The following points highlight the four theories for organic evolution of living organisms. The theories are: 1. Lamarckian Theory 2. Darwinism and the Theory of Natural Selection 3. Synthetic Theory of Evolution 4. De Vries—The Theory of Mutation.
Theory # 1. Lamarckian Theory:
Jean Baptiste de Lamarck (1744-1829), a French Scientist, was undoubtedly the first biologist to propose a definite theory explaining the evolution of living organisms. He published his theory in the year 1809 in Philosophic Zpologique.
Lamarck, as a philosophical zoologist, attempted to explain the forces operating in evolutionary process. Lamarck’s idea makes a compromise of Buffon’s and Erasmus Darwin’s views on evolution.
Lamarck supposes that changes in organisms in course of time are imperative but these changes are, according to him, not the outcome of direct action of the environment, but it acts on the internal structures in an indirect way.
His theory is best known today as the theory of Inheritance of Acquired Characters. Lamarckian analyses of evolution are unacceptable to the present day biologists.
Lamarck and the inheritance of acquired characters:
Lamarck based his theory of evolution on certain biological principles. He believed that the living organisms change in course of time to become more complex. From amoeba to man there is a march of evolutionary progression caused by the transmutability of species. This particular aspect of his theory is universally accepted to be true.
According to Lamarck, nature works to provide incentive for the development of new structures Out of an outcome of new function. Lamarck holds that variations in organisms arise either through conscious efforts or through reaction to a change in the environment or through the effects of use and disuse.
It is supposed by him that the continual use makes a structure greatly developed and disuse, on the other hand, makes the structure atrophied. The variations thus acquired during the lifetime of the organisms are heritable and by this way a new or different species may eventually be produced.
The main theme of Lamarckian doctrine is the inheritance of acquired characteristics. A thorough analysis of his theory reveals some truth of evolution, but most of his interpretations are subjected to violent criticism. His attempt to explain will the phenomena in evolution by his concept of transmission of acquired characters appears inadequate and his arguments in most cases are solely deductive.
Acquired characters originate by change of function in somatoplasm which transmit the characteristics in successive generations. Lamarck regarded the acquired characters as bodily characters initiated in one generation and these are heritable.
The essence of Lamarckian concept:
The essence of Lamarckian concept of progressive evolution thus can be categorised in the following way:
1. Spontaneous generation of simple forms of life was followed by development of more Complex organisms. Living organisms and their component parts tend continually to increase in size.
2. The environment governs the natural tendency of an organism to show structural changes.
3. Use and disuse cause variation in a structure.
4. Characters acquired in one’s lifetime are transmitted to one’s subsequent generation.
The first part of Lamarck’s first law states that the lower organisms were capable of spontaneous generation from matters of inorganic nature and as such there might be some validity in it. But the second line of the law is the statement of a supposed fact which unfortunately is far from truth.
Many groups of organisms show no tendency whatever to produce strains leading to gigantism, though in some restricted groups of terrestrial animals, viz., elephant, horses and camels the increase in size is quite established. Conversely the reduction in size is a prominent feature of evolution in a number of groups.
The second law states that environment acts directly on organisms and causes the production of new characters. Lamarck here believes in the fact that in animals the environment acts through the nervous system or in other words the desire of the animal brings forth the formation of new structures.
To put the idea in its simplest form Dodson (1960) has written “the man who mused—Birds can fly, so why can’t I? Should have sprouted wings and taken to the air”. Lamarck never presented such crude examples but the fact remains that the second law is false.
The first two laws of Lamarck display a vitalistic outlook. These two laws never blossomed further and have been disregarded by later biologists.
Since there is some mechanistic truth in the third and fourth laws of Lamarck they are considered together as a principle of evolution though greater emphasis has always been on the fourth law.
Evidences advanced by Lamarck in favour of his Third Law:
The supposition made by Lamarck that the continual use of a particular structure in the body of an individual causes the enlargement and strengthening of the structure has certain factual bases. Lamarck cited the case of the webbed toes of the aquatic birds. The development of web in them is caused by constant stretching of the skin between the toes.
The length of rabbit’s pinna, that of the neck of giraffe, the long talons of eagle, more development of right hand than the left hand in man are examples of use. The long-necked modern giraffe had short- necked ancestor.
As a response to stretch up the neck further up to take the leaves from high trees, the neck increased in length and this continued for generations until the modern state is reached. Lack of use or disuse causes the degeneration and ultimate elimination of a particular structure.
The instance of modern snakes can be placed here. The ancestor of the existing snakes was lizard-like forms with two pairs of well-developed limbs. By constant disuse of the limbs as a response to fossorial adaptation the limbs completely disappeared in all snakes excepting their vestiges in pythons.
Lack of pigment and degeneration of visual apparatus in cave dwellers due to lack of light in caves are the instances of disuse. Degeneration of lateral toes in the evolution of horse is another example of disuse.
The fourth law: The final proposition arrived at by Lamarck states that the modifications produced by the other three laws during the lifetime of an individual will be inherited by its offsprings, with the result that changes are cumulative over a period of time.
The trial of Lamarckian concept:
Lamarck defended his theses vigorously until his death though he failed to convince his contemporary scientists. But the apparent simplicity of his theory drew the attention of many workers in the line of evolution and an array of experiments followed.
Inheritance of instinct:
William Beebe showed the inheritance of instinct in birds. Birds show many instinctive characters. Instinct is usually defined as the natural impulses by which organisms are guided to perform certain activities. The instinctive characteristic is based upon inherited knowledge.
