Eminent Scientist’s and Evolution of Plants!
Contents
Lamarckism:
The genius of Lamarck would be appreciated all the more today if one remembers the tragedy of his life. He published his views on evolution at a late age, in Philosophie zoologieque in 1809 but it brought little luck to him in his old age. He was snubbed and ignored by Napoleon himself, became blind seven years before his death, lived a life of loneliness and poverty until he was buried in some unknown grave.
Yet, he was an all round scientist of his age—a botanist, a zoologist, a chemist and a physicist. He postulated theories on evolution which, although they may be greatly disputed, were something unknown before his time. His theories tried to explain how new species could have evolved.
These are as follows:
1. The Direct Effect of Environment:
It is common knowledge that environment may influence the form of organisms. Thus, a plant grown in fertile soil will have a luxuriant form while the same plant in a barren tract will be sickly and stunted. In the case of animals the effect of environment may also be indirect as the habits of animals may change in a new environment and a change of habits, according to Lamarck, causes a change of form. Lamarck assumed that such changes of form caused by the direct or indirect effect of environment are inherited and are of cumulative nature. Thus, in course of generations, new species may be formed.
2. Conscious Effort:
Lamarck thought that when an animal feels the necessity of an organ he can develop it by conscious effort. The ‘need’ of an organ causes an organ to be produced. Thus, the need of horns to fight with and teeth to chew with would cause the growth of horns and teeth. This idea of Lamarck is not at all taken seriously today as it baffles scientific reasoning.
3. Use and Disuse:
Excessive use of an organ will cause increase in size or strength of that organ while its disuse will cause its disappearance in course of generations. Thus, horse-like animals transferred to an area where there were only trees and no grass had to stretch their necks for food. This excessive use of the neck caused its excessive development resulting in the evolution of a race of giraffe-like animals. Similarly, disuse of the tails of monkeys has caused their loss in human beings.
4. Inheritance of Acquired Characters:
All such characters acquired during the lifetime of an organism will automatically be inherited by its off-springs. The character will gradually increase in successive generations and such cumulative effect will ultimately give rise to a new species. This is the main as well as the most disputed assumption of Lamarck.
Darwinian View—the Theory of Pangenesis:
Darwin assumed in his Theory of Evolution that variations occur spontaneously and nature selects out the suitable variants. Thus, new species are evolved. At first he did not bother much as to how the variations arise. But, later on, this problem engaged his attention more and more. His views were a modification of Lamarckism. He also thought that variations arise in both plants and animals as a direct effect of the environment and as a result of use and disuse in the case of animals.
He did not think much of the indirect effect of environment on animals by a change in their habits. Similarly, he never thought that the ‘need’ of an organ might cause it to grow. Darwin also thought that some variations might arise due to unknown causes which variations he called chance variations. Like Lamarck, Darwin thought that all new variations, i.e., acquired characters, are inherited.
Darwin realised that the last assumption is not quite easy to accept. A plant is propagated by its seed and any variation, in order to be inherited, must influence the seed or the reproductive parts. Supposing that a leaf or a root is slightly modified by a variation—how can that affect the seed? This problem racked the brain of a genius like Darwin and he gave a purely hypothetical explanation. This is embodied in one of his later books, The Variation of Animals under Domestication, as the Theory of Pangenesis.
This supposes that every cell of the body of an organism is producing heredity- bearing particles called pangenes. These pangenes are passing on to the germ cells (cells that will give rise to the reproductive parts) where these would accumulate and ultimately pass on to the next generation. Thus, any change in an organ during the lifetime of an organism is duly registered in the germ cells and is thus inherited. In the case of animals the blood stream is constantly receiving these pangenes or gernmules which, later, accumulate in the germ cells.
The Theory of the pangenes, however, was based purely on imagination and there was no experimental evidence to prove it. Darwin proposed this theory only ‘tentatively’. Followers of Darwin like Galton noted that blood transfusion did not change heredity and, finding nothing in favour of this theory, ultimately dropped it.
Weismann:
August Weismann (1834-1914) was a devoted disciple of Darwin who was strongly against the Lamarckian doctrine of inheritance of acquired characters. He could not see eye to eye with the Darwinian support to this doctrine and thought that these views of Darwin needed modification. This revised view of Darwinism formed the basis of the neo-Darwinian or modern-Darwinian school as opposed to the neo-Lamarckian or modern- Lamarckian school.
Within two years of the death of Darwin, Fig. 810. August Weismann (1834-1914). H, Weismann began by analysing variations and gradually developed this new idea of Heredity. He said that variations are of two types, some are congenital, i.e., organisms are born with them. Others are acquired during the lifetime of a particular plant or animal. It was with this latter type of variations, the acquired characters, that Weismann was very much concerned. He gradually developed his Theory of the Germplasm to explain that acquired characters could not be inherited.
The Germplasm Theory is based on the concept that living bodies are formed of two fundamentally different kinds of material—the germplasm and the somatoplasm. The germplasm is responsible for the development of reproductive parts and remains in the organism, sealed and secluded from the somatoplasm or the general body of the organism which develops out of the germplasm.
