CLASS XII BIOLOGY
UNIT – VII: GENETICS AND EVOLUTION
CHAPTER 7 Evolution : Theories and Evidences
Syllabus Origin of Life; Evolution of Life Forms-A Theory; What are the Evidences for Evolution ? What is Adaptive Radiation ? Biological Evolution, Mechanism of Evolution; Hardy-Weinberg Principle; A brief account of Evolution; Origin and Evolution of Man
Introduction
A number of changes in flora and fauna have occurred over millions of years on earth. The study of the history of life form on earth is Evolutionary Biology. This chapter deals with origin of life and evolution of life forms of biodiversity on the planet earth, evidences of evolution, mechanism of evolution, Hardy Weinberg principle and human evolution.
The theory of evolution maintains that the different kinds of organisms that we see today have evolved from common ancestors over millions of years. This theory is one of the most important concepts in biology. The distinguished scientist Theodosius Dobzhansky has said : “Nothing in biology makes sense except in the light of evolution”. For more than a century, the theory of evolution has exerted a very strong influence on our thinking about biology, also on developments in other disciplines such as sociology, politics, economics and religion.
Life originated on the earth between 3000 and 4000 millon years ago, in the form the unicellular organisms. How did these simple cells lead to (or evolve into) organisms as large as a whale or a Sequoia (redwood) tree, and structures as complex and delicate as the eye and the brain?
The “Theory of Evolution by Natural Selection”, was put forward by Charles Darwin and Alfred Russel Wallace towards the middle of the nineteenth century. It has provided us with a scientific framework for understanding the evolutionary changes that have occurred, and continue to take place in the biological world.
1. Basic unit of evolution is population. 2. Evolution is a slow, continuous and irreversible process of changes. 3. Earth is about 4.6 billion years old, and the oldest rocks that have persisted in recognizable form are about 3.8 billon years old. For many years, scientists believed that such ancient rocks did not contain any fossils, but they knew that fossils were simply too small to be seen clearly without an electron microscope. 4. The oldest microfossils discovered so far are that appeared 3.3 to 3.5 billion years ago. 5. Massive limestone deposits called Stromatolites became frequent in the fossil record about 2.8 billion years ago. Produced by cyan bacteria, stromatolites were abundant in virtually all freshwater and marine communities until about 1.6 billion years ago. 6. The fossil records indicates that unicellular protists – the first eukaryotes - appeared about 1.5 billion years ago. 7. Origin of Earth : The Big Bang Theory proposes that the universe had an explosive beginning. The universe originated about 20 billion years ago by a big bang (thermonuclear explosion) of a dense entity. About 4.5 billion years ago, our solar system was probably created when the gaseous cloud called Solar Nebula started to collapse under
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the force of its own gravity, until it became a flattened spinning disc of atoms and particles. Its central region heated up and became a star.
According to recent literature the first non-cellular forms of life could have originated3 billion years back. They would have been giant molecules (RNA, protein, polysaccharides etc.) These capsules reproduced their molecules perhaps. The first cellular form of life did not possibly originate till about 2000 million years ago.
THERIES ON THE ORIGIN OF LIFE
1. Theory of special creation states that life was created by supernatural power in the form which has not undergone any change. It was given by Father Saurez. God has created life in six days from meteria prima and man was created by him on sixth day. According to this theory earth is about 4000 years old. 2. Theory of catastrophism was given by Cuvier, according to which after a gap of certain period (called age), the world undergoes a catastrophe (sudden calamity) to kill almost all the living organisms and then God created new generation or new life from inorganic matter. 3. Theory of biogenesis (i.e. life from life, Omnis vivum ex. vivo) was proved by Redi, Spallanzani and Pasteur independently. They disproved (refuted) theory of spontaneous generation (abiogenesis). Francesco Redi (1668) proved that files could not arisa from putrefying meat without their eggs. Spallanzani (1767) demonstrated that putrefaction of meat is due to microbes in the air it can be prevented by boiling and sealing the meat in air tight containers. Pasteur gave a definite proof of life arising from pre-existing life using microbes and sterilization methods. 4. Cosmozoic theory (Theory of panspermia) given by Richter (1865) , Helmholtz (1884), Arrhenius (1908) suggested that life reached the earth from some heavenly body through meteorites. Panspermia (primitive form of life, as suggested by Arrhenius, 1908) consisted of spores or seeds (sperms) and microbes that existed throughout universe and produced different forms of life on this earth.
Abiogenic or Chemical Origin of Life
Majority of the scientists are of the opinion that life originated from inanimate matter. Since the theory of abiogenic origin or chemical evolution of life is the only one that provides an explanation, which can be tested, most scientists have tentatively accepted it.
Oparin-Haldane Hypothesis
1. Alexander 1. Oparin (1894-1980), a Russion biochemist , and J.B.S.H aldane (1892-1964), a British scientist put forward the concept that first organism evolved from non-living material. They also suggested that the sequence of events that might have occurred. In 1923, Oparin postulated that life originated on Earth at some point of time in the remote past, and under the conditions no longer observed. In his book, The Origin of Life (1938), Oparin submitted abiogenesis first, but biogenesis ever since. Oparin’s theory is know as primary abiogenesis. 2. According Oparin and Haldane (1929), spontaneous generation of early molecules might have taken place if the earth once had more reducing atmosphere compared to the present oxidising atmosphere . Oparin and Haldane agreed that the primeval Earth contained little, if at all, oxygen. Perhaps, in the primitive atmosphere oxygen in the free gaseous states was virtually absent. Therefore, no degradation of any organic compound arising in the primeval Earth could have taken place. 2
3. As there was no azone layer in the atmosphere any absorption of UV radiation, that is lethal to our present lives was possible in the primordial Earth. 4. The early gas cloud was rich in hydrogen ,being present in the form of methane (CH4), ammonia (NH3) and water (H2O). 5. Moreover, that atmospheric water vapour condensed into drops of water as rain that rolled down the rock surfaces and accumulated to form liquid pools and oceans. In the process, erosion of rocks and washing of mineral (e.g., chlorides and phosphates) into the oceans were inevitable. Thus, Halane’s hot dilute soup was produced and the stage was set for combination of various chemical elements. 6. Atmospheric chemicals and those in water produced small precursor molecules,like amino acids, sugars, nitrogenous bases etc. These precursor molecules then combined resulting in the appearance of proteins. 7. The energy sources for such reactions of organic synthesis were the UV radiation (solar radiation), cosmic rays, electrical discharges (lighting), intense dry heat (volcanic eruption) and radioactive decay of various elements on the Earth’s surface. Once formed, the organic molecules accumulated in water because their degradation was extremely slow in the absence of any life or enzyme catalysts. Such transformation is not possible in the present oxidizing atmosphere because oxygen of micro organisms will decompose or destroy the living particle that may arise by mere chance.
Experimental Evidence for Abiogenic Molecular Evolution of Life
Haroid C.Urey (1893-1981), an astronomer, accorded the first adequate recognition of Oparin-Haldane’s vview on the oringin of life in 1952,. Urey asked his student Stanely L. Miller, a biochemist, to replicate the primordial atmosphere as propounded by Oparin and Haldane. Miller (1953) made the first successful simulation experiment to assess the validity of the claim for origin of organic molecules in the primeval Earth condition.
1. Miller sealed in a spark chamber a mixture of water (H2O), methane (CH4), ammonia (NH3) and hydrogen gas (H2). He made arrangement for insertion of two electrodes to provide electrical energy (simulation of lightning) to the spark chamber. CH4, NH3, H2 were in 2: 1: 2 ratio and water vapour at 800ͦ C. Electric spark of 75,000 volt was provided to the mixture. 2. The spark chamber was connected to another flask with arrangement for boiling water (provision for evaporation). The other end of the spark chamber was connected to a trap by a tube that passed through a condenser (an arrangement for condensation and collection of aqueous solution, equivalent to rain and Haldane’s soup). The trap, in turn, was connected with the flask for boiling water (arrangement for circulation). 3. The control apparatus contained every arrangement except that it was devoid of energy source. 4. After eighteen days, significant amount of the simple organic compounds (monomers), such as amino acids and peptide chains, began to appear in the aqueous sample of the experimental set. Amino acids found were alanine, glycine and aspartic acid. Therefore, the obvious inference was that biotic synthesis of organic monomers occurred in the simulated experimental condition. By analogy, such synthesis could have occurred in the primitive atmospheric condition. Later on many scientists repeated Miller’s experiment using slightly different starting materials and UV radiation of other energy sources. All of them could successfully synthesis amino acid and related compounds. With hydrogen cyanide (HCN) even adenine and other nitrogen bases were produced.
Miller’s experiment to prove the origin of life in the simulated primeval atmosphere.
Abiotic synthesis of bilmlecules in studied under following headings : 3
1. Chemogeny : Synthesis of organic molecules by chemical reaction. 2. Biogeny : Formation of self replication biomolecules in broth (primordial hot soup or warm little pond) 3. Cognogeny : Evolution of various forms of life.
Enclosing the prebiotic systems
The experiments of Miller and other scientists demonstrate that prebiotic molecules could have been formed under the conditions which most likely existed on early Earth. Still, the formation of prebiotic soup of small molecules does not necessarily lead to the origin of life. For origin of life, atleast three conditions needed to have been fulfilled:
1. There must have been a supply of replicators i.e., self-producing molecules. 2. Copying of these replicators must have been subject to error via mutation. 3. The system of replicators must have required a perpetual supply of free energy and partial isolation from the general environment.
