Mammals

  1. A number of marsupials, each different from the other evolved from an ancestral stock, but all within the Australian island continent.
  2. Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be ‘similar’ to a corresponding marsupial (e.g., Placental wolf and Tasmanian wolf-marsupial).
  3. When more than one adaptive radiation appeared to have occurred in an isolated geographical area (representing different habitats), this shows convergent evolution.

Biological Evolution

  • There are two theories for biological evolution.
    1. Lamarck Theory
    2. Darwin Theory

Lamarckism

  1. Lamarck, a French naturalist had said that evolution of life forms had occurred but driven by use and disuse of organs.
  2. He gave the examples of Giraffes who in an attempt to forage leaves on tall trees had to adapt by elongation of their necks.
  3. As they passed on this acquired character of elongated neck to succeeding generations, Giraffes, slowly, over the years, came to acquire long necks.
  4. This theory is mainly based on the inheritance of acquired characters.

Darwinism

  1. This theory is based on
    • Natural selection
    • Survival of the fittest
  2. Key concepts of Darwinian Theory of Evolution
    • Branching descent
    • Natural selection
  3. There must be a genetic basis for getting selected and to evolve.
  4. Some organisms are better adapted to survive in an otherwise hostile environment.
  5. Adaptive ability is inherited.
  6. It has a genetic basis.
  7. Variation leads to appearance or disappearance of an organ which make organism to fit in that environment.
  8. Fitness is the end result of the ability to adapt and get selected by nature.

Mechanism of Evolution

  1. Evolution is not a continuous process. It may be in forward or backward direction.
  2. On basis of direction of evolution, it may be -
    • Progressive
    • Reterogressive
  3. On the basis of time, it may happen -
    • in one generation due to mutation (Saltation by Hugo deVries)
    • in several generation due to natural selection or acquired characters.

Hardy–Weinberg Principle

  1. Hardy-Weinberg principle stated that the frequency of occurrence of alleles of a gene or a locus is supposed to remain fixed and even remain the same through generations.
  2. Genetic equilibrium -
    • The gene pool (total genes and their alleles in a population) remains a constant. This is called genetic equilibrium.
    • Sum total of all the allelic frequencies is unity.
  3. Allele frequencies in a population are given as
    • p2+2pq+q2=1 i.e. unity
    • This is a binomial expansion of (p+q)2.

Factors affecting Hardy-Weinberg Equilibrium

  • These are
    1. gene migration or gene flow
    2. genetic drift
    3. mutation
    4. genetic recombination and
    5. natural selection

Geologic Time Scale of Evolution

  1. Evolution of life forms correlate with evolution or formation of earth. Hence evolution of life forms is divided into according to Earth's history.
  2. Geologic Time Scale are divided into 4 eras -
    • Azoic or Precambrian Era
    • Paleozoic Era
    • Mesozoic Era
    • Cenozoic Era
  3. These eras are durther divided into geological periods (geoperiods).

Story of Evolution

First cellular forms

  1. About 2000 million years ago (mya) the first cellular forms of life appeared on earth.
  2. Non-cellular aggregates of giant macromolecules could evolve into cells with membranous envelop.
  3. Some of these cells had the ability to release O2 as in photosynthesis.

Multi-cellular life forms

  1. Slowly single-celled organisms became multi-cellular life forms.
  2. By the time of 500 mya, invertebrates were formed and active.
  3. Jawless fish probably evolved around 350 mya.
  4. Sea weeds and few plants existed probably around 320 mya.
  5. The first organisms that invaded land were plants. They were widespread on land when animals invaded land.
  6. Fish with stout and strong fins could move on land and go back to water. This was about 350 mya.
  7. In 1938, a fish caught in South Africa happened to be a Coelacanth which was thought to be extinct. These animals called lobefins.

Amphibians

  1. Lobefins evolved into the first amphibians that lived on both land and water.
  2. These were ancestors of modern day frogs and salamanders.

Reptiles

  1. The amphibians evolved into reptiles.
  2. They lay thick-shelled eggs which do not dry up in sun unlike those of amphibians. Their modern day descendents are the turtles, tortoises and crocodiles.
  3. In the next 200 millions years or so, reptiles of different shapes and sizes dominated on earth.
  4. Some of these land reptiles went back into water to evolve into fish like reptiles probably 200 mya (e.g. Ichthyosaurs). T
  5. he land reptiles were, of course, the dinosaurs.
    • The biggest of them, i.e., Tyrannosaurus rex was about 20 feet in height and had huge fearsome dagger like teeth.
    • About 65 mya, the dinosaurs suddenly disappeared from the earth.
  6. Small sized reptiles of that era still exist today.

Birds

  • Most of reptiles evolved into birds.

Mammals

  1. The first mammals were like shrews.
  2. Their fossils are small sized.
  3. Mammals were viviparous and protected their unborn young inside the mother’s body.
  4. Mammals were more intelligent in sensing and avoiding danger at least.
  5. When reptiles came down mammals took over this earth.
  6. There were in South America mammals resembling horse, hippopotamus, bear, rabbit, etc.
  7. Due to continental drift, when South America joined North America, these animals were overridden by North American fauna.
  8. Due to the same continental drift pouched mammals of Australia survived because of lack of competition from any other mammal.
  9. Some mammals live wholly in water. Whales, dolphins, seals and sea cows are some examples.

Evolution of Man

  1. About 15 mya, primates called Dryopithecus and Ramapithecus were existing.
    • They were hairy and walked like gorillas and chimpanzees.
  2. Ramapithecus was more man-like while Dryopithecus was more ape-like.
  3. 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.
    • They were probably not taller than 4 feet but walked up right.
  4. 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.
    • The brain capacities were between 650-800cc.
    • They probably did not eat meat.
    • This creature was called the first human-like being the hominid and was called Homo habilis.
  5. About 1.5 mya, Homo erectus had a large brain around 900cc.
    • Homo erectus probably ate meat.
  6. 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.
  7. Homo sapiens arose in Africa and moved across continents and developed into distinct races.
    • During ice age between 75,000-10,000 years ago modern Homo sapiens arose.
    • Pre-historic cave art developed about 18,000 years ago.
    • One such cave paintings by Pre-historic humans can be seen at Bhimbetka rock shelter in Raisen district of Madhya Pradesh.
    • Agriculture came around 10,000 years back and human settlements started.

One Mark Questions

Q1. When did the first cellular forms of life appear on earth?

Ans. About 2000 million years ago

Q2. What is founder effect?

Ans. The original drifted population becomes founders and the effect is called founder effect.

Q3. What formed coal deposits?

Ans. Giant ferns (pteridophytes) formed coal deposits slowly.

Q4. What are the first organisms that invaded land?

Ans. The plants

CHAPTER 5 – MOLECULAR BASIS OF INHERITANCE

CHAPTER 7 – HUMAN HEALTH AND DISEASE

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