12th Class Biology blank Theories of Organic Evolution

(i) Lamarckism : Lamarck (1744 –1829) was one of the most brilliant stars on the horizon of the history of evolution. Though he was a man of great intellect yet he had to cut a sorry figure because of poverty, and secondly, as he could not get the approval of the famous and influential Cuvier. His work was recognized only few years after his death.

He was the first naturalist to put forward a general theory of evolution in his famous book.  Philosophic Zoologique published in 1809. His evolutionary theory may be summarised in the form of following laws:

(a) The internal forces of life tend to increase the size of an organism : Lamarck believed that there is some kind of internal force which is constantly working in living beings. Its tries to make the animal bigger in size.

(b) The necessity in animals to produce new structures : According to this law, each organ and structure is the product of some continuous necessity in the animals, to develop it. He believed that it was not due to the direct influence of the environment, but acted through the nervous system, the process being very slow. The plants, he said, were directly influenced by their environmental conditions.

(c) The effect of use and disuse : Lamarck thought that the continuous use of a particular organ or structure tends to increase its size and its development. On the other hand, disuse results in disappearance of that structure, the process being slow and gradual. He supported this by a number of well-known examples. The long neck of giraffe was the result of continuous stretching to obtain food from the trees (long neck was acquired due to excessive use).  The wings of certain birds Ratitae were not used for long, as there were no formidable enemies and food was available in abundance. So they did not fly and consequently lost the power of flight. Their wings became rudimentary (flightless condition acquired due to the disuse of wings).

(d) Inheritance of acquired characters : Lamarcks stated that all the characters, which are acquired in one's own lifetime are inherited by offsprings.

Criticism : The greatest draw-back in the Lamarck's work was that it was too theoretical and there were no proofs to support it. His ideas were theoretically sound but practically they had no standing. He met a severe criticism from various workers –

(1) The most serious blow came from Weisman who put his theory of continuity of germplasm which states that the inheritance is the sole concern of germ cells. Characters introduced in the germ cells will only be inherited and not those which are present in somatic cells.

(2) If the acquired characters were to be inherited, as Lamarck said, the world would have been full of cripples, blinds and deformed persons, as most of these characters are acquired.

(3) Some workers have practically proved that mutilations are not inherited by offspring’s even if practiced for generations.

Lamarck's second and third laws show much truth in them; but they are not the sole cause of evolution. The laws of heredity of Weisman are practically opposed to the fourth law of Lamarck. Nevertheless, there came a number of workers who supported Lamarck and modified his laws. They were known as Neo-Lamarckian and the names of a few of them are : Cope, Haeckel, German savant, Gadow and Spencer.

(ii) Germplasm theory : Germplasm theory was given by Weisman. Later on, Weisman also admitted that the germplasm may become modified to a certain extent by some environmental factors.

• Key point to Lamarck's view about organic evolution is that every offspring inherits characters acquired by the parental generation.
• Law of “Inheritance of acquired characters” is presented by Lamarck.
• Lamarck’s theory was most severely criticized by cuvier who greeted it by calling it “Nouvelle Folie.”

  

Difference between Lamarckism and Neo-Lamarckism

Lamarckism	Neo-Lamarckism
It is original theory by Lamarck.	It is a modification of the original theory of Lamarck in order to make it more suitable to modern knowledge.
The theory lays stress on internal force, appetency and use and disuse of organs.	Neo?Lamarckism does not give any importance to these factors.
It believes that changes in environment brings about a conscious reaction in animals.	The theory stresses on the direct effect of changed environment on the organisms.
According to Lamarckism the acquired characters passes on to the next generation.	Normally only those modification are transferred to next generation which influence germ cells or where somatic cells give rise to germ cells.

 

(iii) Darwinism : Charles Robert Darwin was undoubtedly the first naturalist who put the idea of organic evolution on sound footing. His statements and theories were based upon practical experiences and large number of proofs which he collected directly from the nature. He devoted his whole life for the purpose of finding out proofs in support of the theory of organic evolution.

