It is slow, progressive, continuous and irreversible change of organisms from simple and undeveloped form to complex and developed form in long course of time.
The first theory of organic evolution was postulated by a French biologists Jean Baptiste Lamarck in 1809 A.D., which is popularly known as Lamarckism. Lamarckism is also described under the name of “inheritance of acquired characters”.
FACTS OF LAMRCKISM:
Lamarckism is based upon following important facts:
a) Tendency to grow:
The internal forces of and organism cause to increase in the size of parts of an organism. The new structures appear because of an inner want of the organism. The internal forces of life tend to increase continuously the size of an organism and its component parts. This increase in size is up to a certain limit which is determined by the life itself.
b) Environmental Effects:
The environment in which the organisms liv influences them and this influence leads to change in their habits. Then, the organs of an animal became modified in appropriate fashion in direct response to a changing environment. This results the formation of new organs.
c) Use and disuse of organs:
According to the Lamarckism, the efficiency and development of an organ is directly proportional to its use and disuses. The more continuously used organs become more developed and enlarged while unused ones gradually weakens and finally disappears.
d) Inheritence of acquired characters:
According to this, organisms gain some characters during their life span from the surrounding and the changes in the surrounding. In a long course of time, organisms become entire different from their ancestors with new characters acquired from the pre-existing organisms.
Examples of Lamarckism:-
a) Formation of web toes in water birds:
The present water birds and ancestors were the terrestrial birds or land birds. Some of them could not find food in sufficient amount on the land and migrated into water in the search of food and security. They spread their toes while moving in water. The skin extended between the toes and formed webs for swimming. Hence, the present water birds are accompanied with the web toes.
This illustrates the law, “formation of new organs or environmental effects”.
b) Formation of long necked and legged giraffes:
The horse like short necked giraffes were the ancestors of the present long necked and legged giraffes. They used to graze on ground. But during certain course of time, there became the scarce of the grass on which they feed. Due to this, they had to depend on the foliage of the trees. As the trees were high and big, they continuously have to stretch their neck and legs while eating. Due to continuous use of necks and leg for stretching, their neck and leg gradually became long and strong. In a long course of evolution the present giraffe with long neck and legs are evolved.
This illustrates the law, “Use and disuse of organs” and “Inheritance of acquired characters”.
c) Loss of limbs of snakes:
The ancestors of the present limbless snakes with limbs. In the course of time, those snakes had to live in bushes and burrows. During which their body became elongated as they had to pass through them. For passing through the narrow spaces their limbs were not used continuously and finally, limbs were disappeared by continuous disuse. Thus, present snakes are limbless unlike other reptiles.
This illustrates the law, “Use and disuse of organs”.
Objection to Lamarckism:
The following are the objections to the theory of Lamarckism:-
- Our eyes are continuously used for watching & as told by the law; “use and disuse of organs”, it should be developed. But the eyes become more and moreweaker as we grow up older.
- The organs in the organisms which are useless in the initial stages of life do not vestige but develop later on in the body and become functional.
- The characters acquired by the organisms from surrounding are not found to be heritable. Only those characters affecting germ cells are found to be heritable.
- Mutilations are not inherited from parents to offspring.
- Only the formation & development of organs due to environmental effects have been explained but not their roles in incipient stage. Thus, this theory gives the information on “Organic evolution”. However, it does not stand longer, because it has failed to explain the correct mechanism of evolution. So, it has a little support today.
Germplasm Theory (Neo-Lamarckism):-
This theory is given by German biologists August Weishmann. According to this theory, “in living organisms, there are two types of cells:
Weishmann proved that only the changes in germinal cells are inherited from parents to offspring but not the changes in somatic cells. For this, Weishmann cut the tail of mice for 20 generations and measured the tails in each generation. In all generation of the mouse, he found the shape and size of this tails similar or equal. This was because there were changes in only the somatic cells not in the germinal cells.
This is the most popular theory of evolution given by a British biologist Charles Robert Darwin in 1859 AD. In this theory, Darwin explained about the origin of species by natural selections. So, this theory is also called, “Theory of Natural Selection”.
Facts of Darwinism:-
Darwinism is based on the following important/fundamental facts:
Over population of off-springs:
Nature has provided an enormous power of reproductions or fertility, which can lead to over population of off-springs. This may cause rapid increase in organism number.
