THEORIES OF ORGANIC EVOLUTION: Pre Darwinian, Darwinian and Post Darwinian

The problem of the origin and evolution of man, strictly in its biological viewpoint, needs the review of the origin of life in its broadest sense and of the different explanations given for this phenomenon earlier. Paleontology (जीवाश्म विज्ञान) is one of the sciences which have made the greatest contribution to the knowledge of the genesis of life on earth and of its gradual evolution and complication. The earliest fossils found in the earliest of the earth’s layers of the Archaeozoic era belong to diversified creatures that give evidence to an already existing complex and highly developed organic life.

Many theories were floated to explain the concept of evolution. Some important ones are here.

Theory of spontaneous generation or Abiogenesis: 

The theory of spontaneous generation is as old as human thought. It is well known that life arises only from pre-existing life and assumes that life originated from inert, inorganic matter as a result of a series of Physico-chemical conditions which must have existed at a given moment during the evolution of the earth. According to this theory worms generated from manure; insects from dew, rotten slime, dry wood, sweat and meat; frogs and salamanders from coagulated slime; and toad, snakes, and mice from the mud of the river Nile. The scientists Aristotle, Thales, Plato, and Von Helmont believed this idea of abiogenesis until the 17th century. Later this theory was disproved by Francess Redi, Spallanzani, and Louis Pasteur. 

Theory of Eternity of Present Condition:

This theory argues for the unchangeableness of the universe. It holds that organisms remain unalterable throughout their individual existence and will continue in the same unchanging state throughout eternity(अनंत काल).

Theory of Special Creation or Creationism:

According to this theory, the living organisms on the earth were created by divine power in six days. Father Suarez (1548-1671) is a strong supporter of this theory. The created organisms exist unchanged from the day of their creation. This concept of special creation was followed until the middle of the 19th century.

Theory of Catastrophism:

Cuvier (1769-1832) and Osbinge advocated that the earth was subjected to periodic catastrophes(आपदाओं). These catastrophes destroyed life from time to time and created a new and special form of life after each destruction.

LAMARCKISM:

Lamarckism is the first truly comprehensive theory of evolution. It was proposed by a French-born biologist, Jean Baptiste de Lamarck (1744-1829). This theory is also called the Inheritance of Acquired Characters and explains the origin of new species. This consists of four principles which are: the internal urge of the organism, direct environment and new needs, use and disuse theory, and inheritance of acquired characters.

Lamarck, specialized in animal classification, realized that the various species could be fitted into an orderly relationship that formed a continuous progression extending from the simplest little polyp at one end to the man at the other. He was the first naturalist to become convinced that animals could be modified in order to adapt to the environment and that species were not constant but were derived from pre-existing species. He did not believe, however, that this environmental influence acted “directly upon the organisms but rather that the transformations they underwent were due to use or disuse of organs motivated by individual needs in response to environmental influences. His theory was not well-received and he suffered social and scientific ostracism(वैज्ञानिक अस्थिरता).

In 1809, Lamarck put forth a complete thesis of evolution in his masterwork and classic book Philosophie Zoologique. It is popularly known as the Inheritance of Acquired Characters. He was the first evolutionist to conclude that evolution is a general fact covering all forms of life.

The salient features of the Lamarckism are:

> species change under changing external influence,

> there is a fundamental unity underlying the diversity of species and

> the species progressively develops.

Lamarckism consists of four principles that are briefly discussed.

=> Internal urge: The internal forces of life tend to increase the size of an organism, not only as a whole but in every part as well up to the limit of their function;

=> Direct environment and new desire: Each organ or part is the outcome of a new movement which in turn is initiated by a new and continuous desire or want. Thus, the formation of a new organ (or part) becomes necessary to produce the newly desired movement;

=> Use and disuse: The development of an organ is in direct proportion to its use. Continued use strengthens the organ little by little until its full development is attained, while disuse has the opposite effect, the organ diminishing until it finally disappears;

=> Inheritance of acquired characters: All that has been acquired or altered in the organization of individuals during their life is preserved and transmitted to new individuals who proceed from those who have undergone these changes.

