Evolution: Theory and History of Evolution, Origin of Life, Evidences of Evolution

“It is not the strongest of the species that survives, nor the most intelligent that survives, it is the one that is most adaptable to changes”
~ Charles Darwin

“The phenomenon of creation has two aspects namely 0 Origin of life and (in Origin of different forms of life”

ORIGIN OF LIFE

Life has originated on the earth a long time after origin of earth itself. It is believed that the earth had its origin about 4.8 billion years ago. The earth has been derived from some other heavenly body either in the form of hot gaseous material (Nebular hypothesis) or as a molten mass (Planetesimal hypothesis). Then it cooled slowly through hundreds of millions years and a solid core was formed. In course of time rain water filled the depressions to form the oceans and small water deposits. At that time both land and water were so hot that existence of life was not possible. So life must have been originated much later on further cooling of earth surface.

HISTORICAL THEORIES OF ORIGIN OF LIFE

Some of the earlier views about the origin of life are as follows

1. Theory of Special Creation. 

Until middle of nineteenth century it was believed that life was created by some supernatural divine power. According to Hindu mythology, the world and its plants and animals were created by God “Brahma”. According to Christian mythology, as put forwarded by Father Suarez, the earth and heaven were created on first day; sky on the second day; plants and animals on the third day; sun, moon and stars on the fourth day; birds and fishes on fifth day and finally man and beasts on sÍxth day. At the end, on seventh day a woman was created from the 12th rib of man. In other mythological books similar descriptions have been given regarding origin of life but such ideas lack any scientific truth.

2. Theory of Panspermia (Cosmozoic Theory). 

This theory suggested by Richter (1865) states that protoplasm (Cosmozoa) in the form of resistant spores of some simple organisms might have reached the earth (Cosmos) accidentally from some other planets of the universe So far, there is no evidence of existence of life in any other celestial bodies. Hence, this theory is not acceptable.

3. Theory of Spontaneous Generation. 

According to this theory different living organisms have come up all on a sudden from some non living substances at different times. Worms and tadpoles arise from mud ; flies from decaying organic matter; butter flies from cheese, snakes from horse-hair and so on were bel’eved until seventeenth century. In 1668, Francesco Redio and showed the emergence of flies from rotten flesh from the eggs laid on it but not directly from the rotten flesh.

4. Theory of Biogenesis.

 Accord ing to Abbe Spallanzani (1729-1799) Louis Pasteur (1822-1895) life originates from pre-t xisting living beings’. Hence, the sudden creation of life from lifeless matters as suggested in the previous theory is totally baseless. However, this theory could not explain the origin of initial form of life.

ORGANIC EVOLUTION

The word EV0LUTION is derived from a Latin term ‘EVOLVERE’ which means to unroll. Just as a rolling ball changes its position from time to time, the living and non-living substances change themselves with respect to time, Changes in relation to plants and animals come under “Organic Evolution.” According to Darwin, Organic Evolution is defined as “The descent with modification”. At present, more than one million species of animals exist on the earth and they are modified or changed descendants of somewhat different animals which lived in the past.

Chemical Evolution

At the time of origin of earth, its temperature was 5000 to 6000°C and the elements were in their atomic state. As it cooled gradually the heavy metals like iron and nickel formed the central core, lighter elements like aluminium and silicon formed the middle core and lightest elements like hydrogen, helium, oxygen, nitrogen and carbon formed the outermost surface.When the temperature of earth surface cooled down to 1000°C or even less, highly reactive free radicals, CH, CH2, CN (Cyanogen), metal carbides and metal nitrides were formed. It was a reducing atmosphere without free Oxygen.

(I) Formation of Hydrocarbons. CH and CH² underwent condensation and formed many types of saturated and unsaturated hydrocarbons.

CH+CH →CH CH (Acetylene)

CH²+ CH2 →CH2= CH2 (Ethyene)

CH2+CH2→ CH4+C (Methane)

2CH4→.  CH =CH+ 3H2 (Acetylene)

(ii) Formation of Oxy & Hydroxy Derivatives. 

These hydrocarbons could react with super heated steam to form oxy and hydroxy derivatives such as aldehydes, ketones and acids.

CH= CH+H20→ CHCHO (Acetaldehyde)

                oxidation

2CH,CHO ——-→ CH³COOH+CH³CH²OH

                                  Acetic acid     Ethyl alcohol

                                     polymerisation

CH COOH+CH CH,OH ————→CH³COOCH²CH3 + H20

                                                                         Ethyl acetate

                                  condensation

CH³CHO+CH³CHO————-→CH³CHOHCH²CHO

                                                                            Aldol

(iii) Formation of Carbohydrates, Proteins & Fatty Acids. The simple hydrocarbon molecules along with ammonia and water molecules and their derivatives must have undergone condensation, polymerization, oxidation and reduction and must have given rise to complex sugars, amino acids, peptides, proteins, purines, pyrimidines and fatty acids.

(iv) Hot Dilute Soup. According to Haldane the synthesis of carbohydrates, polypeptides, fatty acids alongwith a variety of other complex organic compounds must have taken place in sea water. Now the sea water along with these compounds was named as hot dilute soup ; the raw material for origin of life.

(v) Formation of Macromolecules. The simple organic substances of hot dilute soup must have come together and reacted among themselves forming new large molecules. In this way more complex organic molecules like polysaccharides, fats, proteins, nucleotides and nucleic acids might have been formed.

(vi) Formation of Nucleoproteins. The aforesaid organic molecules have tendency to aggregate. As such, the nucleic acid molecules must have combined with protein molecules to form nucleoproteins. Formation of nucleoprotein is the boarder line state between living and non-living matter.

All these events upto formation of nucleoproteins and other macromolecules are included under chemical evolution. The events occuring after this can be described under biological evolution.

Biological Evolution

Nucleoproteins had the unique potentiality to multiply by self replication in a suitable environmental condition. This would have led to the following events resulting the origin of first cell.

(I) Formation of Coacervates. Due to condensation and polymerisation of macromolecules in the hot dilute soup, a number of macromolecules could have aggregated forming large well-organised structures called Coacervates These are distinct globular bodies surrounded by fatty acid molecules of lecithin and cephalin forming a boundary membrane. Oparin has named them as “living molecules”.

(ii) Formation of Primary Organism.

The formation of the outer limiting membrane around coacervates did not allow them to mix with surrounding water. Inside the membrane various macromolecules like proteins, enzymes, nucleoproteins and other organic and1

inorganic particles were highly organized. Independent biochemical reactions started among the macromolecules with the help of enzymes (specialised protein molecules). Some of the raw materials required for internal chemical reaction could be absorbed through the membrane from surrounding environment. Nutrients could be collected and deposited in the form of shells.

(iii) First Living Cell.

The coacervate with nucleoprotein at the core, surrounded by nutrient shell and finally covered by membrane was a free living cell similar to a virus. They were named as Protovirus.