 

The natural selection theory may be summarized by a chart devised by wallace

	Facts	Inference
A	1. Enoromous fertility 2. Limited food and space	Struggle for existance
B	1. Struggle for existance 2. Variations	Survival of fittest or natural selection
C	1. Survival of fittest 2. Continuous environmental changes	Origin of new species

 

 

• Charles Darwin wrote Orgin of species.
• Darwin was appointed up on a world survey ship of British government H.M.S. Beagle.

His main ideas about the evolution are given below –

(a) Over-production of offsprings : The power of reproduction is enormous in the living beings. The single Paramecium (Protozoa) divided about 600 times in a years. If all the progeny survive their total amount would exceed that of earth in a few months. Again if all the eggs of a lobster were to produce young ones, in about ten years time the sea would be full of lobsters and there will be hardly any space for other animals. Darwin gave one more interesting example of elephants. An elephant lives for about a hundred years and starts reproducing at the age of thirty. If each female produce six young ones, in 750 years, about 1,90,00,000 would be alive.

(b) Limited supply of food and shelter : The amount of food and shelter is limited in a particular area. It is sufficient only for a definite number of individuals.

(c) Struggle for existence : It is a common experience, that even with the enormous rate of reproduction, the number of species mostly remains stationary. The nature has provided a number of checks over their population. There is limited food, breeding places, shelter, presence of predators and parasites, etc.

There is always going on a struggle for existence among various agencies. The struggle for existence may be–

Inter-specific : When two different groups of animal species are opposed to each other, i.e., lion and deer or birds and insects.

Intra- specific : When there is struggle between the members of the same species, as all of them have same wants and requirements.

Environmental : It is the struggle between the animals and their environment (i.e., climate, vegetation, open spaces, jungles and water, etc.,) A change in climate may affect adversely, resulting in the extinction of some and the survival of others.

(d) Universal occurrence of variations : The “departures form the original pattern” and changes in animals were termed as variations. Darwin believed that continuous and useful variations constitute the raw material of evolution.

(e) Survival of the fittest or Natural selection : In struggle for existance, only those organisms survive which possess the most useful variations. This has been called 'Natural selection' by Darwin and 'Survival of fittest' by Spencer.

(f) Inheritance : The useful variations are inherited by the progeny.

(g) Origin of new species : Favourable variations accumulate over generations to ultimately form a complete new species.

• To explain inheritance of characters from one generation to another Darwin proposed 'Theory of Pangenesis'. According to this theory each somatic cell produce pangene. All the pangenes from body cells accumulate in gametes and transfer characters to next generation.

Most of the biologists agree with the Darwin's theory as the best explanation of organic evolution. But there are a number of objections to this theory.

• By performing the replica plating experiment, Lederberg supported the “Natural Selection Theory”.

Objections

(1) Darwin's theory does not explain that the effects of ‘use and disuse’ of organs are inherited.

(2) He considered the minute fluctuating variations as the cause of natural selection, but most of the variations are non-heritable.

(3) He did not distinguish between germinal and somatic variations. His theory of Pangenesis has no basis at all.

(4) He believed that variations occur in all directions haphazardly. But now it is established that the variations occur only on definite lines of change.

(5) Darwin called mutations as 'SPORTS' but being unaware about genetics he couldn't explain these.

(6) Darwin's theory explains survival of the fittest but not about arrival of the fittest.

(7) It does not explain how natural selection could make use of certain adaptive characters in their initial stages, i.e., what would be the use of electric organs, electric fishes, until they have enough of power to produce a shock.

(8) It does not explain the over-specialised and vestigeal organs. Overspecialization of certain characters proved harmful such as Antlers of Irish elk, teeth of Swedolon, heavy armour of dionosaure.

(9) According to him, only useful characters are inheritable but on the contrary certain useless and non-adaptive characters are also passed on.

(10) Geologists and astronomers think that the time required producing organic world is much more than the actual age of the earth today.

In the light of these criticism and objections various workers after him modified his theory. Most of the work was done after the rediscovery of Mendel's work in 1900. This modified theory of Darwin is known as Neo- Darwinism.