The high temperature prevailing in Earth would have easily fulfilled the second condition, that is, the requirement of mutation. The thermal motion would have continually altered the prebiotic molecules.
The third condition, partial isolation has been attained within aggregates of artificially produced prebiltic molecules. These aggregates called protobionts can separate combinations of molecules from the surroundings maintain an internal environment but are unable to reproduce. Two important protobionts are coacervates and microspheres . Oparin (1924) observed that if a mixture of a large protein and a polysaccharide, with some water, become separated from the surrounding aqueous solution. The later has much lower concentration of proteins and polysaccharide. Oparin’s coacervates also exhibit a simple form of metabolism. As these coacervates do candidate of probable precursors of life.
Microspheres were formed when mixtures of artificially produced organic compounds are mixed with cool water. If the mixture contain lipids, the surface of the microspheres consists of lipid bilayer, reminiscent to the lipid bilayer of cell membranes. Sydney Fox (1950) obtained protenoid microsphere.
There is considerable discussion among biologists as to how the first cells may have evolved. The discovery made in the 1980’s that RNA can act like enzyme to assemble new RNA molecules on an RNA temple raised the interesting possibility that Coacervates man not been the first step in the evolution of life. Perhaps the first macromolecules were RNA molecules, and the initial steps on the evolutionary line were ones leading to more complex and stable RNA molecules. Later, the stability might have been improved by surrounding the RNA within a coacervate. Still other scientists reject the notion of ‘RNA world’, entirely, pointing out that some RNA components are too complex to have been present on the primitive earth.
EVIDENCES OF EVOLUTION
1. Biogeographical Evidence The study of patterns of distribution of animals and plants in different parts of the earth is called Biogeography.
Biogeographic map of the world is that in which the six major biogeographic realms are present. Geologists believe that millions of years ago all the continents we demarcate today were in the form of a single land mass. On account of geological changes, especially movements of crstal plates below the surface of the earth, huge land masses broke off and drifted apart form one another. As these land masses (continents) move away, the seas separated them and acted as 4
barriers to free movement of organisms among the continents. Because of variable environmental conditions prevailing on the different continents, over centuries, plants and animals evolved independently in each biogeographical region. India falls under oriental realm. Two geographical regions separated by a high mountain ranges are Palaearctic and oriental.
Alfred Russel Wallace (1823-1913) divided the whole world into six major biogeographical regions or realms.
1. Palaearctic : Europe and Asia north of the tropics, north- Western corner of Africa, including the Atlas Mountains. 2. Nearctic : North America exclusive of the tropics, Alaska, Canada, United States and Mexico. 3. Neotropical : Central America including low lands of Mexico, islands of the Caribbean and all of South America. 4. Ethiopian : Africa (with exception to the Atlas Mountains), Madagascar and adjacent islands. 5. Oriental : Tropical part of Asia (including India) south of the Himalaya Mountains and eastward through Sumatra, Java, Borneo and the Philippines. 6. Australian : Australia, Tasmania, New Guinea and all islands of the Indonesian archipelago that lie east of Borneo, beginning with Celebes.
Consider, for example, two instances which show similarities in the pattern of plants and animals between two land masses which were once part of a larger land mass.
1. The flora and fauna on each of the Galapagos Island- a chain of 22 islands in the pacific ocean on the west coast of South America resemble those of the South American mainland with the Galapagos islands were once connected. 2. Magnolias, Tulips and Sassafras are now found naturally growing only in the eastern USA and china. The distribution pattern of the present day animals and plants, as well as the distribution of fossils is best explained on the basis of the theory of evolution. Galapagos Islands are chain of 22 Island, present on the west coast of South America. They have the fauna which resembles that of South America, for example the main Darwin’s finch was in South America. The birds in Galapagos Islands show differences in bills and feeding habits. The bills of several of these species resemble those of different, district families of birds on the mainland. All these birds are thought to have evolved from a single common ancestor. The evolution of Darwins finches on the Galapagos Islands is a classic example of species formation and illustrates how adaptation to local conditions produces the divergence , that is the heart of species formation i.e. such an evolutionary process, giving rise to new species adapted to new habitats and ways of life, is called Adaptive Radiation or Divergent Evolution. The clusters of species that have been formed on the Galapagos Islands are thus a clear example of species formation arising by micro evolutionary divergence from an ancestral form occupying different habitats- of microevolution leading to microevolution.
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Furthermore, Australia is the home to the great diversity of pouched mammals (marsupials) but relatively few placental mammals. Darwin explained that adaptive radiation gave rise to a variety of marsupials in Australia in the same process of adaptive radiation as found in the finches in the Galapagos Islands. Natural selection can lead to similar adaption for survival in the similar habitats. This form of evolutionary change is called convergent or parallel evolution, some to the marsupials of Australia resemble equivalent placental mammals that live in similar habitats of other continents. Australia, separated from other from Antarctica and evolved in isolation earlier than placental mammals. Natural selection has favored changes that made the two groups more alike. In other words, their phenotypes have converged. Analogous organs show adaptive convergence that is, development of functionally similar structures in unrelated similar group if organisms.
Parallel Adaptation
Plants and animal communities in widely separated areas are often similar, especially if the areas have similar climates.
This may be true even if the individual kinds of plants and animals are only distantly related to one another. These similarities are difficult to explain as mere coincidence and suggest that parallel adaptation has occurred.
Perhaps the best-known example of parallel adaptation is the resemblance of Australia’s marsupials to the placental mammals found in the rest of the world. Example Tasmanian wolf resembles the placental wolf, their phenotypes have converged.
2. Evidences from Anatomy 1. Homologous organs are those which have the same basic structure and developmental origin. The organisms which possess such organs are said to have originated from common ancestor. Consider, for example, the seal’s flipper, the bat’s wing, the horse’s foot., the cat’s paw and the human hand. In plants, a thorn of Bougainvillea differs from a tendril of Cucurbit in its function, both are located in a similar (axillary) position and have similar origin. Thorns and tendrils are considered homologous. Homologous organs show divergent evolution or Adaptive Radiation. It is the development of dissimilar functional structures in closely related group of organisms. 2. Analogous organs are those which perform the same function but are not similar in structural details and origin. For example, the wings of birds and the wings of butterfly, both of which are used for flying are completely different in their anatomical framework. Neither do they have same origin nor they have evolved from the same organ in a common ancestor . Wing of insect is analogous to wing of pterodactyl extinct flying reptile. Even the strikingly similar analogous organs, such as the eye of an octopus and the eye of mammal, they differ in their retinal position. Flippers of Penguins and Dolphins are also the example of convergent evolution. A stem may be modified to appear like leaves and may perform the function of leaves as in Ruscus. This is a case of analogy. Sweet potato is an underground tuberous root and potato is an underground stem but both are the modification and are meant for storage of food . These are, therefore, examples of analogous organs. 3. Vestigial organs : They are believed to be remnants of organs which were complete and functional in their ancestors. The study of vestigial organs offers an evolutionary explanation of such rudimentary vestiges by stating that adaptations to new environment of the organism have made these structures redundant. Such structures are called vestigial organs. The rudiment of the reptilian jaw apparatus, the rudiment of the hind limbs of python and 6
Greenland whales are some of the examples of vestigial organs. In humans, many vestigial structures indicate a relationship to other mammals including the primates. For instance muscles of the external ear and scalp are rudimentary and often non-functional. But these are common to many mammals where they are functional. The reduced tailbones and nictitating membrane of the eye, the appendix of the caecum, rudimentary body hair and wisdom teeth –all are examples of vestigial organs. 4. The appendix of man is thought to be a remnant of the large caecum- the storage organ for cellulose digestion in herbivorous mammals. Similarly, the non-functional vestiges of the pelvic girdle in python and porpoise show, for instance, that the snake and the porpoise show, for instance, that the snake and the porpoise originally evolved from four footed ancestors. 5. Atavism : Sudden reappearance of ancestral character. Tail in new born human baby.
Embryological Evidence
The sequence of embryonic development in different vertebrates striking similarities. Gill clefts and notochord appear in the embryonic development of all vertebrates from fishes to mammals. The notochord is replaced by the vertebral column in all adult vertebrates. Similarly, gills are replaced by lungs in adult amphibians, reptiles and mammals. Such similarities in embryonic development once again reinforce the idea of evolution from common ancestors. Occasionally, embryonic features such as the tail and gill slits persist in adults.
According to Ernst Haeckel, ontogeny (development of embryo) is recapitulation of phylogeny (the ancestral sequence). This view was summarized by his Biogenetic Law : Ontogeny Recapitulates phylogeny.
Paleontological Evidence
Fossils are the remains and/or impressions of organisms that lived in the past last few centuries and paleontologists have painstakingly built up extensive collections of fossils from all over the world. The fossil record has helped in building the broad historical sequence of biological evolution. Phylogeny, the evolutionary history of the organism, can sometimes be reconstructed with the hip of fossils. Horse, elephant an man are good examples of relatively complete reconstructions of phylogeny. Besides form and structure, the habits and behavior of extinct species can be inferred from the well-preserved fossils. It is also possible to reconstruct the entire habitat of an organism from fossils. Fossils also indicate the connecting link between two groups of organisms. Archaeopteryx shows features of both reptiles and birds.