Selection of bird to demonstrate the inheritance of instinctive traits has certain advantages. These features are attributed to the offsprings by the parents. Beebe showed by experimentation that the birds, raised from incubated eggs, without having parental care, manifest all the characteristic instinctive features of the race.
Kammerer’s experiment on salamanders. Kammerer performed experiments with salamanders to give support to Lamarck- ism. He reared salamanders in different coloured boxes, such as red, yellow, blue, etc. The salamanders according to him afterwards took the hue of the surroundings.
Thus he supported the doctrine of inheritance of acquired characters. Similar experiments were repeated but it could not be proved. Noble and Przibram reported that Kammerer’s experiment was a fraud. It came to light that Kammerer was let down by his associates and as a result he had to commit suicide.
Weisman (1834-1914) started performing experiments by cutting the tails of mice for successive generations (35). He found that even in the last generation the mice grew tails as long as their first ancestor. Crude did long term but similar experiments with the tails of sheep but could not prove the truth of Lamarckian theory.
McDougall (1938) performed experiments on learning in rats. Rats were dropped into a lank of water, from which there were two exits, one lighted and other dark, but not always the same one. The experiment was so devised that a rat leaving by the lighted exit received an electric shock, while one leaving by the dark exit received no shock.
Thus the number of trials required by a particular rat to learn to escape by the dark exit constituted a measure of the speed of learning. These rats were then bred and their descendants were similarly subjected to training.
It appeared that the speed of learning increased from generation to generation. McDougall concluded that learning is an acquired trait and is inherited. He himself demonstrated that the speed of learning varies directly with the intensity of the shock.
Repetition of similar experiments by Crew (1936) for 18 successive generations and that of Agar and his colleagues (1954) for 50 generations failed to produce the results obtained by McDougall. Thus it became clear that effects of training in mice are not inherited. Grew is inclined to ascribe the discrepancy to an insufficiency of controls and an inadequate attention to genetic method on McDougall’s part.
C. C. Guthrie exchanged the ovaries of black and white coloured hen and claimed that a change came about in them. Davenport showed that no such effects are produced.
Gayer and Smith (1918-1924)—It is known that germ cells of the ovary of the female and testes of the male are responsible for inheritance. It is also a fact that germ cells are supplied by blood and the same blood is supplied to other body parts.
It, then, may be presumed that the substances carried by blood might serve as go between conveying to the germ cells the effects of bodily changes. Gayer thought that the role is played by Antibodies. He together with Smith performed an experiment.
A solution of lens substance obtained from rabbits was inoculated into fowls. These fowls produced antibodies against this foreign lens protein. The fowls’ serum bearing these antibodies was now injected into pregnant rabbits.
Some of the offsprings thus born were with degenerated or malformed eyes. When these offsprings were raised and bred, it was noted that the eye defects were inherited in subsequent generations.
Though the experiment gave a positive support, it is questionable whether the case reflects inheritance of acquired characters’ or not. In the experiment it is more likely that the antibodies acted on the eyes of the embryo or on the genes responsible for eye structures of the embryos directly.
If this be correct then it is never a case of inheritance of acquired characters. Pavlov (1923) withdrew his earlier statement that the conditioned reflex in mice is inheritable.
Lamarckism, however, persisted in U. S. S. R. Karl Marx and Engels were strong supporters of Lamarck and opined that Lamarckian way of inheritance would guarantee the future improvement of human race. After the arrival of T. D. Lysenko on the Soviet scene with the vernalization technique Lamarckism got a strong foothold in U.S.S.R.
Lysenko drew big conclusions from a single plant. He and his school were not above falsifying data or censoring classic references, and they systematically opposed opponents at the most overt level of xenophobia and political partisanship.
“It is clear that in the absence of public criticism, with a monopoly of publication and support, surrounded by an atmosphere of repression and fear, the provocative, self-praising Lysenko could prosper” (The rise and fall of Lysenko. Translated by L. M. Lerner, Columbia University Press).
Lysenko’s strange view had cost Russian science and agriculture a generation of progress. Lysenkoist geneticist is a most derogatory compliment.
The Jews and Muslims practice circumcision for centuries and yet there is no reduction of prepuce in them. The feet of the Chinese woman is another example. For generations the feet of the Chinese women were kept bound yet, the practice has failed to bring any modification of the feet of present-day Chinese women.
The work of Metalnikov (1924) on immunity of wax-moths and of Sladden and Hewer (1938) on food preference in stick insects seems to demand prima facie a Lamarckian explanation. Huxley, however, is of opinion that in view of the fate of other claims and of the theoretical difficulties too much weight must not be attached to such isolated ones.
Darwinian concept of Pangenesis— an attempt to tide over the Lamarckian crisis:
Charles Darwin was supporter of Lamarckian concept of inheritance of acquired characteristics. These are acquired anew within the lifetime of an individual. Darwin postulated a theory of heredity to explain the inheritance of acquired characters.
His theory is called the theory of Pangenesis which explains how characters are transmitted from parents to the offsprings. He assumed the existence of imaginary hereditary particles called the pangenes or gemmules.
These particles are produced in the life-time of an individual by each and every part of the body. The pangenes possess all the distinctive characteristics of the particular parts from which they were produced, together with modifications acquired in the life-time of the individual.