Examples of Natural selection

(1) Industrial melanism : Industrial melanism is a phenomenon where the moths living in the industrial areas, develop black colour (melanin pigments) to match the body to soot-covered background, on the bark of trees.

The industrial melanism is observed and worked out by a number of evolutionists like Fisher, Ford and Kettlewell.

Industrial melanism was observed in a peppered moth Biston betularia living, Manchester, in an industrial city of Great Britain and it is the example of 'evolution taking place before the eyes'.

The change of the lighter coloured variety of peppered moth, Biston betularia (typica), to its darker variety (carbonaria) is due to mutation of a single Mendelian gene for survival in smoke-laden industrial environment.

The peppered moths exist in two forms, namely melanic forms and non-melanic forms. The melanic forms are black in colour because they contain melanin pigments. The melanic forms are also called carbonaria. The non-melanic forms are light coloured. The light colour is due to the absence of melanin pigments and are called Biston betularia typica.

(2) Resistance to DDT : The resistance is a character controlled by genes. The resistant flies arise as a result of the application of DDT. They reproduce more and more resistant flies. Thus the resistant populations are evolved as a selective advantage against insecticides.

(3) Resistance of bacterium to drugs : L.L. Cavalli and G.A. Maccacro (1952) experimentally proved that the colon bacteria Escherichia coli develop resistance to the antibiotic chloramphenicol 250 times as great as that tolerated by normal bacteria by exposing the bacteria to increased concentration of the drug.

 

Difference between Darwinism and Neo-Darwinism

Darwinism (Natural Selection)	Neo?Darwinism
It is the original theory given by Charles Darwin (1859) to explain the origin of new species.	Neo?Darwin is a modification of the original theory of Darwin to remove its short?comings in light of genetic researches.
According to this theory accumulation of continuous variations causes changes in individuals to form new species.	Instead of continuos variations, mutations and genetic variations are believed to help form new species.
It believes in the selection of individuals on the basis of accumulation of variation.	Variations accumulate in the gene pool and not in the individuals.
Darwinism does not believe in isolation.	Neo?Darwinism incorporates reproductive isolation as an essential component of evolution.
It can explain the origin of new characters.	The theory can explain the occurrence of unchanged forms over millions of years.
Darwinism cannot explain the persistence of certain forms in the unchanged condition.	Normally only those modification are transferred to next generation which influence germ cells or where somatic cells give rise to germ cells.

 

(iv) Mutation theory

(a) Hugo de Vries (1901), a Dutch Botanist, pioneered the theory of mutations to explain the mechanism of evolution.

(b) The plant on which de Vries had experimented was Oenothera lamarckiana (Evening primrose).

(c) The mutation observed by de Vries in Oenothera essentially was chromosomal number variant.

(d) Mutations are discontinuous variations, called 'sports' by Darwin and 'saltatory variations' by Bateson.

(e) Mutations are generally harmful and recessive.

(f) Role of mutations in evolution is genetic variations.

(g) Mutations are due to changes in chromosomes, genes or DNA.

(h) To be a successful event for evolution, a mutation must occur in germplasm DNA.

(i) Mutations are the changes which may or may not be inherited.

(j) Germinal mutation is a change that is inherited.

(k) Hereditary variations in plants have been produced by use of X-rays.

(l) Frequency of a mutated gene in a population is expected to increase if that gene is selected by nature.

(m) The possibilities of hereditary and evolutionary changes are greater in species that reproduce by sexual means.

(n) Organic evolution would not have taken place if individuals in a population did not show gene variations.

(v) Variations : Dissimilarities between members of the same species are called variations. Tendency to differ helps organisms in their adaptations for different environmental conditions. Heritable variations are responsible for changes in a species to form new species. Variations are thus important in evolution.

Variations are progressive factors in evolution. Members of the same species exhibit variations by structural, physiological or psychological dissimilarities.

Types of variations : Variations are classified in three sets.

(1) Germinal and somatic variations : Germinal variations arise in germplasm of the organism. They occur in the gene pattern and are inheritable. These variation reach the zygote through gametes and hence are inherited from generation to generation. For example colour of eyes or hair occur since birth whereas characters of height and body built develop later in life. Germinal variations are also called blastogenic.