Time Line of Evolution
When scientists first began to study and date fossils, they had to find some way to organize the different time periods from which the fossils came. They divided the earth’s past into large blocks of time called eras. Eras are further sub-divided into smaller blocks of time called periods, and some periods, in turn, are sub-divided into epochs. The major geological eras, with their approximate dates in millions of years as given in the table. The geological time scale.
Table1 : THE GEOLOGICAL TIME SCALE ERA PERIOD EPOCH AGE (MILLION SOME IMOPORTANT EVENT IN THE OF YEARS) HISTORY OF LIFE
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Quaternary Recent 0.01 Historic time Pleistocene 1.8 Ice ages; humans appear Pliocene 5 Apelike ancestors of humans appear Continued radiation of mammals and Miocene 23 angiosperms Cenozoic Origins of most modern mammalian orders, Tertiary Oligocene 34 including apes Angiosperm dominance increases; further increase in Eocene 57 mammalian diversity Major radiation of mammals, birds, and pollinating insects Paleocene 65
Cretaceous 144 Flowering plants (angiosperms) appear; dinosaurs and many group of organisms become extinct Mesozoic Gymnosperms continue as dominant plants; Jurassic 208 dinosaurs; first birds Gymnosperms dominate landscape; first dinosaurs and mammals Triassic 245 Permian 285 Radiation of reptiles, origins of mammal-like reptiles and most modern orders of insets; extinction of many marine invertebrates Extensive forests of vascular plants ;first feed plants; origin of reptiles; amphibians Carboniferous 360 dominant. Age of amphibians Diversification of bony fishes; first amphibians dominant. Age of fishes Diversity of jawless vertebrates ; colonization Devonian 408 of land by plants and arthropods; origin of vascular plants First vertebrates (jawless fishes); marine algae Silurian 438 abundant. Age of invertebrates Origin of most invertebrate phyla; diverse algae
Ordovician 505
Cambrian 544 8
700 Origin of first animals 1500 Oldest eukaryotic fossils Precambrian 2500 Oxygen begins accumulating in atmosphere Or Oldest definite fossils known proterozoic 3500 (prokaryotes) Approximate origin of Earth 4600 Evolution of vertebrates (Hypothetical)
Types of Fossils
Only a minute fraction of the organisms living at any one time are preserved as fossils, most of them in Sedimentary rocks.
Fossils are formed when parts of dead organisms decay with the passage of time and get replaced, molecule for maolecule,b y inorganic, a material like iron iron pyrites, silica and calcium carbonate. This mineral replacement is called PETRIFACTION. Hard parts like bones, shells teeth or trunks are preserved more readily than soft parts. Rarely, however, soft tissues also get petrified. Thin section of fossils when observed under the microscope, reveal the original structure.
A fossil is formed only under certain conditions. The dead organism should not get washed away or completely decomposed by bacteria. Instead, if it is buried near a river or a sea, it may become covered by sand or silt. If petrifaction then sets in, it is turned into a fossil.
Fossils are also formed by processes other than petrifaction. For example, an organism may get buried intact in preservatives like snow, oil, tar, volcanic ash a amber (a resinous secretion from fossil pine tress).
Sometimes what is left behind as a fossil is just an impression of the organism. Such an impression is formed when material surrounding the buried organism hardens followed by decay and subsequent removal of the organic matter. An exact replica of the original structure is left behind. Such impressions are called moulds. A common type of plant fossil is where the internal structure is lost, leaving a thin carbon film which give the outline of the original plant. This kind of fossil formation is called compression. Coprolite : Excreta preserved as fossil.
Pseudo fossils- occasionally rocks present structures resembling plant remains. They are normally products of minerals is rock crevices. Mineral substances crystallize and develop into patterns resembling the outline to the plant.
Fossil Parks
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Our country has rich deposits of fossil plants spanning a gap a 3500 million years. Twenty million years old fossil forest have been discovered and studied by the Birbal Sahni Institute of Palaeobatany, Lucknow ,. These forests need to be systematically studied and conserved for scientific understanding and enlightenment. Some of the excellent localities that can be raised to the status of national fossil parks are;
1. Fifty million years old fossil forests preserved in the sediments between the streaming lava flow that poured out into Deccan country at Mandla district, Madhya Pradesh. 2. One hundred million years old fossil forest localities in Rajmahal Hills, Bihar. 3. Two hundred and sixty million years old coal-forming forests in Orissa.
Microfossils and Fossil Fuel Exploration
Palaeobiological study helps in understanding and locating coal and hydrocarbon sources. Palynofossils -tiny microscopic spores, pollen and other vegetal remains of the past- assist us in interpreting ancient environmental conditions favorable for organic matter accumulation and its conversion to fossil fuels by transformation and subsequent thermal alteration. By quantitative analysis of microfossils, it is possible to determine the approximate location and configuration of near shore marine deposits, which are in turn responsible for formation and accumulation of hydrocarbons. The main source of hydrocarbons are phytoplankton, marine and terrestrial algae as well as lipid-rich plant remains. Thus, the study of fossils plants offers an effective tool in stratigraphical geology and can be exploited in tapping organic fuel resources.
Dating Fossils
Naturally occurring radioactive isotopes of certain elements are employed are employed in this process. Such isotopes are unstable and decay over the course of time at a steady rate, producing other isotopes.
One of the most widely employed methods of dating – the carbon – 14 (14C) method uses estimates of the different isotopes present in sample of carbon. Most carbon atoms have an atomic weight of 12. But a fixed proportion from 12 C as result of bombardment by particle form space. But after an organism dies and is no longer incorporating carbon, its 14 C gradually decays over time, by the loss of neutrons. The common radioactive elements which lose their radioactivity and change into their non radioactive isotopes at fixed rate are :
1. Potassium40 - Argon 40 2. Carbon 14 - Nitrogen 14 3. Uranium 238 - Lead 207 4. Rubidium87 - Strontium87 5. Thorium232 - Lead207
In every 5760 years half of C14 will decay back to N14
Radioactive carbon can be used to determine the age of fossils upto 70,000 years old. Half life of potassium 40 is 1.3 x10 years. Potassium- Argon method is useful because potassium is a common element fount in all sorts of rocks. 9 Potassium decays into Argon extremely slowly. Potassium – Argon method has recently been used to determine the age the hominid fossils in East Africa.
Electron spin resonance method is relatively most accurate method for dating of fossils.] 10
Evolution of Modern Horse
Eohippus (=Hyracotherium). The evolution of modern horse bagan in the Eocene epoch. The first fossil named Eohippus, ‘dawn horse’ was in North America. This horse was about the size of a fox or terrier dog (a type of small dog for unearthing foxes), only 40 cm high at the shoulders. It had short head and neck. The fore limbs were with four complete fingers (2,3,4 and 5) and one splint of first finger and the hind limbs with three functional toes (2,3, and 4) and one splint of fifth toe. Splints are nonfunctional side fingers and were adapted to browsing of soft lush vegetation.
Mesohippus. Mesohippus, the intermediate horse, evolved from Hyrachotherium about three crore years ago during Oligocene epoch. It was of the size of modern sheep, about 60 cm high at the shoulders. Force feet had three fingers and one splint of fifth finger and hind feet possessed three toes, but the middle one was longer than others and supported most of the body weight. Molar teeth had some serrations.
Merychippus. Merychipuus, the ruminating horse, arose from mesohippus in Miocene epoch about two crore years ago. It was of the size of small pony, about 100 cm high at the shoulders. It was with longer neck. Its fore and hind feet had three fingers and three toes, the middle finger and toe being longer than others and supported entire body weight. There was no splint. Teeth were longer with cement. Molar teeth had well developed serrations.
Pliohippus. Pliohippus, the Pliocene horse, evolved from merychippus in Pliocene epoch about one crore years ago. It was the size of s modern pony, about 120 cm high at the shoulders. Its each fore and hind foot had one complete finger and one complete toe and two splints hidden beneath the skin. Pliohippus is, therefore, referred to be first one toed horse. The molar teeth were long with well developed cement and serrations. Teeth were adapted for eating grass.
Equus. This is the modern horse which arose from Pliohippus in Pleistocene epoch about nine to ten lakh years ago in North America and later spread throughout the world except Australia. It is about 150 cm high at the shoulders. It has a long head and a long neck. Each fore and hind foot of the modern horse has one finger and one toe and two splints. The crowns of molar teeth are elongated with enameled ridges, and are highly suitable for During evolution of horse there was :
(i) General increase (with occasional decrease) in size, (ii) Progressive loss of toes, (iii) Lengthening of toes that are retained, (iv) Lengthening of limbs in general (v) Enlargement of brain especially cerebral hemispheres, (vi) Increase in height (vii) Increase in the complexity of molar teeth and an enlargement of the last premolars until they came to resemble molars.
Connecting Links
The organisms which possess the characters of two different groups. Examples
1. Proterospongia : A link between protozoa and Porifera. 2. Neopilina : A connecting link between Annelida and Mollusca 3. Peripatus : A connecting link between Annedlida and Arthropada.
