Darwin’s Theory i.e., Darwinism:
Darwinism, based on natural observations and continuous experiments, is still one of the most widely accepted theories that attracted the scientific world to believe in the doctrine of evolution.
The evolution of living organisms was put forward by an English naturalist Charles Robert Darwin (1809-1882) in his classical book ‘Origin of Species by means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life’ (1895).
He visualised living things in a new light during his memorable voyage in the ship HMS Beagle in the Atlantic and Pacific islands which includes Galapogos.
In that book, he propounded the theory that organisms tend to produce offspring varying slightly from their parents, that the process of natural selection tends to favour the survival of those best adapted to their environment and that by the operation of these factors new species may arise widely differing from each other and also from their common ancestors.
His theory is popularly known as “the theory of natural selection” and is based on several important factors which are as follows :
1. Overproduction i.e., Prodigality of Production
The living beings both plants and animals produce in nature more offsprings than could ordinarily survive. This overproduction of offspring prevents a race of plants and animals from extinction.
It has been noted that plants and animals tend to multiply at high geometrical rates e.g., Capsella bursa-pastoris (Brassicaceae) produces more than 50,000 seeds; Nicotiana tabacum (Solanaceae) produces about 360,000 seeds annually, although all the seeds neither mature nor germinate to give rise to offsprings, still the number of offsprings that reaches maturity is enough to maintain the species.
2. Struggle for Existence
Overproduction causes a continuous struggle amongst living organisms for food, space, light, water and other environmental factors. If all the seeds of a particular plant were to germinate and develop into mature plants, then a vast area on the earth would be covered by them.
If in this way, all these seeds of all the plants were to germinate and develop into mature plants then a keen competition, called struggle for existence, will be set up among themselves for food, space, light etc.
According to Darwin, the struggle for existence is either intraspecific i.e., between different plants and animals of the same species or interspecific i.e., between plants and animals of different species, or environmental struggle between the living organisms and the physical environment like flood, volcanic eruption, earthquake etc.
Those who can adapt themselves as a result of such struggle are called victors and those who are unable to adapt are called unfit or vanquished. The victors can only survive and the unfit perish or become extinct.
Offspring of either plants and animals do not resemble each other, exactly and absolutely; there are always, even slight variations between them.
Darwin suggested that these variations are preserved and transmitted to the offsprings. Such variations help the individuals in the environmental struggle for existence or may not help, rather may be a drawback. Hence, some are more suitably fitted for the struggle while others are ill-fitted.
4. Natural Selection or Survival of the Fittest
This is the principal mechanism of evolutionary change. In the struggle for existence, only the individuals having favourable variations survive eliminating others (with unfavourable variations) which are not well-fitted.
In other words, the individuals best adapted to the natural conditions will survive while others which are unable to adapt will perish. This condition of the survival of the fitted individuals led Darwin to call ‘survival of the fittest.’
Darwin called natural selection as “survival of the fittest” and is a type of selection exercised by nature-during selection, nature has selected only the fittest one and consequently eliminated all the unfit.
5. Heredity and Origin of Species
In the struggle for existence, when the victors survive, the characters of the survived variants are passed to their offsprings in the next generations. So, the variation that caused the fit ones to survive becomes a part of the character.
Darwin believed that these minute variations of fluctuating types piled up generation after generations. Ultimately, these slight variations were accumulated through several generations and thus produced some important modified structure which led to the origin of new species out of old types.
The summary of the main arguments in connection with Natural selection that was given by Darwin and Wallace is shown below:
Darwin based his Natural selection concept on certain facts observed in nature. Out of these observations, he has drawn the deductions. Natural selection maintained the balance of nature.
Alfred Russel Wallace also gave emphasis on “Natural selection” like Darwin and stated that the main agent of evolution is the Natural selection. In support of his idea, he contributed a paper “on the tendency of varieties to depart indefinitely from the original type” (Symposium Linnecan society, 1858). Wallace was a collaborator of Darwin.
Though the phenomenon of natural selection is one of the very important and pioneer factors to explain the cause of evolution, yet Darwinism has not been accepted universally due to the following drawbacks :
(a) Darwinism accounts for the survival of the fittest but not the arrival of the fittest.
(b) It is accepted that by natural selection organisms survived, while others persisted. Darwinism only states the effect of this natural selection but not the causes of degeneracy.
(c) Darwinism only explains the struggle amongst young individuals but does not explain its influence upon adult characteristics.
(d) Natural selection is not always beneficial, as over-specialisation amongst the organisms due to natural selection leads to destruction.