(iv) Early Cell Types. Further reorganization of protovirus led to development of the following two types of organisms

(a) Monerans. In this cellular type nucleoproteins aggregated together in a single nucleoprotein core embedded in nutritive cellular substance. These were the ancestors of modem bacteria and blue green algae.

(b) Protistans. In this type the nucleoproteins condensed in the centre was surroundednby a nuclear membrane. Thus the cell substances remained separate from central nucleus. This was a primitive eukaryotic cell with distinct nucleus and cytoplasm covered over by outer limiting plasma membrane. The origin of living cell is the most surprising and dramatic phenomenon which was ignited some 2000 million years ago. Since then evolution of different forms of life has occurred and has formed one million types animals that we find today on earth.

MILLER’S EXPERIMENT

Experimental evidence in support of biochemical theory of origin of life was demonstrated by Stanley Miller (1953) and Prof. Harold Urey at University of Chicago. Miller devised an apparatus simulating an early primitive atmosphere when synthesis of macromolecules must have started, The apparatus consists of a chamber for electrical discharge, another chamber for producing water vapour and two conducting tubes connecting both chambers. Miller took a mixture of methane, ammonia, hydrogen and nitrogen in the discharge chamber and allowed water vapour to mix with the mixture by constantly heating water chamber. The mixture was subjected to high voltage electric discharge by two electrodes incorporated into the discharge chamber. The process continued for a week and finally the products were collected from the U shaped tube of the apparatus. It was a condensed dark coloured solution and on chemical analysis the compounds identified were amino acids like glycine, alanine, aspartic acid and glutamic acid; a number of other organic compounds and gases 

Like CO and CO². According to Miller, the electric spark is similar to lightening and the mixture of gases are similar to primitive earth’s atmosphere. Thus, he postulated that simple molecules could nave given rise to complex life forming molecules as they were formed in the apparatus

The following conclusions can be drawn from Miller’s Experiment

1. it is possible that in the early climate conditions of the prebiotic earth, organic molecules might have been formed in a similar manner

2. These organic molecules might have collided, reacted and accumulated at several places forming a “Primeval broth” or a “Prebiotic soup” This soup might have contained complex organic substances such as polysaccharides, proteins, fats and nucleoproteins.

OPARIN-HALDANE THEOORY

The modern hypothesis of Origin of life was formulated by Haeckel which states that, The most primitive organism would have been generated from some inorganic matter due to physical forces such as electrical discharge, ultraviolet light and radiations of radioactive elements”

Russian biochemist, Alexander Ivanovich Oparin presented his biochemical hypothesis of origin of life in his book “The Origin of Life on Earth” in 1924. Similar views were also put

forwarded by an English biologist, J.B.S. Haldane in 1929. Experimental evidence in support of chemical evolution was provided by Stanley Miller and Prof. Harold Urey in 1953. The present accepted theory of origin of life is based upon the significant contributions of these workers and is known as biochemical origin of life. This theory is divisible into two major steps such as chemical origin of life and then biological origin of life

What is evolution?

Evolution is the branch of biology which deals with study of origin of organisms and their modification through ages. The theory of evolution is based on the idea that all species are related and gradually change over time. So in other words we can say evolution is the change in the characteristics of a species over several generations and relies on the process of natural selection.

What is the History of evolution?

In the early 19th century Jean- Baptiste Lamarck (1744-1829) Proposed the history of the transmutation of species, the first fully formed theory of evolution. In 1858, Charles Darwin and Alfred Russell Wallace published new evolutionary theory, explained in detail in Darwin’s “On the Origin of Species”. Unlike Lamarck, Darwin proposed common descent and branching tree of life, meaning the two very different species could share a common ancestor. Darwin based his theory on the idea of natural selection, but this mechanism was not widely accepted until it was revived by developments in biology that occurred during the 1920s through the 1940s.

HISTORICAL DEVELOPMENT OF THE CONCEPT OF EVOLUTION

1.According to Hindu mythology, the body of all living beings are made up of five elements or “Pancha Bhuta” namely Kshiti (earth, ap (water), teja (fire), marut (air) and vyoum (sky).bFurther, the concept of “Dashavatara” as explained in “Padmapurana” has described the evolution of vertebrates through ten incarnations and the sequence of “Avataras” includes Matsya, fish like appearance. Kurma, reptile ikce, araha, mammal like, Narasimha, half man halflion, Vamana’ the dwarl man (anthropola, Parsurama’, a complete man-Homo sapiens and ‘Shri Rama’, ‘Balarama, Sri Krishnd ana ‘Kalpr, the last four incarnations depict the formation of clans and communities. dmilar ancient Chinese, Egyptian and Babylonians speculations are found in classical mythologies.

2. Ancient Greck Philosopher Thales (624-548 B.C.) thought life has originated and evolved from sea water.

3. Another contemporary Greek Philosopher Empedocles (495-435 B.C.) put forwarder idea that life arose spontaneously. The body parts ana organs developed independently. There were two forces of nature such a love and hate, The body parts were united at random here two opposite forces and led to creation of monsters. Most of them could not survive becaue their imperfect organization, but a few of them were well organized and survived. Those survived, reproduced and produced their offsprings. This theory provides basic idea of natural selection. Empedocles has been accepted 1s ‘Father of evolutionary ideas.

4. Aristotle (384-322 B.C.) believed that the internal perfecting tendency of organism drives them towards more complex and pertect organization. As a result, simple organism change into well organised forms with passage or ume. He arranged the animals in graded series with man at the top.

5. After a long gap during medieval age again evolutionary ideas were suggested by Bacon (1561-1626), Bonnet (1720-1793), Kent (1724-1808), Oken (1776-1801). Francis Bacon emphasised on variations as the cause of origin of species from old ones. Bonnet proposed theory of performation’ which means that the ova contain a miniature of the adult in preformed state.

6. Linnaeus (1707-1778), Buffon (1707-1780) and Erasmus Darwin (1731-1802) of 18th century were well-known for shaping the ideas uf evolution. Linnaeus believed in theory of special creation and is best known for nis scientific classification of plants and animals. Buffon Suggested that the direct influence of environment modifies the structure of plants and animals and such modifications are preserved through heredity. Erasmus Darwin suggested that modifications arise by the reaction of organism with the environment and these modifications are transmitted to their off-springs.

7. Lamarckism. Jean Baptiste Lamarck (1741-1829) for the first time proposed a scientific evolutionary theory in 1809 published in a book “Philosophie Zoologique”. His theory popularly known as Lamarckism is based on the following propositions:

(i) Influence of Environment. The environment on earth changes constantly which provides force for change in the body of the organism. New body or organs arise in the organism as a response to adapt in the changing environment.

(ii) Use and Disuse of Organs. Body parts or organs used frequently increase in size and are better developed.

On the other hand, less use of an organ leads to its gradual degeneration and loss. So development of an organ is directly proportional to its use.