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4. Lungs fishes, e.g., protopterus, lepidosiren, Neoceratodus are considered the connecting links between the fishes and amphibians. 5. Egg laying mammals; example duck billed Platypus (ornithorynchus) and spiny and eater (Echidna) a connecting link between reptiles and mammals.
Evolution of Vertebrates and Major Groups of Plant
The patterns of evolution of vertebrates and major groups of plants are conspicuously different.
The major groups of vascular plants have left relatively small number of fossils, which even show gaps. There are relatively few major lineages and all the lineages are very distinct from one another. Instead of showing gradual and continuous change through time, the major lineages appear suddenly in the fossil record. After that, they persisted with little fundamental change for hundreds of millions of years. The existence of many of the major subdivisions of the vascular plants living today can be recognized about 345 million years ago on the basis of their distinctive reproductive structure. All primitive land plants reproduce via tiny spores contained in the sporangia. The major taxonomic groups are distinguished by the position of sporangia on the plant.
The sporangia are terminal, located at the tip the plant in the most primitive Psilopsida. These are placed at the base of the leaves in the Lycopsida (represented in the modern flora by Lycopodium and Selaginella). The sporangia are arranged in whorls at the top the plant in Sphenopsida (horsetails). Fossil evidences document that these basic patterns have been maintained for more than 350 million years. Few, if any, intermediates are known between these patterns. The origin of seeds in the land plants was achieved about 345 million years ago in lineages recongnised as ancestral to all more advanced vascular plants. The last major evolutionary advancement among the plants was the emergence of flowering plants (the angiosperms) about 140 million years ago. But the fossil left no clue as to their ancestors. The fossil records also indicate that nearly all the living orders of angiosperms and most of the characters of their modern-day representatives evolved by them.
The continuous change of a character within an evolving lineage is termed as evolutionary trend. A lineage is an evolutionary sequence, arranged in linear order from an ancestral group to a descendant group. The number of trends in any lineage is, same as the number of characters evolving. A trend may be progressive (a general increase in size of organs) or retrogressive (a general degeneration and loss of organs).
BRIEF ACCOUNT OF EVOLUTION
About 2000 million years ago (mya) the first cellular forms of life appeared on earth. The mechanism of how non- cellular aggregates of giant macromolecules could evolve into cells with membranous envelop is not know. Some of these cells had the ability to release O2. The reaction could have been similar to the light reaction in photosynthesis where water in spilt with the help of solar energy captured and channelized by appropriate light harvesting pigment. Slowly single-celled organisms became multi-cellular life forms. By the time of 500 mya, invertebrates were formed and active. Jawless fish probably evolved around 350 mya. We are told that the first organisms that invaded land were plants. They were widespread on land when animals invaded land. Fish with stout and strong fins could move on land and go back to water. This was about 350 mya. In 1938, a fish caught in South Africa happened to be Coelacanth which was thought to be extinct. These animals called lobefins evolved into the first amphibians that lived on both land and water. There are no specimens of these left with us. However, these were ancestors of modern day frogs and salamanders. The amphibians evolved into reptiles. They lay 12 thickshelled eggs which do not dry up in sun unlike those of amphibians. Again we only see their modern day descendents the turtles tortoises and crocodiles . in the next 200 million years or so, reptiles of different shapes and sizes dominated land reptiles went back into water to evolve into fish like reptiles probably 200mya (e.g. Ichthyosaurs). The 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. We do not know the true reason. Some say climatic changes killed them. Some say most of them evolved into birds. The truth may live in between. Small sized reptiles of that era still exist today.
The first mammals were like shrews. Their fossils are small sized. Mammals were viviparous and protected their unborn young inside the mother’s body. Mammals were more intelligent in sensing and avoiding danger at least. When reptiles came down mammals took over this earth. There were in South America mammals resembling horse, hippopotamus, bear, rabbit, etc. Due to continental drift, when South America joined North America , these animals were overridden by North America fauna. Due to the same continental drift pouched mammals of Australia survived because of lack of competition from any other mammal.
Lest we forget, some mammals live wholly in water. Whales, dolphins,seals and sea cows are some examples. Evolution of horse, elephant, dog, etc., are special stories of evolution. You will learn about therse in higher classes. The most successful story is the evolution of man with language skills and self-consciousness.
THEORIES OF EVOLUTION
1. Lamarck’s Theory of Evolution His theory is often called as the Theory of inheritance of Acquired Characters or the Theory of use and Disuse of Organ. The first attempt to explain origin of species and their adaption to the environment was done by Jean Baptiste Lamarck. (1744-1829) was the greatest French naturalist. Lamarck’s theory was published in 1809 (year of Darwin’s birth) in his book ‘Philosophie Zoologique;. According to this theory the organisms undergo changes to adapt themselves to the environment. The changes acquired by the organisms during their life time are passed on the next generation. He took the example of long neck of Giraffe, they continuously stretched their neck of reach to the vegetation on trees. This acquired changes was passed to the next generation. He also gave the principle of Use and Disuse. Use of an organ leads to strengthening of the organ, and disuse will lead to weakening of the organ. Lamarck arranged his theory in the form of four postulates. (i) Internal forces tend to increase size of the body. (ii) Formation of new organs is the result of the need of want continuously felt by organisms Doctrine of Appetency/Desires. (iii) Development and power of action of an organ is directly proportional to its use. (iv) All changes acquired by the organism during its life are transmitted to the off springs by the process of inheritance.
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This theory was discarded by A. Weismann, who gave the Theory of Germplasm. He cut the tail of new born mice generation after generation after generation but the could not get a tailless mice nor the mice developed shorter and shorter tail.
Today again the faith in Lamarck’s theory has been revived, as it is said that if the environment influences the genes of the organisms, the acquired change will be transmitted to the next generation.
2. Darwin’s Theory of Evolution Charles Robert Darwin put forward the concept of natural selection as the mechanism of evolution. The theory was put forward along with Alfred Russell Wallace. Darwin had written the book “Origin of Species’. Darwin was greatly influenced ,by ‘An Essay on Population’ written by Thomas Rev Malthus and he was also influenced by Charles Lyell essays on “Principles of Geology”. Darwin was a British naturalist, in 1831 at the age of 22 he was appointed upon a world survey ship of British government H.M.S. Beagle. For five years on his ship Darwin explored the fauna and flora of continents and islands. Branching descent and Natural Selection are the two key concepts of Darwinian Theory of Evolution. According to Wallace’s Chart, the main points of Darwin’s theory of Natural Selection were as follows : (i) High rate of reproduction (ii) Total number almost constant (iii) Struggle for existence (iv) Variations (v) Survival of fittest (vi) Natural selection
All the successful organisms have high Biotic Potential or Reproductive Rate. The organisms produce a large number of off springs that can possibly survive, example a mice produce a dozen of mice at one time. A rabbit produce 6 young ones in a litter and there are four litter in year. A rabbit starts reproducing at the age of six months.
1. Not all, but only some individual which survive reach adulthood, and those which reach adulthood reproduce at different rates, this is called, as ‘ Differential Reproduction’. 2. The success in survival and reproduction depends upon the characteristic traits of an organism, example only those rabbits will survive which are fastest. There is ‘ Struggle For Existence’ and in this there will be ‘Survival of Fittest’. Was first used by Herbert Spencer. The same phrase was used by Darwin as Natural Selection. So, evolution is the change in the genetic composition of the population which in brought about by natural selection, which acts upon the variability in population.
Causes of Variations
1. Mutation is the ultimate source of variations. 2. At the next level in recombination. 3. Intermingling of two widely separated population.
Weakness of Darwinism
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He was not able to explain the cause of discontinuous variation observed by himself in nature, and the mode of transmission of variants to the next generation.
In 1868, Darwin put forward the Theory of Pangenesis. According to this theory every organ of the body produce minute hereditary particles, called as Pan genes or Gemmules and they are carried through the blood into the gametes.
Weismann’s Theory of Germplasm’ (1862) rejected Darwin’s theory of pangenesis. He established that the germ (sex), cells are set apart from other body (somatic) cells early in the embryonic development, so, only the changes in the germplasm affect the characteristics of future generations.
Alfred Wallace had written the book On the Tendencies of Verities to Depart Indefinitely from the Original Type. Alfred Wallace (1823-1923), a naturalist from Dutch East Indies, was working on Malay Archipelago (present Indonsia).
3.Hugo de vries Theory (Mutation Theory) In 1901, he proposed the Mutation Theory on the basic of his observation on the wild variety of evening primrose Oenothera lamarckiana. 1. Evolution on the grand scale of geological time is called as macroevolution. 2. Evolution at genetic level, ,microevolution. 3. Studies of how individual traits evolve within natural population provide powerful evidence that natural selection can be a powerful agent of microevolutionary change within species. The progressive change in allele frequencies within the population is micro-evolution. 4. Units of evolution, are populations. 5. Unit of natural selection is Individual.
Five basic process affect the Hardy Weinberg equilibrium and cause variation at genetic level. These are
(i) Mutation (ii) Recombination (iii) Gene migration (iv) Genetic drift (v) Natural selection
The Hardy-Weinberg principle states that the proportions of different alleles will stay the same in a large population if mating occurs at random and in the absence of the above mentioned forces.