Salmon: A salmon produces 28 million eggs in one second. If all of them survive, a single species will occupy all the sea after few generations.
Struggle for existence:
Due to increase in the number of organisms, there arises the scarcity of food and space. Similarly, organisms have to struggle for climate and breeding place.
Darwinism tell about the following struggles:-
1.Intro-species: This may occur between the members of same species. For E.g. two dogs fighting for a meat-piece.
2.Inter-species: This occurs between the members of two different species. E.g. cat chasing mouse and mouse being escaped.
The organisms have to struggle against the environmental factors such as extreme heat or cold, lightning, earthquake, volcanic eruption etc.
d) Natural Selection (Survival of the fittest):
Due to overpopulation, the scarce of food, shelter and breeding place takes place. In this course, organisms struggle against different factors in environment. Those who are best acclimatized in the environment will be selected by the nature and other become extinct soon.
Herbert Spencer coined this term for the first time and Darwin termed as natural selection.
Variation and Heredity:
According to this, variation are of two types; useful and harmful variations. Harmful variation cause the organisms unfit in the struggle for existence and cause their elimination while useful variations make them adapted in the environment.
Variation are of 2 types:-
1.Continuous: It is minute variation between generations to generations. For E.g. variation between two brothers etc.
2.Discontinuous: It is the sudden variation which is very different, called as mutation. For E.g. Occurrence of five legs in a new calf.
Origin of new species (Speciation):
In this course of struggle of existence and adaptation in the environment, the successful and advantageous variations are transmitted to the off-springs by the organisms. The changing environment causes variation in generations to generations over a long period of time, and finally results in the origin of new species.
Drawbacks Of Darwinism:-
- Darwin described about slow and small variations during organic evolution but he didn`t describe about usefulness of underdeveloped organs at initial stage.
- He didn`t mention abut use and disuse of organs and also about vestigial organs.
- Darwin didn`t mention about mutation which is major factor of organic evolution.
- Darwin described about survival of the fittest but not about the arrival of the fittest.
- Darwin didn`t explain (differentiate) somatic and germinal variation.
- Darwin did not describe about over specialization of certain organs which may lead to the extinction of organisms.
- The Pangenesis hypothesis of Darwin was kept in dark.
Neo-Darwinism “OR” Modern Synthetic Theory of Organic Evolution:
Neo-Darwinism is the modified form of Darwinism. This theory is modification of Darwin`s theory along with cytological and genetical information regarding the organic evolution. This is based upon the following facts:-
Sudden heritable changes that occur in the chemistry of genetic material of organisms are known as mutation. It can bring drastic change in offspring and lead to origin of new organisms. Basically, mutations are of two types: gene mutation and chromosomal mutation. Both gene and chromosomes determine the characteristics of organisms. So the sudden change in them can lead to variation of characters of offspring. Useful variation leads to better adaptation while harmful variation leads to extinction of the organisms.
Thus, mutation produces variations in offspring.
Genetic recombination is the nature of genetic variations caused by reshuffling of genes during sexual productions or gamete formation. In other words, it is the production of new gene types from already existing genes. Recombination are of the following types:-
- The productions of gene combination containing in the same individuals of two different alleles of same gene or the productions of heterozygous individuals (meiosis).
- The random mixing of chromosomes from two parents to produce a new individual (sexual reproduction).
- Mixing of a particular allele with a series of genes not previously associated with it, by an exchange between chromosomal pars during meiosis, called crossing over to produce new gene combinations.
According to this theory, nature does not itself brings variations in the organisms but supports the useful variations. That means natural selection does not produce genetic change, but once the genetic change has taken place, it acts to encourage some gene over other. It rejects weaker ones and supports the winners during adaptations.
Thus, natural selection guides the population towards adaptation.
According to this, when the potential mates are isolated by various factors such as geographical locations, physiological factors etc. for a long time, then after the long time their inter-breeding capability is lost. This phenomenon is known as reproductive isolation. Migration and hybridization helps to increase the genetic variability to the population which may lead to formation of new species.
This means the random changes in the frequency of gene in a gene pool. It does not occur by natural selection, rather occurs by chance. This is when there is alternation in the number of organisms among small population, due to factors like migration, epidemics etc. genetic drift occurs.