Evolution

 n The study of life forms on earth is known as evolutionary biology.

1. Origin of the Universe

n Universe is around 20 billion years old and comprise of huge clusters of galaxies.

n Galaxies contain stars and clouds of gas and dust.

n The origin of universe is explained by the Big Bang theory.

n According to it, 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.

n Earth belongs to milky way galaxy.

2. Origin of Earth

n Earth was formed 4.5 billion years back.

n Initially, the surface was covered with water vapour, methane, CO2 and NH3 released from molten mass.

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

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

n Cooling of water vapour led to rain which filled the depressions on earth’s surface, forming water bodies.

3. Theories of Origin of Life

n Life appeared 500 million years after formation of earth.

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

(i) Theory of special creation: According to this theory, God created life by his divine act of creation.

(ii) 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. This theory was rejected as spores cannot survive extreme cold, dryness or UV rays from Sun, which are required to be crossed to reach earth.

(iii) Theory of spontaneous generation: According to this theory, life originated from decaying and rotting matter like straw, mud, etc.

l Louis Pasteur dismissed the theory of spontaneous generation and demonstrated that life came from pre-existing life.

l He placed killed yeast in a pre-sterilised flask and in a flask open to air. He showed that life did not originate in the former but new living organisms arose in the latter.

(iv) Theory of chemical evolution or Oparin–Haldane theory: This theory was given by Oparin and Haldane and stated that life originated 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.

n The conditions on earth favouring chemical evolution were high temperature, volcanic storms, reducing atmosphere containing CH4, NH3, etc.

Experimental evidence of chemical evolution/Miller’s experiment

n Experiment was performed by S.L. Miller and H.C. Urey in 1953.

n Experimental set-up: In a closed flask containing CH4, H2, NH3 and water vapour at 800°C, electric discharge was created. The conditions were similar to those in primitive atmosphere.

n Observations: After a week, they observed presence of amino acids and complex molecules like sugars, nitrogen bases, pigments and fats in the flask.

n Conclusions:

(i) It provides experimental evidence for the theory of chemical origin.

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

(iii)It showed that non-cellular biomolecules exist in the form of DNA, RNA, polysaccharides and protein.

4. Formation of First Cell

n First non-cellular life-forms originated 3 millions years ago.

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

n First cellular form of life originated about 2000 million year ago.

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

n Theory of Biogenesis, i.e., first form of life arose slowly through evolutionary forces from non-living molecules. However once formed, the first cellular forms evolved into complex biodiversity of today.

5. Evidences for Evolution

(i) Paleontological evidences

l The study of fossil is called paleontology.

l Fossils are the remains or impressions of past organisms preserved in sedimentary rocks or other media.

l Different-aged rock sediments in earth’s crust indicate the presence of fossils of different life forms which died during the sediment formation.

l A variety of fossils ranging from the modern organisms to extinct organisms can be observed.

l By studying the different sedimentary layers, the geological time period in which the organism existed can be predicted, e.g., Dinosaurs.

(ii) Morphological and comparative anatomical evidences

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

A. Homologous Organs

(a) The organs with same structural design and origin but 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., all have humerus, radius, ulna, carpals, metacarpals and phalanges.

(b) Due to different needs, some structures developed differently. This is called divergent evolution.

(c) Homology indicates common ancestry.

(d) Other examples include vertebrate hearts or brains in animals, thorn and tendrils of Bougainvillea and Cucurbita in plants.

B. Analogous Organs

(a) The organs which are anatomically different but functionally similar are called analogous organs. For example, wings of butterfly and birds.

(b) Due to same function, different structures evolve similarly. This is called convergent evolution.

(c) Other examples include eye of octopus and mammals.

(d) Flippers of penguins and dolphins.

(e) Sweet potato (root modification) and potato (stem modification).

(f) Similar habitat resulted in selection of similar adaptive features in different groups of organs but towards the same functions.

(iii) Biochemical evidence

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

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

(iv) Biogeographical evidence

l Species restricted to a region develop unique features. Also, species present in widely separated regions show similarity of ancestry.

6. Adaptive Radiation

n It is the process of evolution of different species in a given geographical area starting from a common point and radiating to other geographical areas (habitats). Examples:

(i) Darwin’s finches

l Darwin travelled to Galapages Islands and observed many varieties of finches on the same island.

l All varieties had evolved from original seed-eating finches.

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

(ii) Placental animals in Australia

l A variety of placental mammals have evolved which appear similar to a corresponding marsupial. eg. Placental wolf and Tasmanian wolf.

l When more than one adaptive radiation appear 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.

(iii) Marsupials of Australia

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

lThese have evolved from a common ancestral stock, but all within the Australian island continent.

7. Theories of Evolution (Biological Evolution)

(i) Lamarck’s theory of evolution or Lamarckism

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

l 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.

l This theory is no more accepted.