In algebraic terms, the Hardy-Weinberg principle is written as an equation . Its form is what is known as a binomial expansion. For a gene with two alternative alleles, called A and a, the frequency of allele a can be expressed as p and that of alternative allele a as q, because these are only two alleles, P + q must always be equal to one. The equation looks like this.
(p+q)2 = p2 + 2pq + q2
Individuals Individuals Individuals
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homozygous heterozygous homozygous
for allele A for alleles for allele a
A and a
If q is the frequency of the allele a , then the Hardy-Weinberg equation states that q2 = percentage individuals homozygous for allele a say 16% q2 =0.16, q=0.4
Agents of Evolution changes
(i) Mutation (ii) Migration (iii) Genetic Drift (iv) Recombination (v) Natural Selection (i) Mutation Replica plate Experiment of Lederberg and Leaderberg 1. Mutations are random (indiscriminate) with respect to the adaptive needs of organisms. 2. Most mutations are harmful or with no effect (neutral) on their bearer. 3. Mutation rates are very slow.
The Lederberg Replica Plating Experiment , a beautiful example of the genetic basis of a particular adaptation was demonstrated in bacteria by an laboratory by plating dilute suspensions of bacterial cells on semi-solid agar plantes. After a period of growth, discrete colonies appear on the agar plates. Each of these colonies originates from a single bacterium through a large number of cell divisions. The Leaderbergs inoculated bacteria on an agar plate and obtained a ‘master plate’ containing several bacterial colonies. They, then created several replies of this master plate by a simple procedure. A sterile velvet disc, mounted on a wooden block, was gently pressed on the master plate. Some bacteria from each colony adhered to the velvet. By pressing this transferred by the velvet formed colonies on the new agar plates. However, when they attempted to make replicas using plates containing an antibiotic such as penicillin, most colonies found on the master plate did not grow on the replica plates. The few colonies that did grow were obviously resistant to penicillin. How did the bacteria acquire the ability to grow in a new environment (here, agar medium containing penicillin) ? In other words, what was the origin of this adaptation?
A Lamarckian interpretation of this adaption would have been that penicillin somehow induced a change in one or more bacteria, enabling them to grow in the presence of penicillin. A Darwinian view is that there were, in the original suspension of bacteria from which the master plates were prepared, a few bacteria carrying mutant genes which conferred on them the ability to survive the action of penicillin and form colonies. These mutation, which had arisen by chance, and not induced by penicillin, were present only is small numbers in the original suspension.
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Lederberg’s experiment provided evidence that mutations are actually Preadaptive. These kinds of mutations are regarded as advantageous mutations. They appear without exposure to the environment to which the organisms are to adapt themselves.
The new environment does not induce their formation, it only selects the preadaptive mutations that occurred earlier.
(ii) Migration Migration, defined in genetic terms as the movement of individual from one population into another, can be a powerful force in upsetting the genetic stability of natural population. If the characteristics of then arrived animal differ from those already there, the genetic composition of the receiving population may be altered, if the newly arrived individual or individuals can survive in the new area and mate successfully. Gene pool : A total collection of all genes and its allele in a population is called as gene pool. Thus, gene pool will have all genotypes i.e., genes of the organisms. Gene flow : if genes are exchanged between population of species, it is gene flow.
(iii) Genetic Drift (Sewall wright) Effect (Non-directional) factor Natural selection is not the only force responsible to brings changes in gene frequencies. There is the role of chance or Genetic Drift also. Genetic Drift causes the change in gene frequency by chance in a small population. In a small population ,the individual alleles of a gene are represented by a few individuals in a population. These alleles will be lost if these individuals will fail to reproduce. Allele frequencies appear to change randomly, as if the frequencies were drifting thus , a random loss of alleles in small population is Genetic Drift. A series of small populations that are isolated from one another may come to differ strongly as a result of Genetic Drift. Genetic Drift has two ramifications are described below. 1. Bottle neck effect : It is the decrease in genetic variability in a population, example cheetah population in Africa decreased due to hunting. Their, decreased numbers have limited cheetahs genetic variability with serious consequences. The present cheetah population is susceptible to a number of fatal diseases. If that disease attacks the cheetah population, the path of extinction of cheetah cannot be reversed. 2. Founder’s effect : When one or a few individuals are dispersed and become the founders of a new, isolated population at some distance from their place of origin, the alleles that they carry are of special significance. Even if these alleles are rare in the Source population, they will be a significant fraction of the new population’s genetic environment. This effect by which rare, alleles and combinations of alleles may be enhanced in new population – is called the founder’s effect. The founder effect. The founders effect is particularly important in the evolution of organisms on islands, such as Galapagos islands which Darwin visited. Most of the kinds of organisms that occur in such areas were probably derived from one or a few initial founders.
Fixation of new mutations : Genetic drifts fix new alleles, genes that arise by mutation, from time to time and eliminate the original gene, thereby changing the genetic make up of small population.
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(iv) Recombination
Gene recombination is also important source of variations. It occurs during crossing over at the time of meiosis, free assortment (selection) of genes at the time of gamete formation, random union of gametes at the time of fertilization and even chromosomal aberrations. They cause reshuffling of gene recombination which provides new combinations of existing genes and alleles. This is the entity of gene recombination. Gene recombination can occur not only between genes but also within gene pool, it is an important process during evolution which causes variations.
(v) Natural Selection
Causes allele frequencies of a population to change. Depending upon which traits are favoured, natural selection can produce different results.
Forms of Selection : There are three kinds of natural selection
1. Stabilizing Selection (Normalizing selection) : When selection acts to eliminate both extremes from an array of phenotypes, the frequency of the intermediate type which is already the most common, is increased. 2. Directional Selection (Progressive selection) ; When selection acts to eliminate one extreme from an array of phenotypes, the genes determining this extreme become less frequent in the population. The industrial melanism is peppered moth, Biston betualria provides good example of directional selection from nature. 3. Disruptive Selection (Diversifying selection) : In some situations, selection acts to eliminate, rather than favour, the intermediate type. The individuals at both the extremes are favoured.
Examples of Natural Selection-industrial Melanism
First studied by R.A. Fisher and E.B. Ford and in recent time by H.B.D. Kettlewell.
One of the most striking examples, which demonstrates the action of natural selection, is the industrial melanism in England. The peppered moth Biston Betularia, with a dull grey colour or white was abundant in England before the Industrial Revolution. A black coloured form of the same moth (melanic, a dominant mutant differing in a single gene), carbonaria was very rare. Within a couple of hundred years, however the proportion of carbonaria increased to almost 90 per cent.
The moths rest on tree trunks. Before the Industrial Revolution, the tree trunks used to be covered with grey coloured lichen. The dull grey moth easily blended with this background, while the black moth stood out conspicuously, and was therefore more susceptible to predation by birds.
With the advent of the industrial Revolution, large-scale burning of coal became common. The enormous amount of smoke produced resulted in the deposition of particulate matter on tree trunks, turning them black. As a result, the grey moths now became more conspicuous than the black variety, and hence more susceptible to predation. The frequency of black coloured moths in the population therefore increased.
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Gradual replacement of coal by oil and electricity, a well as the improved methods of controlling soot production, reduced the soot desposition on the trees. Conditions then became more suitable for the survival of grey moths, consequently their frequency once again increased. Thus, reduction in pollution is now correlated with reverse evolution.
Industrial melanism, as this phenomenon is called, is thus a particularly interesting example which clearly brings out the action of natural selection. This has been observed in about 70 different species of moths, and in several other European countries as well.
Change in Genotypic Frequencies
If the alleles for grey and black colours are donoted by G and B, the genotypes of the moths would be GG, GB, and BB. Since B is dominant GB and BB will be black. Due to greater predation by birds on the black (melanic) phenotype the proportion of B in the population was maintained at a much lower value than G.
Resistance of Mosquitoes to Pesticides
Mosquitoes have always been a major health hazard, especially as they are responsible for the spread of diseases such as malaria and filarial. When DDT was first introduced to control mosquitoes, it was tremendously successful; most mosquitoes were sensitive to DDT and were therefore killed. However, DDT has now become ineffective against mosquitoes. This is explained as follows:
In the original population of mosquitoes, some individuals were resistant to DDT. However, in the absence of DDT, such resistant individual were few because they had no advantage over the DDT sensitive mosquitoes. However, when DDT was used on a large-scale, only the resistant genotypes were able to survive and reproduce. As a result, over a period of time, almost the entire population came to consist of the resistant type, which made DDT quite ineffective . Evolution is thus a change in gene frequencies in the population in response to changes in the environment-in this case the introduction of DDT. The principle of natural selection thus helps us to understand, why such chemical insecticides would remain useful only for a limited time.
Plants Growing Around Mines
A few plants are now known or grow on the tailings or refuse around mines. Professor A.D. Bradshaw studied one such grass, the bent grass Agrostis tenuis growing on tailings of lead mines in Wales, U.K. He took some of these and planted them in soil from a pasture near by. Similarly, he transplanted live Agrostis plants from the pasture to the lead-rich soil . The bent grass from the mine soil grew very slowly on normal pasture soil. The one from the pasture, on the other hand, could not survive in the lead-rich soil. A very small percentage (three out of sixty), however, could grow in the soil rich in lead. These were undoubtedly the kind, from which the race of bent grass capable of growing in lead-rich mine soil evolved originally. Plants tolerant to selenium such as Astragals and Heplopappus have been reported from the U.S.A. these plants are not only capable of growing in seleniferous soils, but require selenium as an essential element.