Somatic variations are produced due to environmental factors. They develop in somatic cells or somatoplasm and are also called acquired variations. These are non-heritable. Darkness of skin due to working in the sun, development of intelligence by better education, achievements of a musician, muscular body of an athlete, bored nose and pinna in ladies, are some examples of acquired variations. Somatic or acquired variations are not important in evolution.

(2) Continuous and discontinuous variations : Continuous variations are small and graded variations which are found in the members of same species. Darwin called them as fluctuating variations and realised their importance in evolution. Darwin considered all fluctuating variations inheritable from one generation to other to form new species. They were considered important factor for natural selection. For example children of the same class and age show continuous variations in their height and intelligence but they will be much different from children of other classes.

Discontinuous variations arise suddenly and they are distinctly visible in the group. These ungraded variations deviate so much from the average character that they are seen in new form among members of the same species. There are no grades or intermediate stages in such variations. Darwin called these variations as sports while Hugo de Vries called these as mutations. These discontinuous variations are stable and inheritable. Polydactyly or more than five digits in hands or feet in man, occurrence of four horns instead of two in goat etc. are mutations.

(3) Determinate and Indeterminate variations : Determinate variations arise in a definite direction and time and are due to adaptations. These are found continuously and progressively in organic evolution and are affected by strong gene combination from generation to generation. Leaf eating moth Diabrotica soror is a good example of this variation. In this moth many different colours are found other than main colour.

Indeterminate variations do not arise under any special condition and can arise suddenly in any direction. These can develop to any extent.

Causes of variations

(a) Environmental conditions

(b) Inherent tendency to vary

(c) Dual parentage                 

(d) Nuclear reorganization

(e) Change in the gene pattern

(vi) Synthetic theory 

(a) Dobzhansky (1937) in his book ‘Genetics and Origin of Species' provided the initial basis of synthetic theory.

(b) ‘Modern synthetic theory of evolution' was designated by Huxley in 1942.

(c) Some of the important workers who have contributed to the modern synthetic theory are : Th. Dobzhansky, R.A. Fisher, J.B.S. Haldane, Sewall Wright, Ernst Mayr and G.L. Stebbins.

(d) According to synthetic theory there are five basic factors involved in the process of organic evolution. These are :

(1) Gene mutations               

(2) Changes in chromosome structure and number

(3) Genetic recombinations  

(4) Natural selection and

(5) Reproductive isolation.

(e) While the first three factors are responsible for providing genetic variability, the last two are responsible for giving direction to the evolutionary processes.

(f) Besides the five factors outlined above, there are two accessory processes, namely migration of individuals from one population to another and hybridization between races, species and even related genera, which contribute to the evolution.

(g) The most accepted and recent theory of organic evolution is the synthetic theory.

(vii) Hardy-weinberg equilibrium 

(a) Mutations introduce new genes into a species resulting a change in gene frequencies.

(b) G.H. Hardy, an English mathematician, and Wilhelm Weinberg, a German physician, in 1908 established a simple mathematical relationship to the study of gene frequencies.

(c) If certain conditions existed, gene frequencies would remain constant.

(d) The conditions necessary for gene frequencies to remain constant are :

(1) Mating must be completely random.

(2) Mutations must not occur.

(3) Migrations of individual organisms into and out of the population must not occur.

(4) The population must be very large.

(5) All genes must have an equal chance of being passed to the next generation.

(e) According to Hardy-Weinberg concept, the gene frequencies will remain constant if all above five conditions are met.

(f) The distribution of genotypes could be described by the relationship \[{{A}^{2}}+2Aa+{{a}^{2}}=1,\] where \[{{A}^{2}}\] represents the frequency of the homozygous dominant genotype, 2Aa represents the frequency of the heterozygous recessive genotype and a² represents the frequency of the homozygous recessive genotype.

(g) Constant gene frequencies over several generations indicate that natural selection and evolution are not taking place.

(h) Changing gene frequencies would indicate that evolution is in progress.