(iii) Inheritance of Acquired Characters. The changes which appear in the organism due to environmental influence or due to use and disuse are called acquired characters. modifications acquired during Iife time are transmitted to offsprings through heredity and are preserved.

Examples. The development of long neck and long forelimbs in giraffe, loss of limbs in snakes and webbed toes of aquatic birds were explained by Lamarck through his propositions

8. Cuvier (1769-1832) advocated, “Theory of Catastrophism” which states that the organisms are destroyed due to periodic catastrophs basing upon the fossils available. Each destruction is followed by new creations, thus originating new species

9. Charles Darwin (1809-1882) in his book “Origin of Species” in 1859 has stated the origin of new species through natural selection. His theory, popularly known as Darwinism is based upon his observations and the following deductions

(I) Over production, 

(ii) struggle for existence 

(iii) variation and heredity, 

(iv) survival of the fittest, 

(v) natural selection and origin of species.

10. Before publication of Darwin’s findings in 1959, another English Naturalist Alfred Russel Wallace (1823-1913) presented his findings in his manuscript. “On the tendency of varieties to depart from the original type. The facts described in this paper were exactly identical with those of Darwin.

11. Ernst Haeckel postulated “Recapitulation Law” in 1866 which can be briefly stated as Ontogeny (life history of an individual) recaptulates (repeats) phylogeny (the evolutionary history)”. His theory provides embryological evidence in support of organic evolution.

12. August Weismann proposed Germ Plasm theory’ in 1904 which has corrected certain fundamental misconceptions in earlier evolutionary ideas. According to this theory, the body of plants and animals are composed of two types of cells namely somatic cells (somatoplasm) and germinal cells (germplasm) ne gemplasin proauces male and female gametes which transmit characters of parents of offSpings. ACquirea Characters of Lamarck are somatic changes and are not inherited but changes occuring germplasm can be transmitted to offspring.

13. Mutation theory postulated by Hugo de Vries  in 1905 for the first time explained how new characters (variations) appear in an organism. He stated that by sudden change in chromosomes or genes, significant changes could appear in an organism which could be inherited. It was based on his experiment on evening primerose plant (Oenothera lamarckiana). He noticed strikingly different forms appearing all of a sudden among a population of normal plants. Such variations were named as mutations and are inherited to their offsprings. If such variations are selected by nature and preserved generation after generation, a new species could originate.

14. Modern synthetic theory is the present concept of organic evolution. This is also known as ‘Neo-Darwinism which is accepted by and large and is the outcome of contributions made by many naturalists and biologists. Some significant contributors are Theodosius Dobzhansky  (1937), Julian Huxley (1942), Ernst Mayer (1942), G.L Stebines, Simpson (1951), B. Rensch, Muller, J.B.S. Haldane, R.A. Fischer, Sewell Wright and so on. This theory recognizes five basic processes that operate collectively during the processes of evolution. These are 

(i) gene mutations, 

(ii) chromo -somal mutations

(iii) genetic recombinations, 

(iv) natural selection and 

(v) reproductive isolation. 

These are assisted by three additional processes namely 

(i) migration 

(ii) hybridization and 

(iii) genetic drift.

Evolution is an ever continuing process and is occurring more rapidly to-day than in the past ages. Thousands of species of plants and animals have become extinct during the course of evolution and much more numbers of new species have come up in their place. The present discussion establishes the fact that around one million species of plants and one milion species of animals found to-day on earth have come into being by organic evolution and in the

Foregoing chapters we would be still more convinced after studying the evidences in favour of organic evolution.

Types of evolution

1. Progressive evolution: Formation of more complex forms from simple ones.
   e.g. Evolution of amphibians From fishes
2. Retrogressive evolution: Formation of simpler ones from complex forms.
   e.g. Evolution of tunicates.
3. Convergent evolution: Two or more unrelated species become more and more similar in appearance as they adapt to the same kind of environment.
   e.g. Dog fish and whale, wolf and Tasmanian wolf, scaly ant eater and spiny ant eater etc.
4. Parallel evolution: convergent evolution in closely related species.
   e.g. Running habit in deer and horse.
5. Divergent evolution: Members from common ancestor become more and more dissimilar due to development of different functional structures in different environment.
   e.g. Darwin’s finches.

Origin of life : Origin of universe:

Study of universe is called cosmology. It constitutes number of heavenly bodies like stars, planets and satellites etc. Our universe is formed of matter and energy. It is about 20 billon years old. The most accepted theory for origin of universe is “Big-Bang theory” proposed by Abbe Lemaitre in 1931. According to this theory the universe originated by a thermonuclear explosion (big bang) of a dense entity. As the universe expanded, the temperature came down . The gases condensed under gravity and formed the present day galaxies. Our solar system is present in the galaxy called “Milky Way” or ” Akash Ganga”.

Origin of Earth and origin of life in our planet:

Our earth is about 4.5 billon years old. Life appeared on the earth about 4000 million years back i.e, 500 million years after the formation of Earth. There was no atmosphere in the early earth. Water vapour, methane, carbon dioxide and ammonia released from molten mass covered the surface. Water was splitted into hydrogen and oxygen by the UV rays from the sun. Oxygen combined with ammonia and methane and formed water, carbon dioxide, etc. As the atmosphere cooled, the water vapour fell on earth surface as rain drops. When the earth surface was cool enough to hold water, oceans were formed and life was originated in the ocean.

Evidences of evolution

If evolution has occurred, there should many anatomical similarities among varities and species that have diverged from a common ancestor. The evidence for evolution come from the following sources.

A. Evidence from morphology and comparative anatomy:

It is the study how different species can have similar structures.

• The Flipper of a seal, wings of bat, hands of man forelimbs of a cheetah have different functions but same general structure plan.
• Auricle and ventricle are common to heart of the vertebrates like fishes, amphibians , birds and mammals.
• Wings of insects, bats, petrodactyl and Wings of insects, bats, petrodactyl and biretc. All these wings differ in their origin and structure but are adapted for common function i.e, flying.
• Sudden appearance of some ancestral features; a tail in a new born, additional nipples in man etc.

B. Evidences of embryology:

Some fundamental similarities have been observed in the embryonic stage of various animals. All multicellular organisms are developed from a single cell called zygote. They pass through common embryonic stages like morula, blastula and gastrula etc.

EMBRYOLOGICAL EVIDENCES

The sequences of embryonic development is from single celled zygote to blastula through cleavage followed by gastrulation, formation of three germ layers and organogeny. These early stages are strikingly similar in varied groups of metazoans. All multicellular organisms start their development from a unicellular zygote comparable to unicellular protozoans. Similarly early divisions of eggs by cleavage can be compared to phylum porifera. Double layered gastrula is similar to diploblastic colenterata (Hydra). Formation of ecto, meso and endoderm in the next stage is similar to triploblastic acoelomates-the platyhelminthes. Thus, it would be observed that during development ot any the embryo passes through the typical structure of invertebrate phyla in a sequence an Protozoans to higher groups as in the evolutionary ladder. Even the development follows the same pattern. The nearer the relationship in the adult structure, the greaters to similarnty 1n the course of development. During later development, different groups aner Another interesting feature is that when embryos of various vertebrates are comparea, the embryos or higher groups are found to resemble the adults of the lower group. For Example the tadpole of frog resembles the adult fish.