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In our country Professor Y.D. Tiagi discovered populations of impatiens balsamina growing around Zwar zinc mines in Udaipur, Rajasthan. The presence of such plants, which have evolved metal tolerance, can indicate the occurrence of specific metal deposits. Such plants are called bioindicator plants.
Sickle Cell Anaemia is an example of balancing selection.
(i) In few RBC, 1-2% became sickle shaped (ii) The heterozygotes (Hb A/ Hb s), who have one copy of sickle cell allele, coupled with one normal allele are better survivors in the areas where malaria is endemic; because the malarial parasite spends a part of life cycle in the RBC; if they enter itno the RBC which are sickle shaped, they will die. (iii) The women who are heteronygote have higher fertility; that’s why natural selection has not eliminated the allele. (iv) The loss of deleterious recessive genes through deaths of homozygotes (Hb A/ Hb s) is being balanced by the gain resulting from successful reproduction by heterozygotes. For this reason, the selection is called as balancing selection. (v) Heterozygotes enjoy some resistance to malaria, so they survive the malaria parasite more successfully than either normal or sickle cell homozygotes.
Artificial Selection
Some genetic variability is always present in a population. Some alleles make organisms better adapted to the environment, and thus make them more successful in survival and reproduction. As a result, the frequency of such alleles in population gradually increases. This is called selection; these alleles are thus ‘selected’ over the other alleles. This process operating in natural populations is therefore called ‘Natural Selection’.
The process on natural selection, acting on variability inherent in the population, over millions of years, has given rise to the great diversity we see in the biological world.
Man has been using a similar process for improving the qualities of domesticated plants and animals centuries. Plant-breeding and animal-breeding are very similar to the action of natural selection, the difference being that the role of nature is played by man. The criteria for selection are based are based on human interests.
To obtain cows with high milk yield, the diary scientists monitor mild production of large number of cows. Only the calves produced by cows which are high-yielders, are chosen to breed and form the next generation. When this process is repeated (i.e., artificial selection is applied) for many generation, a population of cows with high milk yield is obtained. Here, the work of selection is done by man.
Speciation and Isolation
Speciation is the formation of one or more new species from an existing species. The crucial episode in the origin of species occurs when the gene pool of a population is severed from other population of the parent species and gene flow no longer occurs. Speciation can take place in two modes based on the geographical relationship of new species to its ancestral species.
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1. When a population, formerly continuous in range, splits into two or more geographically isolated populations and form new species., the mode of speciation is called allopatric speciation. This can happen by subdivision of the original population, when a geographical barrier, such as creeping glacier a land bridge (e.g.,Isthmus of Panama) or ocean or mountain, cuts across a species ranga. Alternatively when a small number of individuals colonise a new habitat that is geographically separated from the original range. Examples of Darwin’s finches that formed separate species in the Galapagos islands and the Australian marsupials that radiated to form new species. 2. In the second speciation mode, a subpopulation becomes reproductively isolated in the midst of its parent population; this is sympatric speciation. So, sympatric speciation is the formation of species within a single population without geographical isolation. The usually quoted example of sympatric speciation comes from polyploidy, which is the multiplication of the normal chromosome number. This can happen when chromosomes fail to segregate at meiosis or replicate without undergoing mitosis.] 3. Multiplicative speciation (cladogenesis) : It is the formation of two or more species form a single species, example, allopatric speciation. This can be gradual or abrupt. 4. Fusion species : It is an allogenous transformation, Isolation mechanism may break down due to a mutation. The two species will interbreed and merge to form a single species. 5. Phyletic speciation (Anagenesis) : It is autogenous transformation of a species with passage of time due to pilling up of variations. 6. Species concept : Species is the basic unit of classification. The term was coined by John Ray (1693). Most taxonomists define species as morphologically district and reproductively isolated natural population or group of populations where individuals resemble one another more closely than with members of other species, have a similar anatomy, karyotype and biochemical, interbreed freely and form a genetically closed system. There are three basic concepts about the species. 7. Morphospecies concept : It is the earliest concept of species. Davis and Heywood (1963) have defined it as “assemblage of individuals with morphological features in common and separable from other such assemblages by correlated morphological discontinuities in a number of features.” However, the number of morphological characters chosen for separating species varies from taxonomist to taxonomist. “Lumpers” will combine all the populations with broadly similar traits into a single species while “Splitters’ will separate various populations with even minor morphological differences into district species. 8. Biological species concept : Though first proposed by the Buffon (1753), biological species concept was formulated by Mayr (1942). According to it, a biospecies (=biological species, Biological Species Concept) is sexually interbreeding or potentially interbreeding group of individuals which is reproductively isolated from other species and is therefore, separated from others by absence of genetic exchange. Normally species are district from one another by both morphological traits and reproductive isolation. However, sibling species are those district species which are almost identical morphologically but are district from each other due to absence of interbreeding, e.g., Drosophila pseudoobscura and D. Persimilis. Biological species concept is, therefore, mainly based on absence of cross fertilization between members of two species. Cross fertilization tests carried out by taxonomists between individuals of morphological and geographically separated population have, resulted in revision of species and grouping of many of them into single species, e.g. several species of North American sparrows as subspecies and races of a single song sparrow, Passarella melodia. The only problem of using reproductive isolation is the absence of sexual reproduction in several organisms – prokaryotes, some protists, some fungi, some plants (e.g., commercial Banana) and animals.
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Further, cross fertilization experiments can not be performed on such a large number of species that occur in varied geographical areas. Reproductive isolation cannot be used as a criterion in case of fossils. The living organisms and fossils can be grouped only on the basis of their morphology and biochemistry. Mayr (1987) has named morphologically grouped asexual species as paraspecies while Ghiselin (1987) has named them pseudospecies. 9. Evolutionary species concept : All evolutionary taxonomist have been in search of proper definition of species which is basic unit of classification. One such definition had been given by Simpson. According to Simpson (1961) “an evolutionary species is a lineage (an ancestor – descendent sequence of population) evolutionary role and tendencies . The concept stresses on evolutionary isolation with sexual isolation with sexual isolation being its one aspect it is more dependent on differences which can be morphological, genetical, behavioural and ecological, to know evolutionary distance. However, evolution does not occur simultaneously in all the traits. Neither its rate and direction are the same in which it is occurring. 10. Reproductive isolation may be defined as the existence of intrinsic barrier to the interbreeding on natural populations. Each of these intrinsic barriers is called a reproductive isolating mechanism. According to Mayr (1942), reproductive isolating mechanisms are the biological properties of individuals which prevent the interbreeding of naturally sympatric populations. 11. Reproductive isolation in the form of hybrid sterility is known since long. In the laboratory or in zoos, hybrids can be produced between species that do not interbreed in nature. Horses and donkeys are two different species: a hybrid, mule, is produced from the mating of a male donkey and a mare (female horse). 12. Similarly, mating between stallion (male horse) and female donkey results in a hybrid called hinny. Both mule and hinny are sterile. 13. There are examples of species, which can produce fertile hybrids in captivity. You might have head about the famous “tigons; a hybrid of African lioness (Panthera leo) and Asian tigers (Panthera tigris), which is fertile. No barrier to hybridization between these species has evolved during their long isolation from each other. Natural selection has not fovoured a reduction in hybridisation for the simple reason that no hybridization has been possible. Other examples of species that breed in captivity and produce fertile hybrids are mallard (a duck) and the pintail duck, the polar bear and the Alaskan brown bear and the platy and swordtail fishes. But these species do not interbreed at all in natural condition.
BARRIERS TO HYBRIDISATION
Prezygotic Mechanisms : (Prevent mating or formation of zygote)
1. Ecological isolation : Two species live in different habitats and do not meet. (One may be living in fresh water and the other in the sea) 2. Temporal isolation : Breeding seasons or flowering time may be different in the two species. 3. Behavioural isolation : The males of one animal species are unable to recognize the females of another species as potential mates. 4. Mechanical isolation : The structural difference in genitalia of individuals belonging to different animal species interfere with mating. 5. Gametic isolation : The structural difference species of animals are unable to fuse. In plants, the pollen coming from a different species may be rejected by the stigma.
Post zygotic Mechanisms : A hybrid zygote is formed but in may not devlop into a viable fertile adult. 22
1. Hybrid in viability : Hybrid zygotes fail to develop. In plants, embryos arising from interspecific crosses abort. 2. Hybrid sterility : Hybrid adults do not produce functional gametes. (Mules and hinny are common examples in mammals. Several hybrid ornamental plants are sterile) 3. Hybrid breakdown : The offspring of hybrids are inviable or infertile.
MODERN SYNTHETIC THEORY OF EVOLUTION
The modern synthetic theory of evolution is the result of the work of a number of scientists as T.Dobzhansky, R.A, Fisher, J.B.S. Haldane, Sewall Wright Stebbin. The synthetic theory includes the following factors.
