Lamarckism is the theory of ‘Organic Evolution’.It was proposed by Lamarck.It is also called as inheritance of acquired character.
Principles of Lamarckism :
Lamarckism consist of four principles,they are as follows ;
1. Internal urge of organism
2. Environment and new needs
3. Use and disuse of organ
4. Inheritance of acquired character
1. Internal urge of organism :
i)Animals and plants have the ability to grow and increase in size to attain the maximum size.
ii)Not only the body but also each and every part of an organism increase in volume. According to Lamarck this increase in size is due to an internal urge and inherent ability of the animal itself.
iii)It is due to desire ,some sort of an internal urge of the animal that the size increases.
2.Environment and new need :
i) According to Lamarck , environment plays an important role in evolution.Animals co-operate with the environment for their survival.
ii)They develop adaptive characters to live comfortably in the environment. Whenever there is a change in the environment,the animals also respond to the changes.
iii)The changes in environment,bring about new structure or character.
3.Use and disuse of organ :
i)When an organ is put to greater and constant use,that particular organs develops well;at the same time when an organ is not used for a long time it get reduced and in due course of time it get degenerate, disappear completely from the organism.
ii)This phenomena is called use and disuse theory.
Examples of use theory
e.g Giraffe
i)Lamarck believed that the long neck and long limbs of giraffe are the results of continuous use.
ii)He assumed that the ancestral giraffes were provided with short neck and the fore and hind limbs with uniform length.They were feeding on grass.
iii)At that time there was scarcity of grass.Hence the ancestral giraffes were forced to feed on the foliage of tress.So they tried to stretch their neck and fire limbs to reach the foliage of tress.
iv)This resulted in a slight increase in the length of the neck.
v)The process of stretching the neck continued generation after generation to get the foliage of taller trees and this resulted in longer neck leading to origin of modern giraffe.
Examples of disuse theory
e.g 1. Eyes of cave animals (Proteus anguineous )
i) The cave is characterized by darkness .In the absence of light,the cave animals cannot use thier eyes.
ii) Since the eyes are not used for many generations,the eyes becomes degenerated and extreme cases eyes disappear completely.
e.g 2. Flightlessness
i) Another very good example is the flightless condition in kiwi of New Zealand.Evidence es indicate that kiwi has descended from normal flying birds.
ii) At that time there were no enemies on the land of New Zealand.Hence they led a peaceful life without any attempt to fly and they had no chance to use their wings.
iii) This was happened generation after generation,resulting in the degeneration of wings and loss of flight.
4. Inheritance Of Acquired Character :
i) The transmission of characters,developed by an organism during its life time,to the young ones is called inheritance of acquired characters.
ii) When the environment changes ,animals respond to it.
Criticism Of Lamarckism :
i) Lamarckism does not have any prove to explain the vital force which is present in organism to increase its height or any body parts,rather in contrast to this theory Chinese woman used to wear the tighten iron shoes to compress their feet size purposefully,but this character was not inherited to their offsprings.
ii) As per second principle about the environment and new needs,it’s upto some extent can be truth but any adverse change in structure of an organism.
iii) Use and disuse of organ can be happened only in one organism or generation alone but in the upcoming generation the condition for use and disuse may change
iv) The fourth principle about inheritance of acquired characters is most disputed one,because acquired character is only expressed in morphological way and inheritance is about the relation with genotypic character, therefore theory of germplasm and Mendel’s law discards this idea which was proposed by Lamarck in fourth principles.
New Lamarckism :
New Lamarckism is based on the old idea of inheritance of acquired characters with slight modification.
i) Any deformation in morphological character has no role in inheritance.In a experiment German biologist August Weismann cut off the tails of mice for many generations and realise that it has no effect in their descendants.
ii) Some of the variation acquired by an organism can be inherited to offsprings.
iii) There is no internal driving force which may have the potential for evolution.
iv) The variation occurs within the germ cells leads to the changes in offsprings through inheritance.