Biogenetic Law. These observations has led Ernst Haeckel to formulate “Biogenetic Law in 860. 1nis law is otherwise known as “The Theory of Recapitulation which states the “Ontogeny (developmental history) recapitulates (or repeats) Phylogeny (ancestral history This means that an organism during its development repeats its ancestral history. he development of man can be taken as an example to justify the theory. The fertilized egg or human being is comparable to unicellular protozoa; gastrula represents the Coelenterate development of mesoderm comparable to flatworm stage (platyhelminthes) development of coelomic cavity in mesoderm can be compared with annelid stage. Early human embryo with dorsal hollow nerve cord, dorsal well developed notochord and a series of pharyngeal gill slits represent he fundamental chordate characters. With development of two chambered heart paired aortic arches and a tail it resembles the fish embryo. Later it somewhat resembles the reptilian embryo and finally develops the mammalian characteristics. During 7th month of gestation the human embryo resembles a baby ape. This proves the recapitulation theory and provides evidence in support of organic evolution. Modern biologists are talking of “biochemical recapitulation” ie, during embryonic development the chemical compounds formed in the body show a gradation from lower to higher complexity and show their resenblances with lower forms. For example, fishes excrete ammonia; adult amphibians urea, but their tadpoles excrete ammonia like fishes. Birds excrete uric acid, but their embryos excrete first ammonia, then urea during earlier stages and finally uric acid. Along with providing support in favour of organic evolution, this also suggests how from simple organisation life has gradually evolved more and more complex systems seen in higher animals.

PALAEONTOLOGICAL EVIDENCE

The first two evidences are only circumstantial whereas the present evidence is one direct evidence in support of organic evolution. Palaeontology is defined as a branch of biology dealing with the study of Fossils, Preserved remains or imprints of ancient plants and animals found in the layers of the earth crust are known as fossils. (Lt.fossilium = something dug out).Four general types of fossils are found such as- 

(I) Original fossils-Hard parts like teeth skeleton or shell which are not very old. 

(II)Frozen fossils-Whole body of extinct organisms burried under permanent massive ice deposits without any change (Ex. Wooly Mammoth of Siberia) 

(III) Petrified fossils-Less permanent parts preserved by replacement of their organic parts by mineral deposits and subsequently being converted into rocks, 

(iv) Casts, Moulds and Prints-Wings, skin, leaves etc. leaving their impressions in mud or soft soil which

Subsequently hardened to form fossil.

The earth’s crust is made of inner core OF Igneous rock and outer layer of sedimentary rocks. Mostly fossils are found in sedimentary rocks. this rock has been formed by deposition of sand and mud on the bottom of lakes Or seas over milions of years. The time of formation of each layer of rock has been scientifically calculated. Accordingly Geological Time Scale has been prepared since the origin of life till date. The age of a fossil also can be calculated scientifically. As such, the time of existence Or an exinct species and its biological details can be known.

The fossil records establish the following facts

(I) The time of origin of a species, its course of diversification and its extinction can be known alongwith its habit, habitat, structure, ecology etc.

(ii) The fossils from lower strata are older and simpler in organization in comparison to fossils from higher strata.

(iii) Transitional fossils form connecting links between different groups such as from fish to amphibia, reptiles to birds and reptiles to mammals. For example the Archaeoptery fossil showing both reptilian and avian characteristics is a connecting link between reptiles and birds confirming the evolution of birds from reptiles.

(iv) Fossils exhibit that dominant groups arose near the close of a period when great climatic changes were taking place. These groups enjoyed dominance in the next period because of favourable environmental conditions and finally perished due to again alteration of climate, thereby giving rise to new forms. Thus the fossils provide an accurate explanation for the mechanism of evolution.

The complete fossil records of some mammals like horse, camel, elephant including man have become available, When they are arranged in relation to eras and periods (time), they present a complete series indicating how their evolution has occurred from simpler forms. The best known and most complete fossil record available so far is that of those. The ancestry of horse can be traced since Eocene period (60 million years ago) when it originated from a five-to ungulate eohippus. It was of the size of a fox with 30 cm. in height, four digits in the fore-limbs and three in hind-limbs. It has 44 short crowned teeth suitable for feeding upon soft vegetation. With change of environment into grassy plains in Oligocene period (40 million years ago), the Eohippus evolved into Mesohippus and then into Miohippus which were suited to live in drier conditions. The structural modification in them were towards attaining speed and for grazing on grass, In Miocene (25 million years ago) these were replaced by Merychippus and in Pliocene (12 million years ago) by Pliohippus. Finally in Pleistocene (1 to 2 million years ago) it was evolved into Equus, the modern horse ; with on one digit touching the ground adapted for fast running. This direct evidence suggests beyond doubts that most present day higher plants and animals are of recent origin and compared to total geological time, recent means, only a few hundred thousands years.

EVIDENCE FROM TAXONOMY

Taxonomy is the science of classification of organisms This is a system of grouping the animals with common characteristics into one category. Accordingly the whole animal kingdom is divided into ten major groups known as Phyla. The members of each phylum resemble one another with respect to some fundamental characters. Again, basing upon close similarities, the members of each phylum are further down-graded into classes, orders, families, genera and finally each type is named as a species. The closeness of a group of animals is examined by their common habit, habitat, distribution, morphology, anatomy, physiology and embryology. Had there been no evolution, the animals would have been completely dissimilar and there would not have been any closeness among them. Taxonomy indirectly proves that closely similar animals must have originated from some common parental stock and through organic evolution they are diversified into different species.

EVIDENCE FROM BIOGEOGRAPHICAL DISTRIBUTION

Some species are worldwide in distribution but others are restricted to one or more limited areas. The discontinuous distribution of animals can be explained through organic evolution. For example, Lung-fishes are found in South America, Australia and Africa but not in intermediate regions. Another example is that Madagascar island is only 260 miles from east Coast of Africa but its fauna and flora are markedly different from those of Africa. The first example can be explained by the fact that it had wide range of distribution in the past but due to exinction in the intermediate region its distribution has become discontinuous. In respect of the second example it can be said that once Madagascar was a part of Africa and there was uniform distribution. But later Madagascar was cut off from the main land by extension of sea. So, the plants and animals were isolated for long periods and were subjected to different climatic conditions. Accordingly evolution occurred in different lines establishing different species.

It has been observed that flora and fauna of región of similar climatic conditions significantiy differ ifsuch regions are widely separated by geographical barriers. The example is the plants and animals of South America and Africa. AI these facts provide convincing evidence in support of organic evolution.