(i) Gene mutations (ii) Changes in chromosome structure and number (iii) Genetic recombination (iv) Natural selection (v) Reproductive isolation (vi) Migration (vii) Hybridisation
Today human evolution is being studied by :
(i) Homology in the chromosomes of man and great apes. The banding pattern of human chromosome number 3 and 6 are compared with those of particular autosomes in the chimpanzee. It shows a common origin for man and chimpanzee. (ii) Today, besides the autosomal chromosomes, Y-chromosomes and mitochondrial DNA are being studied, as they are uniparental in origin and do not take part in recombination. (iii) Evidence from blood proteins-It has been proved by the blood protein tests test that man is most closely related to great apes (chimpanzee and gorilla). (iv) Evidence from blood groups- The blood groups A and B are found in apes and not in monkeys. (v) Evidence from haemoglobin- There is 99 percent homology in haemoglobin of man and gorilla.
Neutral Theory of Evolution
According to kimura most of the mutations are neutral not eliminated from the population. This is against natural selection. Kimura proposed that speciation in not due to selection of advantageous genotypes but elimination of deleterious alleles and random selection of neutral alleles. It emphasized that most mutations are of neutral value and genetic drift is responsible for divergence. It is only chance or random drift which delineates a novel collection of mutants into a group divergent from the parental population.
Place of Humans in the Animal Kingdom
Human beings are vertebrates and belong to the class mammalian. Mammals evolved from primitive reptiles in early Triassic period, about 210 million years ago. But for nearly 150 million years, mammals existed as relatively inconspicuous group of small rat-like creatures, completely dominated by the large number of gigantic reptiles of the Mesozoic age. It is only after the great extinction on dinosaurs and other large reptiles, that mammals
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diversified and began to occupy the earth’s many different habitats. Within the class mammalian, human beings belong to the order primates, a group that originated about 65 million years ago and includes not only monkeys and apes but also the lories, lemurs and tarsiers. The Anthropoid apes or the ancestors of monkeys, apes and humans evolved about 36 million years ago and the hominids or the ancestors of apes and humans evolved about 24 million years ago. Today, the apes are represented by two families, namely, Pongidae which include chimpanzees, gorillas and orangutans and Hylobatidae which includes gibbons. The chimpanzee and gorilla are restricted to Africa, whereas the orangutans and gibbons are found only in Asia. Humans belong to the family Hominidae in which Homo sapiens is the only living species.
Early Human Ancestors
Tracing the evolution of human beings, both by fossil hunting and molecular methods, is one of the most exciting and active areas of research in biology. The fossil evidence clearly indicated that such genera as Ramapithecus and Sivapithecus were the forerunners of Hominids. A genus called Astralopithecus appeared in Africa about five million years ago and ultimately gave rise to Homo about two million years ago. But even Astralopithecus had a brain measuring only about 350-450 CM 3. The most important change that must have occurred during the three million years or so, between the appearance of Astralopithecus and that of or genus must therefore have been a phenomenal increase in brain size, all the way up to 1400 to 1450 CM 3, that characteristic of our species. A combination of molecular data and a modern interpretation of the fossil record suggest that the gibbons probably diverged from the main line of Hominoid evolution about ten million years ago, that the orangutan did so about eight million years ago and that the ancestor of gorilla and chimpanzee, about our million years ago. The gorilla and chimpanzee have become separated from each other only 2.3 million years ago.
Place and Sequence of Human Evolution
There is evidence that almost all of Hominid evolution occurred in Africa, and that the evolution of the human species took place in Africa. Several species belonging to the genus Huma can be recognized from the fossil record. For example, Hemo habilis lived in Africa, about two million years ago and was characterised by having a larger brain than Australopithecus, use tools and was bipedal. Another species, Homo erectus appeared about 1.7 million years ago, used fire and is believed to have migrated to Asia and Europe. Fossils of the so- called ‘java man’ and ‘Peking man’, belong to Homo erectus. Homo erectus was replaced by Homo sapiens.
A primitive form of Homo sapiens, called Neanderthal man (Homo sapiens neanderthalensis), was common in Europe and Asia. The Neanderthal men resembled us, though they were relatively short and stocky and more powerfully built. The Neanderthals made tools and used animal hides as clothing. They built hut-like structures for dwelling and buried their dead. There is evidence that an abrupt transition occurred all over Europe whereby the Neanderthal man has wiped out and gave way to the more efficient cousin, the Cro-Mangnon, about 34,000 years ago. The Cro-Magnon people left behind very elaborate cave painting showing the attainment of a form of culture not unlike our own. After the last glacial period (about 10,000 years ago), modern Homo sapiens sapiens began to spread all over the globe, cultivated plants, domesticate animals and reached enormous population sizes
Homo sapiens appeared in Africa about 500,000 years and probably replaced Homo erectus there. But in Asia, Homo erectus appears to have survived for another 250,000 years when is was finally replaced by Homo sapiens migrating from Africa.
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HUMAN EVOLUTION
Place or origin of man : It has been established that Dryopithecus is one of the oldest fossil which in turn evolved into apes and men.
The origin and evolution of man can be studied in the following three major heading : prior to ape men, ape men including prehistoric man and true men including the living modern man.
A Prior to Ape Men
1. Drypithecus. Discovery : The fossil of Dryopithecus africanus was discovered from Miocene rocks Africa and Europe. It lived about 15 million years ago. Dryopithecus and Ramapithecus were hairy and walked like gorillas and chimpanzees. Ramaptihecus was more man like while Dryopithecus was more ape like.
Characteristics : It was ape-like, but had arms and legs of the same length. Heels in its feet indicate its semi erect posture. It had large brain, a large canines. It was without browridges. It was arboreal, knuckle-walker and ate soft fruits and leaves. Dryopithecus africanus is regarded a common ancestor of man and apes (gibbons, orangutan, chimpanzee and gorilla). It is also called as proconsul.
2. proconsul. Discovery : proconsul africanus or D. africanus was discovered by Louis S.B. Leakey in 1948 from the rocks around lade Victoria of Kenya, Africa. It lived in early Miocene epoch.
Characteristics : It was morphologically intermediate between apes and man in many features. It had rounded man-like forehead and long, pointed ape like canines. It moved upon land on all the four limbs and hence is not considered amongst the ancestors of man. Proconsul gave rise to the ancestors of chimpanzee and gorilla in the Pliocene, about 4 million years ago. The chimpanzee and gorilla diverged from each other only about 2.3 million years ago, in the Pleistocene epoch.
4. Sivapithecus. Discovery : This fossil was discovered from middle and late Pliocene rocks of Shivalik Hills of India, hence it is named shivapithecus. Characteristics : It was like Dryopithecus. Its for limbs, skull and brain were like those of monkeys while the face, jaws and teeth resembled those of apes. 5. Ramapithecus. Discovery : It has been established that in late Miocene epoch, Dryopithecus gave rise to Ramapithecus (Rama = The hero of Indian legend, Pithecus = Ape) which was on the direct line human evolution. Ramapithecus survived from late Miocene to Pliocene. Thus, he appeared about 14-15 million years ago. Fossil of Ramapithecus was discovered by Edward Lewis (1932) from Pliocene rocks of Shivalik Hills of India.
B. Ape-men Including Prehistoric Men
1. Australopithecus (First Ape-men) : The early human stock gave rise to Australopithecus. Discovery. Raymond Dart (1924), South African anthropologist, discovered the fossil of Australopithecus africanus (African Ape- man) from Pliocene rocks near Tuang in Africa. He appeared about 5 million years ago. Actually skull discovered by dart was of 5-6 year old baby so it also called “Tuang baby” Some fossils of A.
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africanus were also discovered from Pleistocene epoch. Two mya, Australopithecines probably lived in east African grasslands. Evidence shows they hunted with stone weapons but essentially ate fruits.
Characteristics : Australopithecus africanus was about 1.5 meters high and had human as well as ape characters. It was with bipedal locomotion diet and had erect posture. It had human like teeth, but it had more of an ape brain than a human than a human brain. Its brain capacity was about 500 cc., similar to that of an ape. He lived in caves. Brow ridges projected over the eyes. It did not have chin. There was lumbar curve in the vertebral column. The pelvis was broad. Australopithecus africanus existed until about 1.5 million years ago and gave rise to Homo habilis, bout two million years ago.
Australopithecus africanus also gave rise to man-like apes called Australopithecus robust us and Australopithecus boisei along a separate line that ended blindly (they did not give rise to any another creatures).
In 1981 Donald Johanson , found a 3.2 million years old skeleton of a female human ancestor. He nicknamed it Lucy. Lucy’s scientific name is Australopithecus afarensis.
Six species of Australopithecus are known. These are A. africanus (African Ape man, Southern Ape or Tuang baby), A. afarensis, (Lucy), A. aethiopicus, A. robustus and A. boisei. So we can say that Australopithecus had two main types.
(i) Gracile type : Australopithecus afarensis (Johanson 1976, 5MYA South Africa), represented by fossil Lucy with small brain, small molar teeth pelvic girdles and short fingers like human. (ii) Robust type : A. robustus had heavier body structure and massive check tooth (also called originally Paranthropus) cranial capacity -600CM2 (Other example – Zijanthropus /A. Boise of R. Leakey – Africa, Meganthroupus from -Java).
2. Homo habilis (Able or Skillful man, The tool maker, or ‘Handy man’). Discovery : Loues S.B. Leakey and his wife Mary Leakey (1960) discovered the fossil of Homo hobilis from Pleistocene rock of Olduvai Gorge in East Africa. He lived in Africa, about 2 million years ago. The first human like being the hominid was Homo habilis. They probably did not eat meat.