Darwinism :
The evolutionary idea contributed by Darwin is called Darwinism or Natural Selection Theory.It explains the mechanism of evolution.Darwinism consists of five principles ,they are as follows ;
1. Over production
2. Struggle for existence
3. Variations
4. Survival of the fittest
5. Origin of new species
1. Over production :
i) Animals and plants produce young ones by reproduction.This is called ‘Prodigality Of Production’.This is lead to the over crowding.
ii) Paramecium undergoes binary fission in every 16 hours.If all the daughter paramecium of a single paramecium survive and reproduce at this rate for five years,they will produce a cytoplasmic mass,whose volume will be equal to 10 thousand times the volume of the earth.
iii) A single Oyester lays about 50 million egg in a season.If all these eggs hatch out and if all the young ones reproduce for 5 generations,they will form a volume about eight times the size of the earth.
iv) An elephant,the slowest breeders can produce 6 young ones units life span of 100 years.If all the young ones of a single female elephant survive and reproduce,about 19 milion elephants will be produce in 750 years.
2. Struggle for existence :
i) As the rate of reproduction is high,animals increase in a geometrical ratio.But there is no corresponding increase in space,food and other factors essential for life.
ii) This leads to competition for getting requirements of life.This competition is called struggle for existence.
iii) There are three types of struggle which are describe below ;
a) Intra-specific Struggle
b) Inter-specific Struggle
c) Environmental Struggle
a) Intra-specific Struggle :
i) The competition between members of the same species is called Intra-specific struggle.When members of the same species are living in the same environment,thier needs and requirements are also the same.
ii) Hence the competition is very heavy.
iii) When there is a piece of flesh,dogs fight among themselves to get it.
b) Inter-specific Struggle :
i) The competition between members of different species is called inter-specific struggle.
ii) When there is a dead body of cattle,vultures will crowd arround it to eat the flesh.There will be dogs also trying to get the flesh,but the vultures will chase them away,thus there is a struggle between the vulture and the dogs.
c) Environmental Struggle :
i) The competition between animals and environmental factors is called environmental struggle.
ii) The environmental competition is caused by the following factors.
1.Shortage of food ,water and oxygen.
2.Lack of light
3.Predators
4.Lack of shelter
5.Disease
6.Natural calamities
iii) All these factors constitute the environmental resistance.It checks the reproductive potential of the population keeps the population from growing indefinitely.
3.Variation :
i) Variation is defined as the difference in character between individuals.No two individuals are exactly alike.Variations occurs even between the offsprings of the same parent.
ii) Even in the same organism variation can be observed.For examples,in plants one leaf is different from other in its seze,shape,venation etc.
iii) Variation is the inherent property of life.Darwin believed that the small continuous variations are important for evolution.
iv) Darwin described two types of variation.They are-
a) Favourable variation
b) Unfavourable Variation
a) Favourable Variation :
These are the characters which are useful to the possessor in the struggle for existence.
e.g The strength and aggressiveness of dog ; the high speed of deers .
b) Unfavourable Variation :
These are the character which are not useful to the possessor in the struggle for existence.
4. Survival of the fittest and Natural selection :
i) The organism provided with favourable variation succeed in the struggle for existence.
ii) These organism are the fittest for survival.Other animals are unfit to survive and they perish.
iii) The fitness or unfitness is decided by the environment,the nature.So the environment act as the selecting force.It selects those organism which are provided with favourable variation.This process of selection of organisms with favourable variation is called natural selection.
iii) The organism selected by nature reach sexual maturity and they produce young ones.These young ones receive advantages variation from their parents.