The pangenes are discharged into the circulating blood-stream, from there these are all collected together to form the germ cells. The germ cells eventually unite to form the zygote which gives rise to the new individual. This phenomenon insures the recurrence and development of the parental characteristics in the offsprings together with handing over of the acquired characteristics.
Weismann’s theory of germplasm:
Weismann was not convinced by the concept of pangenesis of Darwin. He advocated that living organisms are composed of two types of materials, the somatoplasm and the germplasm which are quite different.
The foundation of the germplasm theory of Weismann in 1892 directly gave a dread blow to the Lamarckian concept of inheritance of acquired characters. He regarded that every individual in bisexual forms starts its life from the zygote formed by the fusion of male and female gametes.
The zygote constitutes the marvellous bit of hereditary capital which forms the bridge of continuity between generations. So the inheritance of the parental characteristics by the offsprings must pass through the zygote.
In organisms, germplasm becomes isolated in the very early phase of ontogenic development. Boveri has shown the splitting off of germplasm in Ascaris as early as at the 16-celled stage. Of the 16 cells, 15 cells form the somatoplasm and the rest one is set aside as the germplasm.
To strengthen his contention, Weismann performed extensive experiments with white mice. Repeated artificial mutilations, such as cutting the tail of parents fail to produce abnormalities in the progenies. Variations due to mutilations are not inherited.
Any characteristic which is to be inherited, must certainly be stamped in the germplasm. Somatoplasmic changes are not inherited. Castle and Philips have transplanted ovaries of a black Guineapig to a white female whose ovaries have been replaced. The young’s produced were all black.
1. Not a single experiment has so far been able to provide positive evidence favouring the inheritance of acquired characters.
2. Lamarckian concept is not applicable to higher forms. However, it may be true for lower unicellular organisms where there is little or no separation of soma and germplasm.
3. The conscious effort as has ‘been advocated in case of giraffee can hardly be expected in the plant kingdom.
4. Some of the laws speak in favour of vitalism which is antagonistic to materialism in scientific thought.
5. Lamarckism suggests a theory without evidence.
6. It has been made clear that new organisms develop from the germ cells of parents and not from somatic cells. The germ cells are set aside earlier in the growth of an individual and are not affected by body cells.
7. There are numerous cases where Lamarckian inheritance is either impossible or exceedingly restricted. In higher mammals the internal environment is regulated with an extraordinary degree of constancy. The temperature of blood, the salt composition and acidity of blood are kept constant by elaborate and special mechanism.
The reproductive cells like all other cells in the body, are exposed to the internal environment supplied by bloodstream. How then can changes in the external environment be transmitted to them? The regulation of the internal environment provides an effective shock- absorber for alterations which could occur in the external environment.
Yet higher mammals have evolved as rapidly and in as obviously adapted ways as any lower type in which this buffering does not exist.
8. Insects provide a number of hard nuts for Lamarckian cracking. Social
Hymenoptera is such an insect. The bulk of the work in the Hymenoptera colony is, done by neuter females, while reproduction is entrusted to full females and the males which are less abundant. How is it then possible on any Lamarckian view to discover a mechanism by which the special instincts and structures of the workers have been evolved? The workers cannot transmit them because they do not reproduce.
9. Modern biology, taken by and large repudiates Lamarckism.
Modern concept of Lamarckism:
Lamarckism fails as an all-embracing cause of evolutionary process, because it has many shortcomings. It cannot explain the origin of new species and the origin of adaptations remains unsolved. It also cannot give any clue as to how the acquired characters are inherited.
The proofs advanced by Lamarck and some followers of Lamarck are proved to be based not on scientific fact. It will be needless to advance all the proofs of Lamarckian inheritance. Kammerer’s experiment with salamanders are not confirmed by subsequent workers.
Other experimental demonstrations have not been confirmed by subsequent investigations. Certain Lamarckians have advanced the view that inherited effects of function or environmental modification are so slight that they cannot be detected experimentally and require cumulative action through thousands of generation to become obvious.
Payne (1911) has shown an excellent example to demonstrate the failure of production of Lamarckian effects by disuse in Drosophila. Disuse of eyes in Drosophila cannot produce the effect of disuse even up to sixty-nine generations. Thus to plead the impossibility of detection is a counsel of despair which is totally unscientific.
Modern biologists do not recognise Lamarckism as a causative factor to explain the origin of adaptations during evolution. The discovery of genes and their role in heredity throw much light on the origin and inheritance of hereditary variations.
Lamarckian acquired characters are phenotypic variations and they cannot affect the genotypes. The converse is actually the fact. The phenotypic variations are the result of interactions of the genotypes and the environment. Although some characters are well explained by
Lamarckism, there are numerous cases which Lamarckism cannot explain properly. Lamarckism is not sufficient enough to explain the origin of species. Modern evolutionists regard the importance of Natural Selection as the only agent in evolution. Natural Selection works at the gene level. The units of inheritance are the genes and all sorts of changes or characteristics must pass through the pathway of genotypes.
Waddington has developed a new evolutionary point of view which tries to make a compromise between Lamarckian theory and the selection theory. Based on his work on Drosophila, he has advocated that the adaptive characters in Drosophila are genetically assimilated. If this particular concept is substantiated in future, this will revolutionise the entire concept of evolutionary progression.
Lamarckism as such is incomplete in itself to account for evolution. In the opinion of Ray Lankester, Lamarckism is “self-contradictory”. Although Lamarckian doctrine is quite inadequate to explain evolutionary process, it cannot be denied that Lamarck was the foremost figure to contribute much to focus the diverse attention of the biologists upon the central problem of organic evolution.