In support of his theory, Lamarck cited the following examples.

> The most frequently cited example of Lamarckism is that of the long neck of the giraffe. An original deer-like ancestor of giraffe found the supply of grass and herbs inadequate. Hence the giraffes were forced to feed on the foliage of trees. In the process of reaching the leaves of higher branches, its neck got stretched and the forelegs were raised. This process of stretching the neck was continued for generations to reach the foliage of taller trees and resulted in a long neck leading to the origin of modern species.

> The origin of snakes was explained as a loss of limbs stemming from the habit of moving along the ground and concealing themselves among the bushes. This habit led to continued efforts to elongate the body in order to pass through narrow places and, as a result, the animals acquired a long narrow body. Since long legs would have been useless and short legs would have been incapable of moving the elongated body, continued disuse finally caused the total loss of limbs.

> In blacksmiths, biceps muscles in the right arm are more developed because of their continuous use.

It is unfortunate that Lamarck had to give such unnatural explanations. It is indeed unfortunate again that Lamarck is best known for his incorrect theory of heredity by acquired characteristics. But one must remember that his concepts were set forth in the early nineteenth century when the intimate structure of organisms, as well as the true mechanism of fertilization and sexual reproduction, was as yet unknown.

Criticism of Lamarckism

Lamarckism faced severe criticism and Lamarck had to defend them until his death. Some of the objections raised against the Lamarckian theory are as follows.

> The first principle on the tendency to increase in size is true in the case of many organisms. However, this is not universally accepted and there are instances to show a reduction in the size of the organs also.

> The second principle ‘new organs develop upon new desires’ is also not true. If every human being who desires to fly in the air should develop wings, but such is not the case.

> The third principle of Lamarckism, the use and disuse theory, has met with strong objections. If this theory is correct then the size of the eyes of a person should also increase with age who reads the books frequently. Again, we know it is incorrect.

NEO-LAMARCKISM:

A group of evolutionary biologists such as McDougall, Spencer, Cope, Packard, Kammerer, and Sumner, etc. further studied and modified the Lamarckism. This modified version of Lamarckism is called Neo-Lamarckism. The following are some of the evidence for the inheritance of acquired characters.

> In order to prove the inheritance of acquired characters McDougall conducted experiments on rats. During his experiments, he used a tank filled with water and dropped the rats into it. The tank was provided with two exists, one exit was lighted and the other was dark. The lighted exit was provided with electric shock and the dark one without electric shock. The rats tried to escape through the lighted route which was provided with electric shock but could not. After repeated trials, the rats learned to escape through the dark route. These rats were then bred and the offspring were given the same type of training. It appeared that the speed of learning was increased from generation to generation. Thus he concluded that the learning habit was inherited by rats.

> Kammerer did some experiments with Proteus anguinus, which lives in complete darkness. This amphibian is blind and colorless. He observed in Proteus anguinus, the development of normal eyes and color of the skin when exposed to daylight. These somatic characters were inherited to the next generation.

> The white mice were exposed to the higher temperature say 20-30 degree C by Sumner. The body, hind limbs, and tail of the mice increased in length. Further, he observed that this character was transmitted to their offspring.

DARWINISM:

Charles Robert Darwin (1809-1882) was born on 12th Feb 1809 at Shrewsbury in England. As a student, he studied the Greeks and encountered the views of Thales, Empedocles, and Aristotle. Darwin was given an opportunity by the British government in 1831 to travel by HMS Beagle for a voyage of world exploration. He went on the voyage(जलयात्रा) from 1831 to 1833 and explored the fauna and flora of a number of continents and islands of which Galapagos Islands are the most important. Here Darwin found a living laboratory of evolution. 

The publications of T.R. Malthus, Sir Charles Lyell, and Alfred Russel Wallace influenced Darwin very much. 

Darwinism consists of five principles which are: Prodigality of overproduction, Variation, and Heredity, Struggle for existence, Survival of the fittest, and Modifications of species. Darwin published “The Origin of Species”. Darwinism is the term coined for the explanation offered by Charles Darwin for the origin of species by natural selection.  Darwin published his concept of evolution in his book entitled “The Origin of Species”. Darwinism consists of the following five principles.

