Darwin’s theory of natural selection was accepted on account of its direct approach and practical nature but in 1880 doubts started creeping up about its validity and applicability. In this regard Herbert Spencer, August Weismann,Earnest Heckel, Weldon, Cesnala, Davenport, Polution and Sanders, Sukatchew and several others carried out certain experiments to provide support to Darwinism. These supporters of Darwin came to be known as Neo-Darwinians. They had introduced a number of new facts to make the idea of natural selection more acceptable. Thus, Neo-Darwinism has become a modified version of Darwinism.
Some of the experiments conducted in this context are:
- (1) Weldon – his experiments were conducted with shore crabs of Plymouth Sound. In this regard he placed a large break water (barrier) near the mouth of Plymouth Sound. This slowed down the rate of flow of river water. At the same time the deposition of China Clay had increased. This caused the death of numerous crabs. The survivors had a slightly narrow frontum and there was a progressive narrowing of the frontum in succeeding generations. This shows that under changed environmental conditions natural selection operates upon minute fluctuating variations.
- (2) Pollution and Sanders – Both the scientists conducted experiments with They placed these pupae in the wilderness in the plants. Thus, some were put in harmonious background and some others in non-hannonious background. Those kept in harmonious background survived but those in non-harmonious background perished. These experiments show how the protective colouration has a survival value. butterfly pupae in different colours.
- (3) Davenport – He chose chickens with black, white, barred and chequered and left them in the field. It was found that the chickens with plain colours were killed by hawks. While those with barred and chequered colour patterns were spared because they were inconspicuous from the surroundings.
- (4) Cesnola – The role of natural selection was illustrated by Cesnala with preference to Mantis religiosa (preying mantis). Some of which were green in colour and others brown in colour. He fixed them on plants and observed the events. The green ones survived whereas the brown ones perished. That means those having colour markings harmonious with the plants survived, whereas all others were eaten up by the birds.
- (5) Sukatchew – A Russian biologist conducted experiments with dandelions. He collected dandelions from Crimea (located at 45° latitude with temperate conditions), Leningrad (at 60° latitude with sub-arctic conditions), Archangel (at 64° latitude with arctic conditions). Then he planted these plants in mixed pots in Leningrad. He chose 2 methods for planting these plants. Accordingly, he planted the dandelions with 3 cm wide space in between, in 1 plot and 18 cms space left in between in the plants in another plot. The results have shown that 1% of plants planted, in the first plot and within this 1% of survivors, Leningrad variety constituted 11% and Archangel Variety comprised 70%. On the other hand, in the 2nd plot, 1.60% survived. Of this Leningrad variety comprised 96% but Archangel variety only 88%. That means if organisms of the same species are grown in a single way, differences in their survival appear dearly and also changes due to natural selection may not appear strikingly. But they may change the population slowly and gradually. –
- (6) Kettlewell – He wanted to know how natural selection operates in animal populations living in natural conditions. He selected peppered moths (Biston btulania) for the observations in England. Industrial revolution in England started in 1750. Prior to that revolution, the environment in England was very dear. The tree trunks also were very clean. On the tree trunks there were lichens with light white colour intercepted by black dots. The colour of the moths and the colour of the lichens on the tree trunks perfectly matched with each other. For this reason birds could not identify them and eat them. The moths survived. However, some moths became black in colour because of mutations. These moths were dearly visible to the birds, so they ate them. In other words, as the moths were different from the surroundings in terms of their colour, they could not survive, they perished. Kettlewell conducted some field experiments also, centres, Brihimgham Bird Reserve and Dorset Village. In the pollution-laden industrial Brihimgham Bird Reserve, he released 614 moths of which 477 were black ones and 137 were lighter ones. He chose two field In Dorset Village, the environment was clear. He released 969 moths into the village environment. Of these 473 were black and 496 were of lighter one’s, kettlewell concluded that in the pollution-laden Brihimgham Bird Reserve, more black moths were able to survive but in the pollution-free Dorset Village, more light coloured moths were able to survive because of the differential adaptive values and environmental fitness of the two varieties of moths
Conclusions of neo-Darwinians:
On the basis of field experiments and field observations, the neo-Darwinians have given a scientific shape to their theory. They presented their theory as follows:
- (a) Neo-Darwinians accept Darwinism in terms of Universality of variations, over-production and organisms and importance of natural selection.
- (b) Neo-Darwinism does not accept Darwinism Pangenesis (origin from all). August Weismann made Pangenesis untenable by presenting his germ plasm theory.
- (c) Neo-Darwinism emphasized that natural selection accounted everything that is involved in evolution, they went further to secure enough proof towards natural selection.
- (d) Neo-Darwinians could not accept the role of mutations in evolution.
- (e) Neo-Darwinians emphasized that adaptation result from multiple forces and natural selection is only one of these many forces.
- (f) Neo-Darwinians also stressed that characters are not inherited as such but there are character determiners known as determinants, which control only the development. The ultimate character would result out due to the interaction of the determiners, actually of the organisms and the environment during development.
- (g) Some Neo-Darwinians emphasized that natural selection brings gradual changes but not sudden changes. According to them, natural selection by itself does not bring genetic changes.
- (h) They also said that genetic changes occur because of mutations.
- (i) In their view, when genetic changes take place, natural selection favours those which have more adaptive value than others. As a result changes in gene frequencies occur across generations.
- j) This theory considers cooperation also among organisms as an important principle of survival.
Thus, neo-Darwinism shares some features with Darwinism and at the same time introduces certain new elements in the theory of natural selection. However, it is incomplete in the sense that it could not explain all the dimensions of evolutionary theory. Further, it is partly wrong because it lacked present understanding of the genetics.
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 recognised the fact that natural selection involves differential reproduction but he did not equate the two. In the 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 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.