Modern genetics makes the fact crystal clear that whatever changes are induced in the phenotype, the change is not necessarily in the genotype. The failure of Lamarckism is mainly due to the failure of recognisation that the phenotype is the by-product of gene- reproduction.
Theory # 2. Darwinism and the Theory of Natural Selection:
With the rise of scientific methods of inquiry, the concept of organic evolution began to occupy a prominent place in the minds of the thinking men of the world. Although the germ of evolutionary idea has its inception with the Greeks, Charles Darwin was undoubtedly the foremost figure of the evolutionary, thought.
He paved the way for the general acceptance of the truth of evolution by his reasonable and acceptable theory of evolution.
Gradual change is the rule in living world and new species are constantly being produced by the evolutionary dynamics. Darwin advanced the concept of Natural Selection as a force to account for evolution. Natural Selection has been the central strand on which all the main facts of evolution rest.
Natural Selection is regarded as a causo mechanical process of trans-mutating of species ended in inductions and deductions. It is a sort of selective process eternally operating in Nature to improve and maintain the adjustment of living organisms to their surroundings and the way of living, whereby misfits, unfits and weaklings are eliminated.
To explain Natural Selection as a force in evolution, Darwin called attention to certain facts of nature. With the remarkable advancement of modern genetics, Darwin’s original concept of Natural Selection has taken rebirth and the whole of Darwinian concept is embarked upon a phase of modern synthesis.
Evolutionists of today, with the growing scientific advancement, are trying to penetrate deeply into the operations of the biological systems. Although Darwin’s idea of evolution has undergone extensive modifications, his concept is regarded as the most dominating feature in biology.
Foundation of Darwin’s theory:
Charles Darwin (1809-1882) profounded the concept of Natural Selection as the principal cause of evolution and this is universally taken to be true. He created a sensation throughout the world with the magic form of a single idea of the mechanism of evolution.
The publication of his famous book and the Bible of evolution, “On the Origin of Species by means of Natural Selection” in the year 1859 is the crowning event in the rise of evolutionary thought. Darwin came to the conclusion after about twenty years’ of thinking and researches.
He visualised living things in a new light during his memorable voyage in H. M. S. Beagle in the Atlantic and Pacific islands including Galapagos. His voyage was a voyage of discovery. Darwin with unquenchable intellectual curiosity gave shape to his observations and researches into a reasonable theory of evolution. The work of Malthus on population stimulated him very much in arriving at this conclusion.
Alfred Russel Wallace—Co-Discoverer of Darwinism:
The name of Alfred Russel Wallace (1823-1913) in the Scientific literature will always be associated with that of Charles Darwin so far as the publication of the theory of evolution is concerned. He was a contemporary and collaborator of Darwin and contributed a paper “On the tendency of varieties to depart indefinitely from the original type” in a symposium of the Linnean Society in the year 1858.
Wallace came from social origins so poor that he could not aspire to life in college.
He left school at 14 and apprenticed to a railway surveyor. Later he became a school master. The books in the library that were his Oxford and Cambridge. Wallace spent a wanderer’s life. He and Bates decided jointly to see the world.
The British Museum assured them that almost any specimen they might collect could be sold to pay their expenses. They aimed to keep duplicates for their own study as a path towards solving the problem of origin of species.
Off they went to the Amazon. Wallace returned from the Amazon after 4 years. Forced to flee a fire at sea, he spent 10 days in an open boat and lost nearly all his collections. After a year and half writing and talking in London, he concluded that “the very finest field for an exploring naturalist was to be found in the great Malayan Archipelago”.
He spent 8 years in these islands, making 60 to 70 separate excursions. He made his base in the island town of Ternate in a rather ruinous house.
Here he found peace to write the famous brief paper on Natural Selection that he sent out to Darwin to initiate public knowledge of the great theory. Like Darwin, Wallace also gave emphasis on “Natural Selection” as the main agent in evolution.
But the insufficiency of data and his theological inclination in later years brought no honour for him. Wallace also went on an expedition of nature and came to a theory, the working principles of which are exactly the same as that of Darwinism.
Owing to the honesty and generosity of Darwin, a warm friendship grew between Wallace and Darwin. Darwin always recognised the originality of Wallace’s conclusion and their two research papers were combined and published under the joint authorship in 1859. The whole credit went to Charles Darwin, but it should always be remembered that Wallace’s work was almost a paraphrase of Darwin’s work.
Credentials of the Darwin’s theory:
Before Charles Darwin, his grandfather, Erasmus Darwin and Lamarck brought the scientific idea that the present-day living organisms have evolved from simpler forms of the past. Darwin presented large body of evidences to convince the world about the truth of evolution. Heterogeneous collection of facts are marshalled into some deductions which are known as Darwinism.
As stated earlier Darwin discussed his theory of evolution with Natural Selection as the principal factor and supported his theory by elaborate evidences derived from the published research work and personal observations.
It is a piece of doctrine elevated to biological accuracy and a sort of assemblage of diverse biological phenomena. The deductions brought by Darwin from certain biological principles are shown in Fig. 1.15.
Darwin attacked the process of evolution from all corners. He tried to establish the fact of evolution in one hand and also tried to discover the mechanics through which it operates. Darwin based his Natural Selection concept on certain observable facts of nature and he has drawn the deductions from them.
Darwin was convinced by the fact that an enormous discrepancy existed between the number of individuals born and the number of individual survive. The balance of nature is maintained by Natural Selection. To illustrate this fact he has brought a large number of examples.