In course of time, the germplasm gives rise to the offspring in which again the germplasm remains separate from the new somatoplasm that it develops. Thus, the germplasm is a continuous stream from generation to generation while the somatoplasm is formed anew at every generation and is discontinuous.
In bisexual plants and animals, at every generation the germplasm stream becomes a confluence of the maternal and the paternal streams (Fig. 811).
Weismann’s conclusion from the above was that the somatoplasm is a discontinuous entity and so, any variation in it cannot be inherited. The somatoplasm does not influence the germplasm. Weismann did not believe in Darwin’s Theory of pangenesis which tried to explain the influencing of the germplasm by the somatoplasm.
The independence of the germplasm from the somatoplasm is further shown by the modern technique of grafting the ovary from a guinea-pig of one variety to that of another variety. It is found that such a grafted ovary passes its original characters to the progeny and is not influenced by the body of the new guinea-pig to which it had been grafted and on which it is growing.
Weismann’s Theories of Heredity:
After a clear statement of his theory of the germplasm, Weismann tried to find out the mechanism of inheritance. Fortunately, at the time of Weismann more was known about the structure and division of cells than during the lifetime of Mendel. It was known that chromosomes are stable bodies within the nucleus, their number for a species or variety is fixed that they are equally divided during mitosis.
Weismann declared that heredity is determined by certain determiners which are located within complicated bodies called ids. The ids again are components of the chromosomes. These determiners cannot be changed so that the heredity of an organism does not change.
A complete conviction in the theory of the germplasm led Weismann to believe that no acquired character, as such, could be inherited. But, he was confronted with certain difficulties in the form of variations which he knew to be heritable. These he explained by supplementary hypotheses.
Firstly, he admitted that there are certain variations in organisms that could be inherited. He explained this by his hypothesis of parallel modification of germplasm and soma. He said that under certain circumstances there is a variation of the organic body (soma) which also involves a parellel variation of the germplasm substance.
This variation is inherited. The second difficulty was even greater. Certain organisms have different forms under different circumstances. The amphibious plant Ranunculus aquatilis develops dissected eaves when grown in water and broad entire leaves when grown on land. This is called polymorphism.
Some butterflies and moths have different forms according to the environment and the nutrition they receive. Among bees, it is well-known that the same egg may develop into a queen bee or a worker bee according to the nutrition supplied to it. These phenomena cannot be explained by Weismann’s theory of the determiners.
To avoid this difficulty Weismann developed another hypothesis—that of germinal selection. He now said that the germplasm has got not a single set of determiners but a number of different sets of determiners. It is the environment that decides which set of determiners will express themselves. Thus, there are any numbers of determiners struggling for expression within the germplasm and finally one set is selected out by the environment.
While the earlier theories of Weismann (germplasm and chromosomes) were well received and still form the basis of Genetics, his last theory about germinal selection met a different fate. This last theory died out in course of time like Darwin’s pangenesis as, likewise, this also was based on mere speculation. While Darwin thought of a centripetal theory (pan-genes from soma to germplasm), this was a centrifugal (determiners from germplasm to soma) one. Weismann gave the first concrete form to the modern conception of inheritance through chromosomes but went too far in his speculation with determiners.
Darwin to De Vries:
The period from the death of Darwin to de Vries and the rediscovery of Mendelism was occupied by extreme speculations of the Weismannian type. But even in this period, some people were not satisfied with speculative theories alone but were bent upon observing more closely what was actually happening in nature.
Galton and Bateson closely studied variations and concluded that there are two kinds of variations— continuous and discontinuous. Continuous variations are the common graded fluctuating variations such as one finds when measuring the lengths and other measurable (quantitative) attributes of organs like leaves, fruits, etc. Here the extremes are connected by a graded series of intermediates.
The other type of variations is discontinuous. These arise all on a sudden and are separated from normal ones by big gaps—there being no intermediate form. Darwin called these sports but he did not consider them to be of much significance. On the other hand, even before de Vries, Galton, Bateson and Korjinsky believed that it is the discontinuous type of variations that plays the real part in species formation and evolution. The continuous types are unstable and not strictly inherited while the discontinuous ones are very stable as well as hereditary.
Galton sought to explain this by his famous analogy of a polyhedron. If a polyhedron is slightly tilted it comes back to its original position as soon as the hand is removed. This is the case of continuous variations. But, if the polyhedron is moved too far it settles on another of its faces and cannot come back to its previous face. This is what happens in discontinuous variations. The importance of discontinuous variations forms the core of the mutation theory of de Vries.
Along with the realisation of the importance of discontinuous variations another science was being developed by Galton and Pearson. This was the science of Biometry which involves actual measurement and statistical study of variations and heredity.
De Vries—Mutation Theory:
Hugo de Vries (1848-1935), a Dutch botanist, pointed out the line of evolution which has now become the most important. Although Darwin was aware of the ‘sports’ he did not stress their importance. Both Galton and Bateson realised how important they were. But, it was de Vries who was their real champion.
De Vries made an exhaustive study of variations and in course of this he used to collect variations growing wild and to plant them in his own nursery.