Evidences from biogeography:

Fossils of Africa and South America- some fossils are common to both Africa and South America. However plants and animals of these two places now differ. The Africa now supports old world monkeys and giraffes etc. While the South America supports new world monkeys and jaguars. It explains that these land masses were once joined and species of that original land mass had been dispersed to different parts and adapted to the respective environment

BIOCHEMICAL EVIDENCE

Uniformity in physiological and biochemical organization in the cellular and subcellular levels can be interpreted through organic evohuticn.

(i) The protoplasın, nucleic acids (D.NA. and RNA.), Chromosomes, A.T.P. (the biological energy nolecule), Trypsin (protein digesting enzyme), Amylase (carbohydrate splitting enzyme) are uniform from Protozoa to Mammalia.

(ii) The plasma proteins of blood provide most convincing evidence for establishing closeness between varied animals. The degree of similarity can be tested by antigen

antibody reaction. When the immunized blood serum of man is fixed with the serum of monkey, sheep, dog, cat, bird, reptile, there will be different amounts of precipitation If the precipitation is more, there is close afinity between the two.

The direct and indirect evidences have established the validity of biological evolution beyond doubt and confirm the mechanism of evolution as explained by modern syntheic theory.

Connecting link &Missing link: The animals which occupy intermediate phylogenetic position between two different groups of animals are called connecting links. Such connecting organisms may be existing in living condition till now or they might have become extinct. Such extinct forms are made available in fossil state and these fossil connecting links are called Missing links. Both of them provide good evidence in support of organic evolution

because they serve as intermediate transitional connecting forms and establish continuity in evolutionary ladder. A few examples of connecting/missing links are as follows:

1. Euglena-Connecting link between plant & animal kingdom.

2. Peripatus Connecting link between Annelida &Arthropoda.

3. Prototheria-Connecting link between Reptiles & Mammals.

4.Archaeopleryx-Missing link between Reptiles & Birds.

What is theory of evolution?

Theories of evolution: Lamarck and Darwin ; two divergent vision

The theory of evolution is the shortened form of the term ” theory of evolution by natural selection” which was proposed by Charles Darwin and Alfred Russell Wallace in the 19th century. Other than that there is also another concept of revolution. We can say early concept of evolution which was proposed by Jean Baptiste de Lamarck. So let’s discuss about both of these theories and their contributions.

THEORIES OF EVOLUTION

CONCEPT OF ORGANIC EVOLUTION

The lirst problem of creation i.e., how the life is originated has been dealt in the previous section. The second problem ofcreation is how different forms oflife have come into existence from very simple primitive form of life. The present chapter is devoted to explain the mechanismof organic evolution.

Herbert Spencer first used the term ‘evolution’ in the field of Biology which refers to the development of more complex forms of life from simpler and earlier forms. The term ‘Organic is used to distinguish the evolution of organisms from the evolution of non-living systems

Facts about Evolution

1. All present day plants and animals have descended from pre-existing forms. (A Spallanzani)

2. The present day complex forms oflife have developed from simpler and earlier forms. (H. Spencer)

3. Evolution involves gradual sequence of changes from simpler to more complex state. (j M. Savage)

4. Evolution is an unimaginably and incredibly very slow process. (.B.S. Haldane)

5. Evolution is “descent with modification'” (Darwin)

Lamarckism: Theory of inheritance of acquired characters

French philosopher and naturalist Jean Baptiste de Lamarck is one of the most pathetic figures of the evolutionary history. He was the first man who gave a complete theory to explain the mechanism of organic evolution. Lamarck thought that new forms arise from pre existing forms. To explain this he published his theory of “Lamarckism” or “Theory of inheritance of acquired characters” in his book ” Philosophie Zoologique” in 1809.

LAMARCKISM

Jean Baptist de Lamarck (1741-1829) was a French naturalist. He was born on Ist August, 1741. Hewas trained for priesthood, commissioned as a lieutenant and was retired on account of ill health. In 1809 Lamarck elaborated his doctrine of evolution in his master piece “Philosophic Zoologique”. In spite of his impaired sight, Lamarck wrote Histoire naturelle de animaure sans vertebrate. Lamarck became blind in his later years and died blind in Paris on Dec. 18, 1829.

LAMARCKISM (or Inheritance of Acquired Characters) Lamarcks theory of evolution was published in “Philosophic Zoologique” in the year 1809. It is popularly known as “The inheritance of acquired characters in organisms” and comprises of four assumptions or propositions.

(a) New needs. Every living organism is found in some kind of environment which 15 always working on it, The changes in the environmental factors like light, temperature, medium, food, air etc. or migration of animal from one area to another area leads to the origin of new needs. The living organisms have to exert special efforts like the changes in the habits or behaviour accordingly.

(b) Use and Disuse of organs. The new habits involve the greater ans use areatly of certain affects other the organs which are of no use in new conditions. This use and disuse ot orgas ao the form, structure and functioning of the organs. Continuous extra us ir deueneration and them more encient while the continued disuse of some other organs lead to their degeneraion and ultimate disappcarance.  So the organisms acquire certain new characters due to direct and indirect environmenta effect during its own life span and arc called acquired characters.

(C) nherilance of acquired characters. Lamarck believed that acquired characters are inheritable and are transmitted to the offspring; so that these are born fit to face the changed environinent conditions and the chances of their survival are increasea.

(d) Speciation. Lamarck believed that in every generation, new characters are acquired ana transmitied t0 next generation, so that new characters accumulate generation alter generation. Alter a number of generations, a new species is fornied. So according to Lamarck, an existing individual is the sum total of the characters acquired by a number of previous generations and the specification is a gradual process.

Evidence in favour of Lamarckisma

1. Ancestors of modern horse lived in soft ground in jungles. There werc browsers and had plantigrade foot posture. When thick jungles were replaced by dry grassy plain, these changes in the environment were accompanied by changes in the molars and premolars, reduction in the number of digits and lengthening of the legs. The foot posture giadually changed to unguligrade which was suited for swift running over hard ground

2. Giraffe: Obtained its long neck by stretching it upwards to reach the available food in the form of leaves from tall trees

3. Clasping Birds: Through constant perching on the twigs or branches of trees clasping birds developed sharp and curved digits.

4. Water Birds (duck etc) developed their webbed feet by constant stretching ofthe skin of their feet during swimming over the water surface.

5. Eyes are reduced in moles because they live underground.

6. Muscles of Pinna are reduced in man but are wcll developed and functional in rabbit, dog and clephant ctc. Because these nve njungies and use their pinna etc. to collect Sound from the surroundings.

7. The limbs are absent in snakes and other burrowing animals, because these are of no use in crawling and burrowing, ratherproduced hindrance. So these became gradually reduced and finally disappeared. Criticism of Lamarckism: Lanarckism was exposed to severe criticism and Lamarck tried to defend it through out his lile, It lailea to neet the test ol observations. Cuvier and Weismann were the great crilics of Lamarckisin.