5.Origin of new species or Speciation :
i) Darwin stated that new species arises by the co-operation of all the above principles.
ii) Over production leads to struggle for existence.Animals are provided with a bulk of variations.Environment selects the animals with favourable variations ; these animals are allowed to survive .
iii) They mate and during mating the favorable characters are transmitted to the next generation.
iv) Generation after generation the character succeedingly transmitted and adapt the favourable variation or characters.
v) Thus after many generations the descendants appear to be different from the original animals.Hence these descendants can be classified into a separate group or unit called species.
Criticism of Darwinism :
Charles Darwin’s natural selection theory is favoured by many biologist.Still there are criticism for and against it.
1.Darwin stated that all variations are inherited but he did not distinguish between heritable and non-heritable variations.
2.He did not explain the origin of new character.He explained only the progressive improvement of the existing organs.
3.Darwin did not explain the actual process of inheritance.
4.Darwin’s theory explain only the existence of useful variations,but does not give any idea about the useless variations like the vestigial organs.
5.Natural selection says that a new species is formed by the gradual accumulation of useful variations.If it is true then there should be intermediate forms.But in most of the cases intermediate forms are not recognised.
6.Darwin’s theory can not explain the existence of over specialization of certain structure .
7.The theory cannot explain neutral flowers and sterility of hybrids.
Supplementary Theories Of Darwin :
Darwin proposed three additional theories to explain certain facts which cannot be explained by the natural selection theory,they are :
1.Sexual selection theory
2.Artificial selection theory
3.Theory of pangenesis
1.Sexual Selection Theory
i)In some species of animals,males differ from the females of the same species in their morphology.This is due the presence of certain specialized characters in males one.
ii)This specialized characters or organs assist in reproduction.Hence they are called secondary sexual characters.
iii)It is very specialized in matting and role in evolution.Hence Darwinism proposed the sexual selection theory to explain the existence of all these characters.
iv)It states that females select males for mating.The selection is based on the presence of desirable qualities or attractive characters.As a result of which,males get chance to mat and produce the individual of adaptive characters which will be suitable for environment.
v)Salient feature of sexual selection theory are:
a)In any species ,males are more numerous than females.This helps the females to select the males.
b)Males are more attractive by the development of secondary sexual characters.
c)The males which are provided with briliant colour.Concipicous ornamentation,pleasing behaviour,mating dances,melodious songs and sounds are given more preferences by the females.
d)In some species like quarreles are fight among themselves for the possessions of females.
e)Sexual selection operates on only one sex of the species i.e in males.Hence the males alone develop more desirable characters . But natural selection operates on both the sex equally.
f)Sexual selection is concerned with reproduction.While natural selection is concerned with the survival of the individual.
2.Artificial Selection Theory
i)It is another supplementary theory proposed by Darwin.It is similar to natural selection theory.In natural selection theory ,the environment is the operative force and it selects the animals with favourable and desirable characters.
ii)In artificial selection man selects the animals and plants with desirable characters.The desirable characters which are useful for the welfare of human beings.
iii)For example the production of a large numbers of eggs by hen,the production of more quantity of milk by cow,good yield in crops etcetera are the desirable characters.
3.Theory Of Pangenesis
i)This theory explains the process of inheritance.It states that each cell of the body produces minute particle called pangens or gemmules.
ii)Gemmules are produced by all the cells throughout the life of an individual.These gemmules are transported and deposited in the germ cells by the blood stream.Thus each germ cells contain the minute replica of parents present inside the germ cell.
iii)During development each gemmule controls the development of the cell or organ from which it originated in the parents.This theory is not at all accepted now a days.
New Darwinism :
Neo-Darwinism is the modern theory of natural selection.
i)The recent supports of Darwinism modified the theory according to the latest developments in biology.
ii)In Neo-Darwinism the major defects of Darwinism are rectified.
iii)The Neo-Darwinism has the following ideas;
A.Experimental evidences.
B.Explanation to the objections.
A.Experimental Evidences :
The natural selection theory is supported by a number of experiment conducted by biologists.A few are given below ;
1.Industrial Melanism
i)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.
ii)Industrial melanism was observed in peppered moth Biston betularia living in Manchester an industrial city of Great Britain.