His study revealed two important facts:
1. The Occurrence of Elementary Species:
Among many wild and domesticated species there are varieties with minute differences that breed true. These differences are not very marked and de Vries called these varieties elementary species. Even the fluctuating’ variations really included some ‘elementary species’ of this type.
2. Mutation in Oenothera:
Oenothera lamarckiana or Evening Primrose is a garden plant with yellow flowers which open towards the evening. In Holland, this plant often grows in a wild state. In an abandoned potato field, de Vries found certain variants of this plant which looked very different from the normal. De Vries brought these to his home and on cultivation found seven types that bred true.
These he called mutants. After breeding for eight generations de Vries classified his mutants as follows:
(a) Progressive species: due to addition of certain characters: to be considered as elementary species:
(1) O. gigas (stouter with larger leaves and flowers)
(2) O. rubrinervis (red veined leaves and stems).
(b) Retrogressive species: due to loss of some characters.
(3) O. nanella (dwarf)
(4) O. laevifolia (smooth, narrower leaves)
(5) O. brevistylis (shorter style).
(c) Degressive species: very weak forms; not likely to survive in nature.
(6) O. albida (pale whitish)
(7) O. oblonga (oblong leaves on weak, dwarfish plants).
Besides these seven, de Vries also got two other forms which did not breed true and were giving rise to mutants like the parent O. lamarckiana.
These he grouped together as:
(d) Inconstant species:
(1) O. lamarckiana (the parent form)
(2) O. lata (with broad leaves)
(3) O. scintillans.
Starting with nine plants from the field and growing them for eight years de Vries found that the same seven mutants arise from the stock parental forms year after year.
De Vries, therefore, concluded as follows:
1. New elementary species appear suddenly and attain full constancy at once.
2. The same new species are produced in a large number of individuals. We get the same mutants again and again so that there is a greater chance of selection by nature.
3. Mutability is something fundamentally different from fluctuating variability.
4. Mutations take place in nearly all directions. This is because a mutation may mean either a gain or a loss and may involve any character.
Although de Vries formulated an extremely important theory which stands well established today, the material on which his thesis was based was later on found to be rather abnormal. Even de Vries had noticed that Oenothera lamarckiana was an ‘inconstant species’ unlike the more usual true species. The reason of this inconstancy has now been found out cytologically. The chromosome set of O. lamarckiana (2n = 14) is derived from two sets—the gaudens complex and the velans complex.
These plants give rise to two types of gametes—gaudens and velans. Among the zygotes formed, the gaudensgaudens and velans-velans homozygous true breeding forms fail to survive while the gaudens-velans ‘balanced heterozygotes’ only live to form the common O- lamarckiana plants. Thus, O. lamarckiana is a hydrid, now known as a ‘complex heterozygote’ (also called ‘structural hybrid’) which is unstable and is prone to throw out chromosomal mutations.
Many of the mutants obtained by de Vries were not mutants in the de Vriesian sense (gene mutations) but mere unstable chromosomal ‘aberrations’. Of his mutations, O. gigas is now known to be a tetraploid (4n=28); 0. rubrinervis and O. nanella (a balanced lethal) are recombination plants (2n = 14); O. albida, O. oblonga, O. lata and O. scintillans are unstable trisomies (2n + l =15), while O. brevistylis and O. laevifolia are true gene mutants. So, de Vries proved his theory with a very unnatural species but, nevertheless, these facts do not in any way diminish the importance of the theory.
Present Position of Darwinism and the Role of Mutation in the Mechanism of Evolution:
Darwin named his book Origin of Species but really sensible criticism are levelled today only against that part of his theory which tries to explain the origin of new species. Darwin’s Theory established the fact of Evolution but still there was a serious gap. Darwin said that Evolution was the result of natural selection of variations but he could not explain how these variations arose. Darwin’s variations were, mainly, acquired characters. But, Weismann convinced most geneticists that acquired characters as such could not be inherited. This assertion caused a great confusion.
Then where were those heritable variations which were necessary for the evolution of new species? In the words of de Vries, “Natural selection may explain the survival of the fittest but it cannot explain the arrival of the fittest.” The explanation came after the mutation theory became established.
It is now understood that some of the variations, which form the raw material on which natural selection works, may be selections from natural hybrids but the more important variations are the mutants. Thus mutation has assumed a great importance today as the principal mechanism moulding evolution. The present-day idea of mutation is somewhat different from the idea of de Vries. Different types of mutation are recognised today.
Many such mutations, like the trisomic aberrants, are unstable. Even de Vriesian mutations (gene mutations) need not be very prominent but may involve slight changes of characters looking like ordinary fluctuating variations. These mutations are just the variations mentioned by Darwin.
Such mutated variations are inherited and an accumulation of a number of such insignificant mutations may give rise to a new species. Environment may be a factor is causing and selecting such mutations. Thus, in the mutation theory we have found a way of explaining the Darwinian view of Evolution.
The second point in Darwinism which has been subject to criticism in his doctrine of the survival of the fittest. This part of his theory did not take into sufficient account catastrophic destructions of whole races of organisms (when both the ‘fit’ and the ‘unfit’ are destroyed) and the role played by chance when some so-called ‘unfit’ organisms may also survive.