=> Over-production: Many more individuals are born each generation than will be able to survive and reproduce. 

=> Variation and Heredity: There is natural variation among individuals of the same species. Many of the favorable adaptations are hereditary and

are passed on to the progeny of future generations. 

=> Struggle for existence: Organic beings increase by a geometrical ratio, while food production only increases in an arithmetic ratio. So that in a very short time, an area will be overpopulated with any one species, unless something happens to check the increase. As a result, there is a struggle for existence which is threefold as given below.

> Intraspecific struggle: The intraspecific struggle is found among the individuals of the same species. The competition is heavy in case of intraspecific struggle because the needs and requirements of the members of the same species are the same who live in the same environment. It is the most severe check on the rate of reproduction.

> Interspecific struggle: It is found among organisms of different species living together. Members of one species struggle with other species for similar requirements i.e., food, shelter, and mating.

> Struggle with the environment: Living organisms struggle with adverse environmental conditions like floods, cold waves, heat waves, and earthquakes, etc.

=> Survival of the fittest or natural selection: Individuals with certain characteristics have a better chance of surviving and reproducing than others with less favorable ones. 

=> Modifications of species: Gradual modification of species could have occurred over the long periods of geological time through additive processes occurring in the past in the same manner as they are occurring in the present.

Criticism of Darwinism:

Several objections were made to Darwin’s theory.

> Darwin’s explanation is inadequate because selection creates nothing. It merely eliminates or preserves already existing variations without indicating their cause, which is the main question.

> Individual differences that may give rise to variations affect the reproductive cells little or not at all.

> Overspecialised organs such as the huge antlers of the Irish deer cannot be explained on the basis of natural selections.

> It cannot account for the degeneracy(पतन) of certain characters.

> There is doubt about the struggle for existence being as fierce(भयंकर) as it had been supposed to be.

NEO-DARWINISM:

The modern theory of the origin of species or Evolution is known as Neo-Darwinism. The Neo-Darwinists like August Weismann, Earnest Heckle, Lyell, Huxley, Wallace, and Simpson supported the natural selection. Later R.A Fisher, Sewall Wright, and J.B.S. Haldane explained natural selection by neo-Darwinism.

There are many lines of evidence supporting Darwin’s theory that natural selection is the basis for evolutionary change.

=> Fossils: Geology and paleontology have contributed one line of information that supports current concepts of evolution. Evidence embodied in fossils shows gradual changes associated with geological change. This has led to conclusions about the progression of life forms that have evolved during the history of the earth and about the evolutionary relationships between various phyla and species.

=> Adaptation in living organisms: Perhaps the strongest evidence, and certainly among the most interesting in support of the theory of natural selection, consists of the innumerable examples of structural variations and behavior patterns found in animals and plants, which are of particularly adaptive advantage to the lives, habits, and environment of the individual. Examples are (1) parasitic birds: cuckoo birds and sparrow bird, (2) adaptation in coloration: moths in different environments, and (3) physiological adaptation: animal’s native to desert regions, rats and camels.

Difference between Darwinism and Neo-Darwinism:

It must, however, be noted that the modern concept of natural selection has been considerably broadened and refined and is not quite the same as Darwin’s. Darwin recognized the fact that natural selection involves differential reproduction but he did not equate the two. In modern theory, natural selection is differential reproduction, plus the complex interplay in such phenomena as heredity, genetic variation, and all the other factors that affect the selection and determine its results. In the Darwinism system, natural selection was mainly elimination, death of the unfit, and survival of the fit in a struggle for existence, a process included in natural selection as it is now known but not forming all or even the major part of that process.

Conclusion:

The evolution that has taken place in living things i.e., plants and animals is called organic evolution. Various concepts (theories) explain the process of Organic Evolution; the important ones are Lamarckism, Darwinism, and Synthetic Theory of Evolution. The former consists of four principles of evolution. Darwinism consists of five principles viz., the prodigality of overproduction, variation, and heredity, struggle for existence, the survival of the fittest (natural selection), and modifications of species.