Prodigality of production:
Power of reproduction is one of the basic characteristics of living organisms. The living creatures reproduce at a terrific rate. Phenomenon of reproduction is a matter of fact and is a power bestowed on all living things. Darwin was acquainted with the famous doctrine of Malthus and found it applicable to the animal world. Organisms increase in a geometrical ratio, but the food production and space almost remain constant.
Without some checking mechanism, organisms will exhaust the available food and space. The following examples will show the rate of egg production in some animals. A female Salmon produces 28,000,000 eggs in a season, an Ascaris lumbricoides passes 700,000 eggs in 24-hours, an oyster liberates 114,000,000 eggs at a spawning and Rana catesbeiana produces 20,000 eggs annually.
Darwin has calculated that in oyster, if all the eggs produced by a single female, survive and multiply, the shells will heap up to eight times the size of the earth. He has also calculated that starting with a single pair of elephants (the slowest breeder amongst mammals) about nineteen millions of descendants would be produced within 750 years.
Constancy of food and space:
Without artificial means of increasing, the food and space can hardly be changed. Extraordinary power of productivity on the part of all living organisms will result over- crowdedness. Darwin believed that the prodigality of reproduction is very important because it results into a crowded condition and to the sharp competition for the necessities of life.
Darwin recognised that this over-crowdedness results into the struggle for existence.
Struggle for existence:
As a result of the prodigality of production and the constancy of food and space a competition for survival amongst the animals results, because numerous young’s are always being produced than can be sustained. The struggle for existence is a sort of competition and is threefold.
It may be
(1) Intra- specific struggle,
(2) Interspecific struggle and
(3) Environmental struggle.
The intraspecific struggle is the competition against the organisms of their own kinds. The interspecific struggle is the competition between members of different species. Instances of such type of struggle is universal in living world. Almost every living form depends directly or indirectly upon other living forms.
The environmental struggle is usually against the physical environments such as against excess of moisture or of drought, against extreme temperature (cold or hot), against other geological conditions.
No living organisms are alike and they vary appreciably. Darwin observed slight variations occurring in all parts of the organisms and these variations are the primary cause for which the children do not resemble exactly their parents. Geology also furnishes innumerable instances of changes in nature during the phylogenetic development of a particular form of animals in course of time.
As the struggle for existence is universal and the variations become the rule in nature, some’ variants become advantageous in the struggle for survival and others are disfavoured. Favourable variants will have great chances for survival and the unfavourable variants will fail to survive.
Survival of the fittest:
In the struggle for existence the organisms with variation in structure, habits or instincts may be better adapted to new conditions and will have better chance of survival. Consequently their offsprings will inherit those variations.
In course of time, these organisms will inevitably acquire superiority over the rest and less adapted forms. Variations bring organic diversity in response to environmental dynamics and result into adaptation.
Species are plastic and dynamic entities that have been moulded over long periods of time through Natural Selection. Species do not come de novo. Natural Selection enforces adaptation amongst organisms. Nature will select good varieties and others are crushed out. Natural Selection operates amongst the fittest and the new forms are stabilised and thus leads into speciation.
Darwinian concept of natural selection:
Darwin conceived that the consequence of potential increase in number will result merciless competition amongst and between the species. Any quality possessed by an individual which offers an advantage over others in the struggle for. existence would be insured to the survivors.
In subsequent generations a large number of individuals with advantageous characteristics. will be produced. With the repetition of selection in each generation all the survivors would eventually be of the favoured types.
Darwin had the idea that the improvement of organisms is actually the work of Selection. The ultimate result of Natural Selection is that each kind of organism tends to become more and more improved in relation to its condition. Such improvement inevitably leads to the gradual advancement in the organisation of a greater number of living beings.
The struggle for existence and the Natural Selection are highly metaphorical terms. The struggle for existence merely signifies that a number of individuals in each generation is bound to die before they -can reproduce and the Natural Selection is responsible for the differential survival and the reproduction of the variants.
A hundred years’ history of Darwinism:
Darwinism has stood the test of more than one hundred years and it underwent through differential phases— until its pervading influence makes its status high in the intellectual thought. Its hundred years’ history is very dynamic in nature.
Phase of ready acceptance:
After the enunciation of the theory of Natural Selection by Charles Darwin in 1859, it was accorded a warm reception. It was proclaimed as a masterpiece and was readily accepted by all the people of the world.
Before him, no one had forwarded any concrete suggestion as to how evolution could have occurred. Weismann was the greatest supporter of Darwinism and regarded Natural Selection as the principal cause of evolution.
Eclipse of Darwinism:
In late nineteenth century, the evolutionary studies became stereotyped and resembled the concept of obvious because it is essentially a materialistic concept.
Efficacy of Neo-Darwinism:
Neo- Darwinism has enabled us to analyse the working of Natural Selection in a way that was unimaginable in Darwin’s time. Modern concept of selection not only includes the idea of rejection but also the maintenance of the status quo of the species.
Natural Selection operates in a most contradictory fashion. It is actually a biological duality and has got two aspects. These are: Stabilising aspects. This aspect of Natural Selection keeps down the deleterious mutant genes as well as the gene combinations. By this way Natural Selection protects and stabilises the developmental pattern of the species. It results homeostasis.