(ii) Darwinian theory of evolution

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

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

l There has been gradual evolution of life forms with new forms arising at different periods of history.

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

l 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).

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

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

n Natural selection is based on following factual observations:

l Limited natural resources.

l Stable population size except seasonal fluctuation.

l Varying characteristics of members of a population.

l Most of the variations are inherited.

l Limited population size means there had been competition for resources.

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

n Theory of special creation has three connotations:

(a) Organisms we see today were created as such.

(b) The diversity was always the same since creation and will be same in future also.

(c) Earth is about 4000 years old.

l But these ideas were challenged during 19th century based on Darwin’s observations as stated above.

l Alfred Wallace, worked in Malay Archepelago, obtained similar conclusions as Charles Darwin. All existing life forms share similarities and common ancestors as well but these ancestors were present at different periods in history of earth (epochs, periods and eras)

l Conclusions:

(a) Earth is very old, not thousand of years but billions of years.

(b) Geological history of earth closely correlates with the biological history of earth.

n Examples of natural selection.

(a) Industrial melanism:

l In England, before industrialisation in 1850’s, white-winged moths were more in number than dark-winged moths.

l But after industrialisation in 1920’s, dark-winged moths became more in number than white-winged moths.

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

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

l In mixed population, those that can better adapt, survived and there was increase in their population size.

(b) Chemical resistance:

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

l Microbes are also becoming resistant to antibiotics and drugs because of same reason.

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

(c) Man has bred selected plants and animals and thus created new breeds with in short period of times.

(iii) Mutation theory of evolution

l This was put forth by Hugo de Vries based on his work on evening-primrose (Oenothera lamarckiana).

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

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

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

l Evolution was gradual for Darwin while de Vries believed saltation.

8. Hardy–Weinberg Principle

n This principle states that allelic frequencies in a population are stable and remain 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.

n Sum total of all allelic frequencies is 1.

n It can be expressed as p2 + 2pq + q2 = 1 where p and q are frequencies of AA and aa respectively, and 2pq is of Aa.

n Disturbances in genetic equilibrium result in evolution.

9. Factors Affecting Hardy–Weinberg Equilibrium

n 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.

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

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

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

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

(a) Stabilisation: Larger number of individuals acquire mean character value so peak gets higher and narrower.

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

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

10. Brief Account of Evolution

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

n Some of these cells are said to release oxygen by splitting water with the help of solar energy captured by light harvesting pigments(as in light reaction in photosynthesis).

n Single-celled organisms eventually became multicellular.

(i) Evolution of plants

l Plants invaded land before animals.

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

l The first vascular plants developed in Silurian period.

l Sea weeds and few plants existed around 320 mya.

l Giant ferns (pteridophytes) fell to form coal deposits slowly.

(ii) Evolution of animals

l Around 500 mya, invertebrates originated and were active.

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

l In 1938, a fish caught in South Africa, Coelocanth, which was thought to be extinct and were called lobefins.

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

l Around 200 mya, reptiles dominated the earth. They lay thick-shelled eggs which do not dry up in Sun, unlike those of amphibians.

l Some of land reptiles went back into water to evolve into fish-like reptiles 200 mya (e.g., Ichthyosaurs).

l The land reptiles were dinosaurs, of which Tyrannosaurus rex was biggest. Tyrannosaurus rex was 20 feet height and had huge fearsome dagger-like teeth.

l Dinosaurs suddenly disappeared around 65 mya. Some say reptiles evolved into birds.

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

l Some mammals lived only in water, e.g., whales, dolphins, seals and sea cows.

11. Evolution of Man

 

Human Ancestors	Time of Origin	General Features
1. (a) Dryopithecus (b) Ramapithecus	15 mya	Ape-like, hairy, arms and legs of same length, large brain, ate soft fruits and leaves, walked like gorillas and chimpanzees. More man-like, walked more erect, teeth like modern man.
2. Australopithecus	2 mya	Fossils found in Tanzania and Ethiopia, man-like primates, 4 feet tall, walked upright, ate fruit, hunted with stone weapons, brain capacity was 400–600 cc.
3. Homo habilis	2 mya	Fossils found in East Africa, first human-like being, brain capacity 650–800 cc, did not eat meat.
4. Homo erectus (Java man)	1.5 mya	Fossils found in Java, brain capacity 900 cc, ate meat.
5. Homo sapiens neanderthalensis (Neanderthal man)	100,000–40,000 year ago	Fossils found in east and central Asia, brain size 1400 cc, used hides to protect body, buried their dead.
6. Homo sapiens (Modern man)	75,000–10,000 years ago 18,000 years ago 10,000 years ago	Developed cave art, agriculture, started human civilisation. Prehistoric cave art developed Agriculture started.