(1)  The first principle, the tendency to increase in size, has been noted in many forms, but evolution shows reducton 1n SIze. Persons constantly busy in reading their eyes more than others, often developed impaired sigat. Why their eyes do not become more elmfficient?

(2) The second principle that new organs develop where the organisms feel their need is also not true. If the development of new organ or structure depends upon the desire why man having long desire to fly like birds has not developed wings 

(3) Lamarck explains the importance ot new character or change, but does not explain its utility in its initial stages.

(4) His third principle, reaction to the environment, may have same weight, since organisms do react to the environment, it causes temporary changes in organization and these changes can not be inherited to the off’spring. Similarly it could not be understood that how use or disuse of an organ will produce a change in its size and how these changes will be inherited to the offspring.

(5). Experiments have discarded his theory of inheritance of acquired characters eg.if a parent becomes blind or deaf or lame before producing the offspring, they do not produce lame offspring. Mutilations and wounds of parents do not appear in the offspring. The adaptive characters of plants and animals which superficially appear to be the direct result of use or disuse or effect of environment are actually ofgerminal origin.

(6) Chinese women use iron shoes to keep their feet short in young ones but young ones at birth have normal size. A child always learns the language which his parents taught him, but never acquires it

(7). The deadliest blow to the Lamarckism came from experiment conducted by Weismann He removed the tail of mice continuously for about 22 generations and even the offspring 22nd generation has a tail which was as long as in the original parents. 

Weismann differentiated between changes occurring in the somatoplasm and the changes Occurring in the germplasm which are all inherited by the offspring. Neo Lamarckism. A group of scientist further studied Lamarck’s theory and approved it modified form, which is known as Neo Lamarckism. 

A few of the Neo Lamarckists are Spencer, Cope, Packard-Naigali, Gadew, Dali, Mc-Dougil etc.

1. Bonner carried out numerous transplantation experiments within native and unnatural environment and found that variations produced were inherited to their further generations.

2. The white mice which were reared at a high temperature (20- 30° C) by F.B. Sumner were found to develop longer body, a long tail and longer hind limbs. And this abnormality was found to be transmitted to the offspring generation after generatio

3. Brown sequared described that certain diseases like exophthalmia, haematoma and dry gangrene are caused by injuries in the restiform bodies of the brain and are inherited to the offspring.

4. McDougall trained rats to follow certain “escape routes” from a tankof water and the training was given for about 45-50 generations. It was found that there was decreast in number of errors made in learning the problem, generation after generation.

5. Deilefson showed that rats kept for several generations in cages became adopted hoti rotating conditions. It has been realized now that through Lamarckism is incomplete in itself, and unable explain all the cases of evolutionary changes, it holds good to certain extent because the bou characters of the organisms are not single attributer. but the result of interaction of inheritab factors (genes) and environmental conditions.

Lamarckism was based on following assumptions:

• Living organisms increase their body parts due to internal force of life.
• New organs develop due to need of the organism.
• Constant use of an organ leads to better development and its disuse makes it vestigial.
• Characters acquire by an organism during its lifetime are inherited to its offsprings.
  So let’s discuss briefly about the above points:

1. Influence of environment:
   Lamarck said that the environment influences the form, habit, and life style of the organism; hence, finally affect their structural organization. Since the environment is not static, there is change in structural organization of the organism in order to adapt with the changing environment.
   Ex. Leaves of beech tree growing on sunny side develop two layers of palisade cells and those growing in shade condition grow single layer of palisade cells.
2. Use and disuse of organs:
   More the use leads better development and less the use leads vestigial development.
   Used organs: Long neck and front legs of giraffe, webbed feet in duck, better developed right arm of blacksmith etc.
   Disused organs: Legs of snake, eyes of underground animal and wings of a flightless bird etc.
3. Inheritance of acquired characters:
   All characters acquired by an organism during it’s life time must be inherited to its offsprings.
   Ex. Ancestors of giraffe had equal forelimbs and hindlimbs. When the grassland flora was replaced by tall plants they stretched their neck and front legs to reach the leaves of the tree. These characters are preserved and inherited to the next generation. Hence giraffe had developed long neck and front legs by elongating these organs for many

Generations. Neo Lamarckism :

Lamarckism was modified by Mc Douglas, Spencer, cope, sumner and Agar etc to make it acceptable to the students of evolution.

Essential features;

• Body parts don’t increase due to internal force of life.
• New organs don’t develop due to desire of the organism.
• Changing environment has a direct effect on the organism.
• Change in the germ cells or somatic cells which give rise to germ cells are heritable.

DARWIN’S THEORY OF EVOLUTION

Outstanding contributions of Charles Robert Darwin (February 12, 1809-April 19, 1882) in the field of biological evolution has given a proper shape to the concept of organic evolution. Basing upon his own observations of nature and influenced by the ideas of contemporay biologists Darwin l had formulated the principles of origin of species.

LIFE AND WORK OF DARWIN

Darwin was born in 1809 at Shrewsbury in England. His grand father, Erasmus Darwin was a noted naturalist and his father, Robert was an eminent physician. After completing his early education at Shrewsbury he was sent to Edinburg University at the age of fifteen to study medicine. Since he did not show his interest in medicine, he was sent after two years to Cambridge University to study theology. There too, he was a failure. However, he developed interest in natural history. His friendship with Prof. Henslow and Sedgwick stimulated his interest in Botany and Geology

On the recommendations of Prof. Henslow, Darwin was appointed as a naturalist for a voyage of world exploration in a British ship H.M.S. Beagle. Ths ship left Plymouth of England on December 27, 1831 and visited

Charles Darwin. many islands of Atlantic Ocean, coasts of South America and some islands of south pacific including Galapagos Islands. During his expedition he was

so much impressed with the dynamic forces of nature that he was transformed from a passive observer into an evolution oriented thinker. He realised that Galapagos island is a living laboratory of evolution. He came across giant tortoises and finches (kinds of birds) markedly different in different islands and mainlands. He recorded his observations and carried samples of his collections on his way back to England.

From 1836 he was busy in analysing his observations and collections and accumulation of related facts from other scientists till 1944. Finally he wrote an elaborate essay of 230 pages but did not publish it. In order to reach at a final conclusion he continued work up_to 1958. While he was giving a finishing touch to his theory, he received an identical theory from Wallace.

Darwin prepared an abstract of both of their findings and published them in the “Proceedings of Linnean Society” in 1858. The next year, in 1859, Darwin published his whole ideas in a book, “The Origin of Species which was judged by large section of scientists to be the most important book of nineteenth century.

CONTRIBUTION OF WALLACE

Alfred Russel Wallace (1823-1913), a young English naturalist after travelling widely in tropical SouthAmerica, Southeastern Asia and Malay Archipelago came into independent conclusion of mechanism or origin of species through natural selection. He submitted his essay “On the tendency of varieties to depart indefinitely from the original type” to Darwin for eritical review. To the utter astonishment of Darwin, he found that his findings were exactly identical to those of Wallace and Darwin could not become the only authority of the theory.