This aspect of Natural Selection allows the species to maintain its hold on the changing ecological condition and also to control and overcome new ecological opportunities. It establishes new genotypes quite suited to the new conditions. The dynamic aspects of Natural selection operate in two ways—one is directional and the other is disruptive. This aspect is dealt separately in the last part of this topic.
The Neo-Darwinian concept of selection gets the greatest support from Mendelian laws of heredity. There is no organism without genotype and the genes constituting the genotype are the fundamental particles of life. It is the corporate genotype that determines the course of development in an environment. The genotypes should be changed to give an efficient functionating organism.
A variety environment is open to the genotypes and the genotype will choose that in which their fitness is highest and will eventually maintain a condition of equilibrium. The selection mechanism gives shape and order to these variabilities.
Neo-Darwinian concept of selection maintains that selection not only includes the idea of rejection of ‘unfits’ but also of the maintenance of ‘fits’. The fitness is due to the selection of genes. The efficacy of Neo-Darwinism is represented in Fig. 1.16.
Environment—the perennial source of genetic variability’s:
Every organism exists at the expense of an environment. New environments are constantly being produced. To survive, an organism must react to the environmental changes by homeostatic modification of the developmental pattern. Environment is a directive force in evolution. Environment does not change the organism as such, but it offers challenges to which organism may respond by its genotype.
Reproduction produces variety in the genotype:
The main consequence of reproduction is to produce an endless variety of genotypes. Some of them may have different longevity and viability and thus Natural Selection always gets new materials to work with. Fig. 1.17 shows the schematic representation of the relationship between reproduction, mutation and Natural Selection.
Mutations—source of hereditary variation and provide the material basis:
Any change in the hereditary materials is known as mutation. Evolution of diversified living organisms in this world is due to hereditary changes. Hereditary changes are essentially due to ‘mistakes’ in the process of replication of DNA molecule.
Mutations are classified into two broad categories:
A. Point or Gene mutations: When one or a few nucleotides within a gene are affected.
B. Chromosomal mutations: When the number of chomosomes or the number or the sequence of genes in a chromosome are affected. Chromosomal mutations are of different types, viz.
1. Changes in the number of chromosomes:
Changes in the number of chromosomes are due to:
(i) Fusion— involving the fusion of two non-homologous chromosomes into one resulting in loss of a centromere,
(ii) Fission—involves splitting of one chromosome into two with the acquisition of an additional centromere,
(iii) Aneuploidy—When one or more chromosomes of the normal set may be lost or present in excess and
(iv) Haploidy and polyploidy—Most organisms are diploid having two sets of chromosomes in somatic cells.
Some organisms are normally haploid possessing only one set of chromosomes. Polyploidy arises when more than two sets of chromosomes are present.
2. Changes in the number of genes in chromosomes:
Changes in the number of genes are due to:
(i) Deletion: when a segment of DNA containing gene/genes is lost from a chromosome, and
(ii) Duplication: when a segment of DNA containing gene/genes is present more than once in a chromosome.
3. Changes in the sequence of genes in the chromosomes: These are due to:
(i) Inversion—When the sequence of a block of genes is inverted in a chromosome and
(ii) Translocation—Change of the sequence of a block of genes in chromosomes.
Heredity is a conservative process while the genotype has the inherent property of stability. But the genotype of an organism may be changed by mutation and recombination. Mutation provides the raw materials for Natural Selection to work with.
Evolution is impossible without mutation, because mutation introduces novelty into the genotype and evolution is impossible without new organisation. Mutation is accident and Natural Selection converts accident into apparent design, randomness into organised pattern.
Natural selection—stabilises the species:
Existence of a species depends mainly upon its genetic quality and equilibrium. Random mutations are likely to cause damage to the living organisms. But the improvements as well as the adjustments are established by Natural Selection by perpetuating the useful ones.
Selection mechanism always tries to fix the frequencies of gene- mutation at values that are most advantageous under the environment the organisms live.
Selection is essentially a process of conservation. Mutations provide new evolutionary opportunities and selection mechanism stabilises those mutations so that continuous productions of new species will occur in nature. It must be regarded that mutations are not the final offer to a species, it must be subjected to the action of Selection.
The researches on industrial melanisation reflect the operation of directional natural selection and also the instance of the origin of adaptiveness in biological organisation. The development of dark melanic species of moths in the industrial regions of Western Europe furnishes the most striking example of evolutionary changes.
The case of a peppered moth, Biston sp. of Manchester can be stated below. The genus Biston has two species:
(i) Biston betularia is the lights- variety and
(ii) Biston carbonaria is the darker variety.
Before industrialisation in Manchester, B. betularia was very prevalent and B. carbonaria was absent. But with the advent of industrialisation in Manchester, the lighter variety vanished and the darker variety became dominant. The sequence of changes is stated in Table —Evolution-2.
The two species of moth differ in single Mendelian gene. The darker variety is usually dominant, but the higher varieties are the normal ones. Industrialisation causes the removal of lighter types and the darker varieties become prevalent. The darker varieties are actually the mutants of the lighter forms caused by the deposition of melanin.
The most possible interpretation of the spread of melanic variety is that the darker forms are more vigorous than lighter varieties. Abundance of lighter varieties in lighter environment is due to the fact that they are camouflaged by being protectively coloured to match the hues of surroundings and thus escape the eyes of the enemies.
In the dark surrounding caused by industrialisation, the darker varieties nearly match the colour of their surroundings.
Industrialisation removes the disadvantages of the dark varieties which become widespread and eventually suppress the lighter varieties. In this particular case the change in environmental condition causes the change of gene responsible for colour production. The darker forms are the mutated forms of the lighter varieties.
