Introduction
Biological evolution is defined as any genetic change in a population that is Inherited over several generations. Till now we discussed about the concept of evolution and its different evidences like fossils, homologous structures, artificial and natural selection, etc. Now we will be dealing with the theories of evolution out forward by various biologists.
His theory is often called the Theory of Inheritance of Acquired Characters or the Theory of use and Discuss of Organs.
The first attempt to explain origin of species and their adaptation to the environment was done by Jean Baptist de Lamarck (1744-1829). He was the greatest French naturalist. Lamarck’s theory published in 1809 (year of Darwin’s birth) in his book ‘Philosophie Zoologique’.
According to this theory, the organisms undergo changes to adapt themselves to the environment. The changes acquired by the organisms during their life time are passed on to the next generation. He took the example of long neck of Giraffe, they continuously stretched their neck to reach the vegetation on tress. This acquired change was passed to the next generation. He also gave the principle of Use and Disuse. Use of an organ leads to strengthening of the organ, and disuse will lead will lead to weakening of the organ.
Lamarck arranged his theory in the form of four postulates:-
(i) Internal forces tend to increase size of the body.
(ii) Formation of new organs is the result of the need or want continuously felt by organisms Doctrine of Appetency/Desires.
(iii) Development and power of action of an organ is directly proportional to its use.
(iv) All changes acquired by the organism during its life are transmitted to the offsprings by the process of inheritance
He said that evolution of life forms has occurred but the driving force according to him was use and disuse of organs. Use of an organ lead to the strengthening and disuse leads to the weakening of that organ. He gave the example of giraffes and explained that the ancestor of giraffe were bearing a small neck and forelimbs
But in an attempt to eat (forage) the leaves on tall trees, they had to stretch their necks. Lamarck proposed that this acquired character, i.e. stretching of neck was passed on to the next generation resulting in the present time long necks. Nobody believes this conjecture any more.
The main concept of Darwinian theory is natural selection. He presented his theory after Lamarck’s theory. It is possible that the work of Thomas Malthus on ‘populations ‘influenced Darwin.
Thomas Malthus, an English economist is widely known for his theories on populations and the increase or decrease in size of populations in response to various factors. One of the factors he studied was natural resources.
Salient feature of Malthusian theory are:-
(i) Every population has an inherent capacity to increase its number exponentially if every individual of that population reproduced maximally. It means that if all born individuals of a population reach their reproductive ages and reproduce at their maximum rate, then there are changes of that population to rise many a times. This fact can be seen in a growing bacterial population as how fast it grows on getting optimum conditions
(ii) Nature keeps a control over the size of population by posing several challenges like disease, food shortage, death etc.
(iii) Natural resources like forests, water, air, food, etc. are limited in the nature and their excessive exploitation badly affects the size of populations.
(iv) The populations remain nearly stable in size except during some seasonal fluctuations.
But the fact is that even after full probabilities of populations to grow exponentially, they do not grow so. The population sizes in reality are limited.
Malthus interpreted that if population sizes are limited then there had been a competition or struggle, for acquiring the resources, among individuals of same species or of two different species. The one who had ‘better” adaptability survived and produced more progeny. A concept of ‘fitness’ and natural selection’ was thus, reflected in his theories.
According to Wallace’s Chart, the main points of Darwin’s thory of Natural Selection were as follows:-
(i) High rate of reproduction
(ii) Total number almost constant
(iii) Struggle for existence
(iv) Variations
(v) Survival of fittest
(vi) Natural selection
All the successful organisms have a high Biotic Potential or reproductive Rate. The organisms produce a large number of offsprings that can possible survive , example a mice produces a dozen of mice at one time. A rabbit produces 6 young ones in a litter and there are four litter in a year. A rabbit starts reproducing at the age if six months
1. Not all but only some individuals which survive, reach adulthood, and those which reach adulthood, reproduce at different rates, thisis called ‘Differential Reproduction’
2. The success in survival and reproduction depends upon the characteristic traits of an organisms, example only those rabbits will survive which are fastest. There is ‘Struggle of Existence’ and in this there will be ‘Survival of Fittest’ was first used by Herbent Spencer. The same context was asserted by Darwin as ‘ Natural Selection’
So, evolution is the changein the genetic composition of the population which is brought about by natural selection which acts upon the variability in population
Causes of variations:-
Mutation is the ultimate source of variations.
At the next level is recombination.
3. Intermingling of two widely separated populations
Weakness of Darwinism:- He was not able to explain the cause of discontinuous variations observed by himself in nature and the mode of transmission of variants to the next generation.
In 1968, Darwin put forward the Theory of Pangenesis. According to this theory, every organ of the body produces minute hereditary particles, called Pangenes or Gemmules and they are carried through the blood into the gametes.
Weismann’s theory of germplasm’ (1892) rejected Darwin’s of pangenesis. He established that the germ (sex), cells are set apart from other body (somatic) cells early in the embryonic development, so, only the changes in the germplasm affect the characteristics of future generations.
Alfred Wallace had written the book ‘On the Tendencies of Varienties of Depart Indefinitely from the Original Type. Alfred Wallance (1823-1923), a naturalist from Dutch East Indies, was working on Malay Archipelago (present Indonesia).
Darwin’s novelty:it is a possibility that these above mentioned ideas of Thomas Malthus inspired Charies Darwin to frame his own theory of evolution based on the natural selection. However, there was a novelty in the concepts of Darwin along with this. Darwin asserted that variations which are heritable and which make resource utilization better for few (adapted to habitat better) will enable only these to reproduce and leave more progeny.
Darwin said that the ‘appearance’ of new life forms is due to accumulation of variations from one generation to another.
Darwin also talked about the rate (speed) of appearance of new forms. He said that the appearance of new forms is linked to the life span of that organism. For example, the microscopic microbes have more chances to evolve faster. But if we talk about any other organism like fish or fowl, they will take millions of years to evolve because their life spans are much longer (i.e in years) than microbes.
To explain avolution, Darwin talked about two things:-
(1) Branching descent
(2) Natural selection
Branching descent explains the “pattern” of evolution and natural selection explains the ‘mechanism’ of evolution. Branching descent and natural selection are the two key concepts of Darwinian theory of evolution
Branching descent:- He said that the evolution is a process With time, new life forms appear, and these new life forms arise as ‘branches’ from the previously existing life forms. Hence branching descent points towards the concept of common ancestry
Natural selection:- He emphasized that the selection of ‘useful variations’ is the main (but not exclusive) mechanism for evolution to occur, which has already been discussed.
Note:- Neo-Darwinism concludes that new species develop through mutations with natural selection.
Mutation Theory:- In 1901, Hugo de vries proposed the Mutation Theory on the basis of his observation on the wild varienty of evening primrose Oenothera Lamarckian.
According to mutation theory, new species originate as a result of large, discontinuous variations which appear suddenly.
The main features of mutation theory are as follows:–
(i) Mutation arise from time to time amongst the individuals of a naturally breeding population.
(ii) Mutations are heritable and establish new forms or species.
(iii) mutations are large and sudden and are totally different from fluctuating variations of Darwin, which are small and directional.
(iv) Mutations may occur in any direction
2 thoughts on “biological evolution”