Darwinism can be discussed under the following headings:

DARWINISM

The evolutionary theory of Darwin (and Wallace) is known as “Origin of species by natural selection” or “Theory of Natural Selection” or “Descent with modifications” and is also shortly known as “Darwinism. This theory is based upon the following observations of nature from which certain logical deductions have been made.

1. Enormous Multiplication Rate. Every species has a natural tendency to multiply by reproduction in geometric progression (1: 2:4:8for continuity of race. According to population theory of Malthus plants, animals including human beings tend to double in each successive generation. For example a rabbit produces six young ones in a litter and four litters in a year and the young one starts breeding at the age of six months. The elephant, which is said to be the slowest breeder, will produce about 19,000,000 elephants in a period of 750 years. For all the offsprings of any species remain alive and reproduce they would soon overcrowd the earth.

On the contrary, we observe that overcrowding never takes place. The population of each species remains fairly constant. This is because a large number of offsprings perish before attaining adulthood and capability of reproduction. Sometimes, the path of life-cycle from egg to adulthood is so much full of hazards that most of the developing individuals perish and only a few eggs reach upto the adult stage (example- liver fluke).

2. Struggle for Existence. While individuals multiply in geometric ratio, the total quantity of food and habitation remain almost censtant on earth. So Darwin concluded that each organism has to strive or struggle with others in order to fulfil its requirements for life. This struggle continues from zygote stage till death. Large number of individuals are destroyed in the struggle. Therefore each species has to reproduce large number of offsprings, so that, some may survive to continue the race. 

The active competition or struggle for maximum amount of food and better shelter takes place in the following three areas.

(I) Interspecific. Struggle between organisms of different species living together for food and space. A tiger struggles to kill a deer for food and the deer struggles to escape from the tiger. This is an active struggle of “Kill-or-be-killed” example.

(II) Intraspecific. Struggle between individuals of the same species for food, habitat and mating partner (alongwith mental and moral struggle in case of human beings only).

(III) Environmental. Struggle between organisms and the environm ental conditions such as excess of rain or drought, intolerable heat or cold, natural calamities like

earthquakes, cyclone, lightening, volcanic eruption, flood, pollution and so on, results in death of large number of higher plants and animals

3. Variation and Heredity. No two individuals of a species are exactly identical (except identical twins). This is due to sinall or large differences observed among the members of one species and is known as Variation. Some of the variations may be advantageous and increase the chances of survival of their bearers. But harmful or neutral variations do not help in the B struggle for existence. For example, a variation which increases the speed of a herbivore will help it in escaping the predators and increase its chances of survival in the plains. On the other and, a variation with chlorophyll deficiency in a plant would result in lower rate of photosynthesis which would be unfit for survival.

some variations are temporary and are not transmitted to their offsprings but others appeanng in the parental generations continue to appear in the progeny generation after generation. The later type of variation is called heritable variation and form the raw material for evolution.

4. Survival of the Fittest (or Natural Selection). Individuals possessing favourable and heritable variations are more successful in the struggle for existence. They win in the struggle and are said to be fittest. The concept of “survival of the fittest” was originally the idea of Herbert Spencer and the same phenomenon is named as “Natural selection by Darwin. He conceived the idea of natural selection from his observation of artificial selection. He pointed out: that the breeders of domestic animals perform selective breeding to raise new varieties of horses, cattle, dogs and cats with desirable characteristics. Nature also selects such individuals with better variations for survival and preservation. Similarly, useful variations occuring in nature are preserved because nature selects them. Hence, the fittest individuals survive through natural selection.

5. Continuous Environmental Change. Darwin observed that the environment with respect to time and space is never constant. So a particular variation which is best suited in a given environment may prove unfavourable in the changed environmental conditions. Hence an organism cannot reach perfection in the changing environment. The before, in order to survive in the struggle for existence, an organism must be capable of changing in response to the changing environment. The extinction of gigantic reptiles due to environmental change during mesozoic era is a suitable example.

6.Origin of Species. Based upon the above observations Darwin concluded the origin of species through Natural selection. Beneficial variations coupled with environmental change when selected by nature and the process continues slowly and steadily through millions of

years lead to emergence of such descendants which considerably differ from their ancestors. Ultimately these descendants form new species. When variations occur in different directions, two or even more species may arise from a single ancestral stock. This process accounts for the

one million species of animals existing to-day on earth and much more number of species extinct since the time of origin of life.

OBJECTIONS OF DARWIN’s THEORY

Darwin’s theory received widespread approval from 1859 to 1900 and this period is known as romantic period of Darwinism. In 1900 Mendel’s principles of heredity came into light through De Vries, Correns and Tschermack. Since then, the validity of Darwinism was subjected to serious doubts and criticism. Some of the important objections are noted below.

1. Darwinism explains the survival of the fittest but does not explain the arrival of the fittest.

2. This theory is based upon small variations but does not explain the usefulness of an organ in its initial stage

3. Darwin thought minute fluctuating variations as the principal factor for natural selection but they are mostly non-heritable. He considered ‘sports’ or ‘saltation yáriations’ as rare but they are more frequent as we see to-day.

4. The theory does not explain the effect of use and disuse and the presence of vestigeal organs

5. He did not differentiate between somatic and germninal variations

6. He could not explain overspecialization of particular structure through natural selection. Example-Antlers of extinct Irish deer.

7. Darwin thought the somatic cells ofthe bodyproduce minute particles-the pangenes which are carried by blood and accumulated in the germ cells, So each germ cell is a

minute copy of the parent body which could develop into the adult through reproduction. But such hypothesis is totally baseless. Hence, Natural selection is not the only pathway for evolution. Neither, it is a creative force. But it is just a guiding factor.

MODERN SYNTHETIc THEORY (OR NEW-DARWINISM)

In the light of our recent knowledge in genetics, molecular biology and cell biology, Darwinism has been modified and is accepted today in a synthetic form, known as Neo- Darwinism. Germplasm theory of Weismann, Mutation theory of De Vries, Mendel’s laws of inheritance, Wagner’s (1868) factor of isolation and population dynamics have remodelled original Darwin’s theory. Dobrhansky (1937) in his book “Genetics and Origin of Species” has provided the basis of the theory and Huxley (1942) has designated the theory as “Synthetic Theory”. The contributions of Muller (1949), Fisher (1958), Wright (1968), Mayr (1970) and G.L. Stebbins (1966-76) have givena final shape to the theory.

The following five basic processes and two accessory factors are the chief ingredients of the theory

1. Gene Mutation. Sudden change in gene (DNA) due to radiations, light, temperature etc. brings about permanent phenotypic change in the individual.

2. Chromosomal Mutation. Change in chromosome number (Chromosomal abberation) and chromosome struclure als0 contribute significantly in phenotypic change.