Full industrialisation makes the surrounding darker and the darker varieties are stabilised by Natural Selection because of their greater survival value in that particular environment. The lighter varieties vanished due to their non-adaptiveness.
Adaptation the outcome of Selection:
Mutations that improve the adaptive value of the species in an environment are tied up by Natural Selection and subsequently become incorporated with the genotype. The gene combinations may produce new potentialities which are perfected by Natural Selection. The interaction between the genetic variabilities
and Natural Selection produces greater adaptive plasticity. The dynamic aspect of Natural Selection establishes new genotypes adapted to new conditions.
Darwinism has a long checkered history of success and eclipse, until very recently it is fully vindicated and shines high in the intellectual fabric of modern thought. Darwinism has undergone evolutionary transformation but the essential cognancy remains still undisputed and operates with new facts and factors unknown to Charles Darwin.
Neo-Darwinism is the lawful product of modern researches which has largely retained the selective implication as shown by Darwin.
Neo-Darwinian school establishes the intimate relationship between Mutation and Natural Selection. The importance of genotype in the realm of evolution is most vital. Mutation and Natural Selection work hand in hand in causing evolution.
Mutations provide the raw material and Natural Selection directs the way. Mutations, in wide sense, are the changes in the genotypes and evolution is possible because of the plasticity of the genotype.
The genotypes give the ground- plan on which selection mechanism composes the various themes of evolution. In the realm of evolutionary dynamics, Natural Selection establishes new genotypes adapted to new condition and thus leading to progressive evolution.
The advancement of physiological genetics modifies the original concept of selection to a great extent. Selection mechanism is universal but it works at the level of genes. The present status quo of organisms rests on the balance between environmental flux and adaptive intensity where selection mechanism directs the way of evolution.
Theory # 3. Synthetic Theory of Evolution:
All modern Biologists are unanimous, to accept evolution as a fact. But no Biologist has actually observed the origin of major groups of organisms. Because major evolutionary steps have taken millions of years to be completed. The evolutionary processes which gave rise to major evolutionary groups, took place in the remote past—long before the origin of man.
So there were no people to observe them. But the process of evolution can be studied in two ways:—Indirect method by observing the comparative sequence of events in the past and Direct method- based on the experimental causes of evolution from the events in the world at present.
Efficacy of modern synthetic theory:
Since the publication of the theory of Evolution by Charles Darwin more than a century ago, biologists studied the subject in two ways. Most of the earlier biologists were interested in the course of evolution. But the modern biologists tried to explain the process in a different way.
They wanted to explain the processes and causes of evolution. The synthetic theory of evolution tries to explain the processes and mechanisms of evolution. This theory is based on five basic processes.
(i) Gene mutation,
(ii) Changes in the structure and number of chromosomes,
(iii) Genetic recombination,
(iv) Natural selection and
(v) Reproductive isolation.
The first three agencies cause genetic variability while the remaining two lead a population into adaptive channels.
Charles Darwin provided the evidences to establish that evolution has actually taken place. He put forward the concept of Natural Selection to explain it. His ignorance of hereditary mechanism caused the rise of conflicting group—the early Mendelian geneticists.
The synthetic evolutionists in the years 1920-30 made a compromise view on the whole problem. They removed all the obstructions left between the Darwinian naturalists and early Mendelian geneticists. The strongest support on this issue comes from the researches in population genetics.
Natural Selection establishes the link between environmental change and organic evolution. Darwin first recognised that the link between variability or constancy of the environment and evolutionary change or stability is established by Natural Selection.
This process is the result of the following characteristics of population:
(i) genetic variability,
(ii) genetic recombination,
(iii) hereditary continuity,
(iv) ability for mutation,
(v) excess reproductive ability,
(vi) integration of the genotype, and
(vii) limitations of genepool of a population.
Genetic variability and cross fertilization may produce enormous number of genetically different kinds of individuals.
A new generation may possess only a sample of the aforesaid combinations. The interaction between living organisms and their environment may or may not involve struggle for existence between different organisms. But it will affect the genetic composition of the sample. The pathway in which natural selection guides the population depends on the environmental change and the content of the genepool.
The diversity of population is due to the fact that, natural selection can either promote constancy, direct continuous change or promote diversification depending upon environmental changes.
Natural selection—its three kinds:
Based on the organism-environment relationships three types of selection have been recorded.
Normalising or stabilising selection:
This selection promotes constancy in a population. If environmental interactions remain constant (or unchanged) through time, normalising selection prevails and thus evolutionary change is arrested. The interaction between mutation and natural selection is essential.
Mutation is essentially an accidental change in genetic system. Mutations are usually of no use to a species living in a ‘normal’ environment. Most of the mutations range from neutral to deleterious to lethal. Useful mutations are incorporated in the gene- pool of a species.
Some mutations become beneficial in heterozygous condition but become deleterious in homozygous state. Normalising selection checks the accumulation of such mutations that would lower the fitness of a species in an environment.
Mongoloidism (Down’s syndrome), chondrodystrophy (Achondroplasia), Phenylketonuria, Albinism arise repeatedly by mutation in human population. Mongoloidism is a dominant lethal mutation in man which results due to duplication of chromosome-21.
Mongoloid individuals have 47 chromosomes instead of the normal number of 46. Most of the mongoloid individuals survive, but seldom possess reproductive ability. Chondrodystrophic dwarfism in man is one of the many dominant mutations in man. Phenylketonuria and albinism are two instances of numerous deleterious recessive mutants in human populations. Both of them occur very rarely (1: 10,000 and 1: 20,000).