Characteristics : He was about 1.2 to 1.5 metres tall. He had bipedal locomotion and moved erect had about 650-800 cc. cranial capacity. The teeth were like that modern man. Homo habilis (habilis = mentally able or skillful) was the first tool maker and used tool of chipped stones extensively. It is also called handy man because heaps of tools found with these fossils included sharpened stones which indicate for the young ones.
3. Homo erectus (Erect man) : Homo erectus appeared about 1.5 million years ago, in middle Pleistocene Homo erectus probably ate meat. He is called as middle Pleistocene man. H. erectus evolved from Homo habilis. He was 1.5-1.8 metres tall. Homo erectus males probably larger than females. He had erect posture. His skull was flatter than that of modern man. He had protruding jaws, projecting brow ridges and small cannines and large molar teeth. The cranial capacity was 900 cc. Cranium was domed to accommodate the large brain. He was omnivorous. He made more elaborate tools of stones and bones hunted big game and perhaps knew use of fire. 26
(a) Java Ape- man. Discovery : In 1891, Eugene Dubois discovered a fossil Pleistocene rocks central Java (Island of Indonesia). Eugene Dubois named it as Pithecanthropus erectus. Pithecanthropus means ‘ape man’ Mayer in 1950 assigned it as Homo erectus erectus.
Characteristics : Body 1.65 to 1.75 metres tall weight about 70 kg. Legs long and erect, but body slightly bent when moving. Inconspicuous chin and somewhat broader nose. Forehead low and receding but brow ridges high, like those apes. Skull cap thick and heavy, flattened in front. Cranial capacity 800 to 1000 cc. (average 950 cc). Lower jaw large and heavy. Teeth large, but quite like those of modern man, except larger canines of the lower jaw. Lips thick and protruding. Ha was omnivorous and cannibal perhaps, he was the first prehistoric man to make use of fire for hunting, defense and cooking.
(b) Peking Man. Discovery : W.C. Pei (1924) discovered the fossils of Peking man form the l ime stone caves of Choukoutein near Peking (Bijing – capital of China was formerly known as Peking) and named them Sinanthroupus. Davidson Black (1927) named ist Sinanthropus pekinensis. Mayer (1950) renamed it as Homo erectus pekenensis (a subspecies). The Pleistocene rocks from which fossils of Peking man were excavated are about 6 lakh years old.
Characteristics : Placing Java ape man and Peking man as subspecies of Homo erectus has a sound basis, because of close similarities between these. The body structure was quite similar in both. Being about 1.55 to 1.60 metres tall, Peking a\man was slightly shorter and a little lighter and weaker. The only noticeable difference of Peking man from Java ape man was its large cranial capacity, which ranged from 850 to 1100 cc. Like Java ape man, the Peking man was omnivorous and cannibal .There is a clear evidence of use of fire by it. It has been confirmed that both Java and Peking men used to live in caves in small groups or tribes. The tools of Peking man were relatively more sophisticated.
Heidelberg man. Discovery : In 1908 one of the most perfect fossil jaws belonging to middle Pleistocene was found by workmen working near Heidelberg, Germany. It was shown to Otto Schoetensack, who gets the credit for its discovery. It was named Homo erectus heidelbergensis.
Characteristics : It had lower jaw with all the teeth. The teeth were human like. The massive jaw was ape-like. He used the tools and fire. The cranial capacity is believed to be about 1300 cc; which is intermediate between those of erect man (H. erectus) and Neanderthal man (H. sapiens neanderthalensis ). Thus, it is regarded as intermediate between pithecanthropines and Neanderthalers.
C. True Men Including the Living Modern Man
(1) Neanderthal Man (Homo sapiens neanderthalensis ). Discovery : Fossil of Neanderthal man were first obtained from Neanderthal valley in Germany from the late Pleistocene epoch by C. Fuhlrott (1856). Later, many fossil were excavated in various countries by different paleontologists.
Characteristics : He had slightly prognathous face. Neanderthal man walked upright, as we do, and had low brows, receding jaws, and high domed heads. If there was anything truly different 27
about them, it was that they were much stockier than we are. The cranial capacity was 1,300 to 1,600 cubic centimeters. Neanderthal man existed half a million years ago, but was most numerous from about 100,000 years ago. Became extinct 30,000 years ago. Neanderthal man was the legendary cave dweller, having heavy brow ridge and huped back.
He was adapted to a cold environment, who encountered a succession of glaciers that passed over most of the northern temperate regions of the world. He was not only skilled hunter but true predator, a specialization that did not occur among hominids before or after them. The Neanderthal man was cannibal and fashioned the skin into clothing to protect himself against the harsh climate. Natural caves became campsites that were illuminated and heated by fire. It is believed that he buried his dead with flowers and tools. The may have had a religion.
It is usually considered that Homo sapiens neanderthalensis did not evolve into Homo sapiens.
(2) Cro-magnon man (Homo sapiens fossils). Discovery : It has been known as cro-magnon man, because its fossils were first discovered in 1868 form Cro-Magnon rocks of France by MacGregor. Cro-Magnon man emerged about 34,000 years ago in Holocene epoch. Thus, he is regarded as most recent ancestor of today’s man.
Characteristics : The Cro-Magnon man had, like us, about 1.8 metres tall, well-built body. His face was perfectly orthognathous with an arrow, elevated nose, broad and arched forehead, moderate brow-ridges, strong jaws with man-like dentition, and a well developed chin. His cranial capacity was, however, somewhat more than outs, being about 1620 c.c. It is , therefore, believed that Cro-Magnon man was somewhat more intelligent and cultured than the man of today. It could walk and run faster and lived with families in caves. It made excellent tools and even ornaments, not only of stones and bones, but also of elephant tusks. Its tools included spears, bows and arrows, as he was omnivourous. Use of the Skin clothes by this man is also confirmed. A number of cave paintings done by Cro-Magnon man have been discovered. The Cro-Magnon man was the direct ancestor of the living modern man. Prehistoric cave art developed about 18,000 years ago.
(3) The Living Modern Man (Homo sapiens sapiens). Discovery : Further evolution of man after Cro-Magnon involves the evolution of culture rather than of anatomy. Homo sapiens sapiens appeared about 25,000 years ago in Holocene epoch and started spreading all over the world about 10,000 years ago. Agriculture came around 10,000 years back and human settlements started.
MODERN HUMANS
Homo sapiens
The evolutionary journey to modern humans ends with the appearance, about five hundred thousand years ago, of Homo sapiens (“wise man”), our own species. We are newcomers to the human family-H. sapiens has not been around nearly as long as H. erectus was. Still. Humans have changed quite a bit, since those first days.
Out of Africa- Again
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The origin of human races is a much-debated point among scientists studying human evolution. Many argue that the difference races evolved from H. erectus independently, and that each adapted to a different place-Orientals in Asia, Caucasians in Europe, Aborigines in Australia, and so on. Others believe that the same species would be unlikely to evolve more than once and argue that human races appeared after H. sapiens evolved from H. erectus. Recently, scientists studying mitochondrial DNA from living humans all over the world have argued that their research shows that all human races originated from one H. sapiens ancestor in Africa. Scientists looked at mitochondrial DNA to study evolution, because the DNA within mitochondria is transmitted only by females. Females eggs carry many mitochondria within them that become part of a new baby, while sperm contribute no mitochondria a to the new baby. Sperm carry their mitochondria wrapped around their tails and so do not inject them into the egg during fertilization. For that reason, particular versions of a mitochondrial gene can be traced back through a family tree, fro, mother to grandmother to great-grandmother.
Human races evolved only recently in the evolutionary scale of things, and there has not been enough time for many mitochondrial DNA difference to accumulate, so the exact human tree cannot be reliably traced using this approach. So far, however, the greatest number of different mitochondrial DNA sequences occur among modern Africa. Since DNA accumulates mutation over time, the oldest DNA should show the largest number of mutations. This result thus argues, that human have been living in Africa longer than on any other continent. While researchers are not in complete consensus, this line of investigation appears to suggest that H. sapiens evolved in Africa and that the human races evolved after that, and not independently from separate species of H. erectus. If this is correct, then H. sapiens was born in Africa and from there spread to all parts of the world, retracing the path taken by H. erectus half a million years before.
Homology in Chromosomes of Man and Great Apes
The somatic cells of humans contain 46 chromosomes (44 autosomes and 2 sex- chromosomes). Human chromosomes are usually obtained by culturing certain types of white blood cells from the peripheral blood. They can then be treated with specific stains to produce characteristic bands along the length of each chromosome. The pattern of banding so obtained is unique for each pair of chromosomes. Banding techniques enable the identification of individual chromosomes and their parts. The diploid number of chromosomes in gorilla, chimpanzee and orangutan is 48. Comparisons have been made between banded chromosomes of man and those of the great apes. The total amount of DNA in human diploid cells and that of the great apes are not dissimilar. But what is most interesting from an evolutionary viewpoint, is that the banding pattern of individual human chromosomes is very similar and is some instance, identical to the banding pattern of apparently homologous chromosomes in the great apes. Diagrammatic representations of the banding pattern of human chromosome numbers 3 and chimpanzee. This remarkable similarity in the fine structural organisation of the chromosomes is understandable only in terms of a common origin for man and chimpanzee.
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