3. Recombination. Reshuffling of genes due to crossing over lead to genetic variability ofbthe total gene pool’ and thus results in the variation of characters

4. Natural Selection. Variations due to mutations ifsuitable for the existing environmental conditions are selected by the dynamic forces of nature and lead to speciation.

5. Isolation. It is the mechanism by which a species is divided into small populations and each population lives independently in a well defined territory. Isolation may be geographical reproductive or physiological. Such barriers do not permit interbreeding among different populations, Finally each population may lead to emergence of a new species.

6. Genetic Drift. In small population the gene frequencies are found to fluctuate purely by chance factors, This random changes in gene frequencies is not under the control of natura selection and is called genetic drift by Sewal Wright. This force has atendency to eliminate less frequent alleles, to attain homozygosity and lowers the amount of variability. As a result, either the population would be extinct or if it withstands the selection pressure, would lead to iformation of a new species

7.Hybridization. Interbreeding among the individúals of different populations of a species results in the introduction of new genes in the gene pool of a population. Hybridization could also take place between individuals ol closely related species. The offsprings exhibition characteristics somewhat difterent irom both the parents and may forin new species.

Evolution is a never-ending process. It would go on operating so long as there is mutation and organisms possess heritable variations. The linal goal is to reach perfection. But no organization is perfect. Improvement in body design is limitless and timeless. However, this not applicable for human beings. Biological evolution is being tempered and arrested through “Cultural evolution of human beings.

EVIDENCES OF EVOLUTION

Various theories of organic evolution conclusively reveal that different species of plants and animals have come into existence from pre-existing forms through a continuous, slow and gradual process of change. The pre-existing forms were simple, unorganised whereas the present day forms are varied, complex and well organised. The process is so slow that nobody has witnessed directly the entire spectrum. The lifespan of human being is insignificant compared to time taken for even one step of modification. Under such circumstances one has to look for evidences to confirm that biological evolution has really occurred. Except a few direct evidence, most of them are indirect as they are based upon experimental, circumstantial or logical derivations. However, such proofs are convincing and are drawn from different branches of biology. The following evidences in support of organic evolution could satisty our doubts about the process.

ANATOMICAL EVIDENCES

Comparative study of morphology or anatomy of any particular organ o” organ system in various members of a given phylum or class reveals basic similarities. The skeletal, muscular, alimentary, nervous and excretory systems of all classes of vertebrates exhibit striking resemblances suggesting the fact that all of them have originated from some common ancestor. A few concrete examples may be presented

A.  Homologous and Analogous Organs. Organs similar morphologically and embryonically but differing in functions are homologous organs. They have common origın and are built upon the same fundamental plan but due to different functions they superficially appear to be different. For example, the flippers of seal, wings of bat, fore-wings of bird, foreleg of horse and hand of human though appear to be dissimilar are homologous as they are made of the same bones-humerus, radius-ulna, carpals, metacarpals and phalanges along with same muscles, nerves and blood vessels arranged on the same plan with similar mode of origin and development. This homology convincingly proves that they have a common

Evolutionary Origin.

On the contrary analogous organs are morphologically and embryoncially different but 94 functionally are apparently similar. For example, the wings of a bird, a butterfly, a bat and a pterodactyl (a flying reptile) support the body in flight but their basic structure is totally different  The wings of an insect is lifeless, formed of thin flap of chitin and tlaps by muscles attached at the base. In pterodactyl, the wing is an enormous fold of skin supported by elongated 4th fingure. In bird the entire forelimb is modified into wings. In bat, a mammal, the wing is formed of a fold of skin supported by the elongated phalanges of last four digits. Similarly body shapes of fish, inchthysaurs and whale are analogous having no common ancestry,

B. Vestigeal Organs. Organs functional in ancestors but in the present forms are functionless and occur in degenerated condition are known as vestigeal organs. For example the wings of flightless birds rudimentary eyes of cave dwelling animals, splint bone in the leg of horse. Man has many vestigeal structures such as nictitatig membrane in the eye, vermiform appendix at the end of caecum, coccyx or tail vertebrae, nonfunctional muscles of pinna and a lot more. All such organs are useless to their possessors but their presence can be explained only by assuming their inheritance from ancestors where these organs were functional. Man who is terrestrial and bipedal needs no tail but he has evolved from arboreal monkey like ancestors through semiarboreal apes. Monkeys possess a functional tail and ape and man possess only its vestiges (coccyx bone). Hence, the vestigeal organs prove that the present day forms have evolved from dissimilar ancestors.

Darwin’s theory of evolution: (Darwinism)

Darwin’s work on evolution is known as Darwinism which is the most accepted and popular among all the theories of evolution.

1. Over production:
   Living beings have an internal desire to produce their own offspring. So the number of organisms increases in geometric ratio(G.P.). If the number of organisms increases in this rate, the earth would be covered with organisms of few species only within a short period of time. This overproduction is a driving force in evolution, as it can lead to adaptation and variations in a species due to struggle for existence.
   Examples:

• Many species of fish lay millions of eggs at one time.
• A single oyster produces 60-80million eggs per year.
  It represents tremendous reproductive potential found in organisms.

1. Struggle for existence:
   According to Darwin, due to over production, number of organisms
2. Struggle for existence: According to Darwin, due to over production, number of organisms don’t increase. Hence the carrying capacity of the nature doesn’t allow these organisms to grow indefinitely. So a struggle occurs in the organisms for living. It may be individuals of same species, between members of two different species mainly for shelter, light, offence and defence and it may be a struggle of organisms with the environment against extreme cold, heat, storm, earthquake etc.

3. Survival of fittest and natural selection:
   Natural selection is the final outcome of Darwin’s thought. According to him due to struggle for existence better adaptive individuals are produced and survive but less adaptive individuals perish. So the nature selects the organisms with useful variations and better adaptive to changing environment. Hence only useful variations are preserved. The word ” natural selection” was replaced by Herbert Spencer as ” survival of the fittest”.
4. Origin of species:
   As a result of struggle for existence organisms becomes better adapted to its environment. These adaptations are preserved in Species generation after generation. Accumulation of small variations for many generations results origin of new species. ” Branching descent” and “natural selection” are two key concepts of Darwin’s theory of evolution.

Darwinism vs. Lamarckism

Darwin and Lamarck both had speculated the evolution process through their theories. These two theories provide varying suggestions and obvious differences ranging from the mechanism of evolution to vast diversity on earth.

Here we represent some key differences of Darwinism and Lamarckism-

• Darwinism discredits the internal vital force theory where as Lamarckism strongly builds on the notion of internal vital force in organisms.
• According to Darwinism Development of disappearance of an organ happens due to continuous variations; but Lamarckism says from an evolutionary perspective organs develop if it is constantly used. If unused, it could disappear.
• Darwinism emphasises on the struggle for existence. Lamarckism discredits struggle for existence.
• Darwinism was built on the principle of survival of the fittest whereas Lamarckism was not built on the notion of survival of the fittest.

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