Phenylketonuria is a disorder in the metabolism of phenylalanine (an amino acid). Albinism causes inability to synthesise pigment in skin, hair and iris of eyes. In human population all these disorders arise by mutation but normalising selection checks their undue accumulation by lowering Darwinian fitness from minimum to zero.
Genetic variability and normalising selection are common in the living world. Normalising natural selection is a conservative force which purges the genepool of a population of deleterious genetic variants and thus tries to keep the species unchanged or constant. Genetic variability is one of the prerequisites for evolutionary change and mutation is the source of variabilities. Mutations are mostly deleterious.
Deleterious mutation in one environment may be useful in another. So deleterious and useful mutations are not fixed categories. Use of insecticide causes resistance to insect population. The insecticide resistance is imperative for survival when an insect population is being regularly treated with insecticide.
This results change in genes, i.e. emergence of genetic variability. Genetic variability enables a population to adapt to changing environments.
This selective force directs changes. Directional selection occurs when environment interactions change in one direction. When specific interactions have been established (such as predator- prey, grazing animal-forage etc.) between two different types of organisms this type selection force is likely to operate. Directional selection operates by repeated feedback interactions between predator and prey.
This is well illustrated in evolution of horses on the plains of North America. The grasses having hardest leaves possess Darwinian fitness as they were least likely to be damaged by grazing.
The horses having hardest teeth with complex patterns of enamel were well equipped to feed maximum number of such grasses. Instances of such co-evolution stimulated by feedback interactions are plenty in the evolutionary history of vertebrates.
Directional natural selection can be demonstrated in industrial melanisms in many species of moths. This has been extensively studied by H. B. D. Kettlewell, 1961; Ford, 1971, and many other workers in England.
The peppered moth, Biston betularia, is a normal lightly pigmented form and its melanic (darkly pigmented variant) form is Biston carbonaria. These two species of the genus, Biston, are different due to a single gene. The ‘dark gene’ is dominant. Since the middle of last century the melanic forms have become widespread in industrial areas where the vegetation became blackened due to pollution.
In polluted regions due to shoot and other wastes, the normal light forms have been replaced by melanic forms. But in non-polluted regions the normal light forms are still present. Predation of the moths by birds is the chief selective force which promoted industrial melanism.
The melanic forms are protectively coloured on polluted blackened vegetation. They become prominent in non-polluted regions where the light forms are well protected.
Diversifying or disruptive delection:
Diversifying natural selection promotes diversification in living organism. If a previously homogeneous habitat becomes diversified, the interactions between biotic populations and their environments diverge from one other. This initiates the process of adaptive radiation. Evolution of reproductive barriers to make the diverging lines genetically separate will ultimately lead to the permanency of such divergence.
A species or a population lives in an environment. Environment is hardly completely uniform. Natural environment is actually mosaics of more or less similar or different habitats or sub-environments. The habitats or sub-environments may be suitable for some genotypes while others for competing genotypes.
Diversifying natural selection enhances the adapted- ness of populations living in heterogeneous environments. Genetic diversity is an advantage in changing environment.
Theory # 4. De Vries—The Theory of Mutation:
A Dutch Botanist, Hugo de Vries (1848- 1935) published the mutation theory in the year 1901 to explain the process of evolution. He based his theory on the results obtained from the breeding experiments on a plant called the evening primrose, Oenothera lamarckina.
In his experiment, de Vries observed certain striking differences in the forms appeared suddenly amongst a population of normal evening primrose, de Vries regarded this phenomenon as mutation (L. mutare=to change) or saltation (L. Saltare=to leap) and these changed forms as the mutants.
The conservative process of heredity causes a mutant to breed true and the progenies are produced like the parents. The term mutations or saltations means sudden large changes or discontinuous variations in the organisms and that are heritable.
Saltationist school is of opinion that National Selection has nothing to do in species formation and mutation is the only causative agency in Speciation. Saltationists regard mutation as the sole agency of evolution. Mutations are best regarded as the initiating force in evolution and Natural Selection as the limiting force.
Mutation is actually the change in gene. The genes are highly stable and faithfully duplicate with exactness in the process of reproduction. Despite such phenomenon, genes may mutate. Mutation results through some fault in gene reproduction. Muller established mutation on a solid foundation. Mutation can be caused by X- radiation. Auerbach has produced mutation by chemical mutagens.
Mutation in non-reproductive cells of an organism has no evolutionary significance, but such changes in the reproductive cells produce significant effects on the species. In fact, most of the mutations produce detrimental effects on the organism but few may have favourable effects on the organism in combination with other genes.
The genes that contribute to form the genetic make-up of an organism form a highly co-ordinated system.
Mutation is a random affair and takes place in all directions. Genes are giant molecules and slight alteration in their structural organisation results mutation. Selection mechanism plays the most vital role in evolution. Selection incorporates favourable mutations into the hereditary constitution. Thus it is the genes in the chromosomes that are to be selected.
Mutations would be meaningless until these are selected by Natural Selection. The coordinating action of Natural Selection and mutation gives rise to evolutionary directiveness. As already discussed mutations provide raw materials for Natural Selection to work with.
Usually the favourable mutations appearing in any species are incorporated in the stable genotype by the action of Natural Selection. By this way a species may change for the better by accumulating advantageous mutations.