The peppered moth exist in two forms, namely melanic forms and non-melanic forms.
iii)The melanic forms are black in colour because they contain melanin pigments.
The non-melanic forms are light coloured.The light colour is due to the absence of melanin pigments.
iv)In Manchester all the moths were non-melanic before 1845.In the year 1845 only one melanic form appeared.There after the number of melanic forms gradually increased.
v)In 1848 it formed about 1% of the population and the remaining 99% was nkn-melanic forms.In the year 1898,the population and the non-melanic form was reduced to 1%.
vi)If there are white coloured moth in the soot-covered area,they will be easily seen by the predators and captured and eaten by them.On the other hand,if there are black coloured moths in the soot-covered areas,they match the colour of the surroundings and hence easily escape from the predators birds.
2.Resistance to DDT
i)In agriculture,insects,pests are controlled by the spraying of insecticides like DDT.However ,DDT cannot brings about 100% deaths of any species of insects.A certain number of individuals survive.The surviving individuals are resistance to DDT.
ii)The resistance is a character controlled by genes.
iii)After generation to generation more resistant varities are formed which have the advantages against insecticides.
B.Explanations To The Objection :
Neo-Darwinism tried to give convincing evidences and explanations against the objections and criticism raised against Darwinism which are as follows;
1.Germplasm Theory
i)The important objection levelled against Darwinism was that Darwin did not distinguish between heritable and non-heritable variations.
ii)This objection is answered by ‘Germplasm Theory’ proposed by August Weismann in 1904.
iii) According to this theory,the characters appearing in the somatoplasm disappear with the death of the possessor,so they are not inherited.The characters or changes appearing in the germplasm alone are inherited generation after generation.
2.Variation
A second objection against Darwinism is that it does not explain the origin of variation.
Now the origin of variation is clearly understood.It is produced by the following factors ;
a.Mutation. e.Isolation
b.Recombination. f.Founder Principle
c.Genetic Drift. g.Migration and Gene flow
d.Hybridization
a.Mutation :
i)Mutation is a spontaneous changes in a gene or in a chromosome ,this changes may produce an alteration in the character.
ii)The change that occurs in gene is called gene mutation or point mutation,the change that occurs in a chromosome is called chromosomal mutation or chromosomal aberration.The mutation produces a different character.Thus mutation produces variations.
b.Recombination
The formation of gene combination not present in the parental type is called recombination.It is a process of mixing of the available genes.
c.Hybridization
i)Hybridization refers to the crossing of two species resulting in the formation of hybrids.
ii)The hybrids share the genetic materials from two different species.Hence they develop new characters.
e.g Mule is a hybrid of horse and donkey.
d.Genetic Drift
i)In small population genes frequency change or fluctuate purely by chance,the smaller the population the greater the chance for changes.
ii)The random changes in gene frequency by chance in a small population is called genetic drift.
e.Isolation
i) Isolation is the separation of individual of a species by some barriers,which prevent interbreeding.
ii)Geographical isolation causes two populations to be separated.The geographically separated population are called allopatric populations.
f.Founder Principle
i)This principle was proposed by Mayr (1963) and Sheppard (1960). It states that when a new population is formed in isolation it’s gene pool is not identical with that of the parent population because of sampling error.
ii)This difference is further improved by the different types of evolutionary forces acting independently on the two populations.This leads to genetic divergence or variation.
g.Migration and Gene flow
i)The gene pool of a population is much affected by migration .This happens when the isolation is incomplete or when the isolation disappear or when the animals exhibit fast migartion.
ii)In any case ,animals belonging to different populations or species are brought together and they interbreed .Thus there is a occurence of gene flow from one population to another.This brings about variation.
iii)Gene flow is a source of variation.Gene flow from adjacent related population,which introduces new genes and gene combination into given population.
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