(e) This theory did not take into sufficient account of the destruction of whole races of organisms irrespective of their fitness by natural calamities – in this respect, we find also the survival of the weakest.
(f) Natural selection is unable to explain the existence of some structures present in plants and animals which are not helpful in their survival.
(g) Variations, that occur at random, can never be of the utmost importance and can not explain the production of highly organised and specialised structures.
Modern Concept of Darwinism i.e., Neo-Darwinism :
The natural Selection as formulated by Charles Darwin in his ‘Origin of Species’ could not satisfactorily explain the origin of new species. His theory established the fact of evolution but due to the lack of large mass of evidence, there was a serious gap.
Darwin’s idea was that evolution was due to the natural selection of variations but he, unfortunately, could not explain how these Variations arose.
Since then, the mutation theory and other findings from genetics, systematics, ecology, morphology etc have accumulated and a theory acceptable to most of the modern evolutionists has been derived from Darwin’s theory—this is known as neo-Darwinism.
Neo-Darwinism may be stated simply as the revised view of Darwinism which includes the combination of ideas of Darwin, de Vries, Huxley, Fisher, Haldane, Muller, Dobzhansky and other modern biologists.
In his book “Process of Organic Evolution”, Stebbins stated the factors on which the “Synthetic theory” has been based. The factors are stated below :
(a) Gene mutation (b) Structural changes and changes in number of chromosomes. (c) Genetic recombination (d) Natural selection (e) Reproductive isolation. The first three factors provide genetic variability and the last two factors provide direction to the evolutionary process.
Stebbins explained synthetic theory in a popular way. He has drawn analogy of gene mutation with fuel of auto vehicle, genetic recombination with the engine of the vehicle, Natural selection with driver, chromosomal changes with the accelerator, reproductive isolation with driving signs and speed limits of the highway.
1. Stebbins considered that the elementary unit of the evolutionary process is population and the statement that the elementary evolutionary act tends to modify the population is unconditionally true.
2. The role of zoo and phytogeographical structure of species and the dynamics of size in microevolution, the primary stage in which a change in genetic structure of population takes place is made clear by the synthetic theory.
3. This theory explained evolutionary process on the basis of firmly established genetic mechanism.
4. Analysis of the manifestations of genetic automatic processes the distinctive development of such a population that depends on the action of selection on its relatively improvised gene pool, explains in a better way the results of experiments on model population and gives a natural explanation of many phenomena observed in fully and partially isolated natural population.
Synthetic theory is considered as the development of Darwinism based on the data of modern genetics. Neo-Darwinism fundamentally differs from Darwin’s original theory, Dobzhansky called neo-Darwinism as biological theory of evolution because this theory is the result of synthesis of the findings of genetics, cytology, systematics, ecology, palaeontology, comparative anatomy, morphology etc.
According to this theory, “evolution is a result of mutation (genic changes), with the recombination of genes through biparental reproduction under the influence of natural selection over long periods of time.”
The main features of Neo-Darwinism to explain the origin of new species are as follows :
Different types of variation or variability occurring within a species are not uniformly distributed in the entire population of a species, rather somewhat restricted to a local population.
Such population having a distinctive type of variable character is given the rank of a subspecies which again is considered as an incipient species. This incipient species i.e., subspecies ultimately becomes converted into a species through natural selection.
In each subspecies, there is a complex of gene-determined characters and these various genes in a complex are derived more or less from the same ancestors by mutation.
The genes of a particular sub-species are not specific to it but are present in varying degree in different sub-species. Within each subspecies, a particular combination of genes is present-natural selection operates on this gene combination and ultimately these combinations are screened out by ecological factors.
Then random mutation takes place within each subspecies, as a result, different alleles arise in different subspecies. These sub-species are geographically isolated from each other, sometimes they may also be physiologically and ecologically isolated.
Next, new mutations appear in an isolated sub-species which (i.e., mutation) can not be distributed uniformly within the entire population of a species. In this way, new species are ultimately formed by gradual mutation.
According to neo-Darwinism, very small mutations serve as the raw materials of evolution; large mutations are generally disadvantageous, hence can not serve as raw materials.
Biological or synthetic theory of evolution is based on data from all biological science. This theory maintains the mutation and sexual recombination furnish the raw materials that the natural selection fashions from these materials, genotypes and gene pools; and that in sexually reproducing forms, the arrays of adaptively coherent genotypes are protected from disintegration by reproductive isolating mechanisms.