Theories of Organic Evolution (Part - 1) - Notes, Botany, Class 12 PDF Download

THEORIES OF ORGANIC EVOLUTION

LAMARCKISM –

First theory of evolution was proposed by Jean Baptiste de Lamarck (1744-1829) Book : Philosophie Zoologique (1809) Lamarck coined the terms – Invertebrates, Annelida.

The term Biology was given by Lamarck & Treviranus.

Theory of Inheritance of Acquired Character 

Basic Concepts of Lamarckism – 

(i) Internal Vital Forces : Some internal forces are present in all organisms. By the presence of these forces organism have the tendency to increase the size of their organs or entire body.

(ii) Effect of environment and new needs – Environment influences all type of organisms. Changing environment gives rise to new needs. New needs or desires produce new structures (doctrine of disired/Appetency) and change habits of the organism.

(iii) Use and disuse of organs.

If an organ is constantly used, it would be better developed whereas disuse of organ result in its degeneration.

(iv) Inheritance of acquired character – During the life time of an organism new characters develop due to internal vital forces, effect of environment, new needs and use and disuse of organs.

These acquired characters are inherited from one generation to another. By continuous inheritance through many generation these acquired characters tend to make new generation quite different from its ancestors resulting in the formation of new species.

Examples in support of Lamarckism : 

1. Long neck and high fore limb of Giraffe.

2. Aquatic birds stretched their toes and developed web.

3. Snakes lost their legs.

4. Deers became good runners by the development of strong limbs and streamlined body.

5. Retractile claws of carnivorous animals.

Criticism of Lamarckism – 

1. According to first concept organism tends to increase their size but it is not universally true. For example among angiosperm the trees seem to be primitive and the shrubs, herbs and grasses  have evolved from trees but the size was reduced during evolution.

2. Second concept is false. Can we sprout wings wishing to fly like birds.

3. The third concept is some what true like the well developed biceps muscles of blacksmith and less developed wings in flight less birds. But this concept also have many objections like the eyes of a student/reader do not increase in size and power with increasing age, the constantly beating heart maintains a constant size through generation.

4. Fourth concept is completely false because acquired characters are not inherited.

• Weismann – Weismann cut off the tails of rats for about 22 generations but there was no reduction in the size of tail.

On the basis of this experiment Weismann proposed the theory of continuity of germplasm.

According to Weismann. 

(i) Two type of matters are present in organism, somatoplasm and germplasm. 

(ii) Somatoplasm in somatic cells and germplasm in Germinal cell.

(iii) Somatoplasm dies with the death of organism while germplasm transfers into the next generation.

(iv) If any variation develops in germplasm, it is inherited, while if variation develop in somatoplasm it is not transmitted.

•  Pyane : Pyane kept drosophila in dark up to 69 generation, but there was no reduction in the size or sight of eyes.
•  Boring of ear and nose in Indians.
•  Iron shoes of chinese.

Neolamarckism –Term by Pakard Although Lamarckism remained controversial but some scientists gave the following evidences in favour of Lamarckism. They are known as neo-lamarckians.

According to neolamarckism environment effected the inheritance of acquired character. According to it changing environment give rise some physical and chemical changes in organism, which effect their germplasm, and these acquired characters are definitely inherited.

1. Sumner's Experiment – Sumner kept white rat in warm temperature resulting in elongation of body, large pinna and long tail. These features were inherited by the offspring.

2. Kammerer's Experiment – Kammerer kept salamander in dark background. The black spots found on skin were widely spread. In lighter background the skin became yellow with limited black spots. These character were inherited by the offspring.

3. Mac Dugal's Experiment – Mac Dugal trained white rats to cross a tank of water following a definite route. These trained rats were mated and their offspring were again trained. It was observed that there was decrease in the number of errors by offsprings of white rats.

Special Points :-

1. Allen's law :- It states that animals which live in very cold climate, their extremities such as ears, tails, etc. become progressively smaller.

2. Cop's law :- It states that there is a tendency of animals to increase in size, during the long course of evolution.

DARWINISM

Charles Robert Darwin was born on 12th Feb. 1809 in England.

Darwin travelled by H.M.S. Beagle.

The ship left on Dec. 27, 1831 and returned on Oct. 2, 1836.

He travelled South America, South Africa, Australia and Galapagos Islands.

Darwin was influenced by two books. "Principles of population" of Malthus. "Principles of Geology" of Charls Lyell.

Alfred Russel Wallace : He travelled South eastern Asia and South America. The idea of natural selection striked in his mind. Wallace wrote an essay and sent it to Darwin. "On the tendency of varieties to depart indefinitely from original type". Wallance was a naturalist working in Malay archipelogo (present indonasia).

There was striking similarity between the views of Darwin and Wallace.

Wallance's Chart : Wallace presented a chart to explain main points of theory of Darwin :

"Darwinism" or "The theory of Natural Selection" was proposed jointly by Charles Darwin and A.R. Wallace.
This theory was later on explained by Darwin in his book 'On the origin of species by the means of Natural Selection' (1859).

The main features of theory of Natural Selection are as follows – (1) Over Production : All organisms have capability to produce enormous number of offspring, organisms multiply in geometric ratio.

e.g.
– Plants produce thousands of seeds.
– Insects lay hundreds of egg.
– One pair elephant gives rise to about six offspring and if all survived in 750 years a single pair would produce about 19 million elephants. Thus some organisms produce more offspring and other produce fewer offspring This is calleddifferential reproduction.

(2) Struggle for existence – Every individual competes with others of the same and other species for basic necessities like. space, shelther and food. It is called struggle for existence and it continues for the whole life from zygote stage to its natural death.

The struggle for existence is of three types –

(i) Intra-specific struggle : It is competition between the individuals of same species for same needs like food, shelter and breeding (most acqute type of struggle).

(ii) Inter-specific struggle : It is the struggle between the individuals of different species for food and shelter.

(iii) Environmental struggle : This struggle is between the organism and their environment. All organism struggle with cold, heat, wind, rain, drought and flood etc.

(3) Variations and heredity – Except the identical twins no two individuals are similar and their requirements are also not same. It means there are differences among the individuals. These differences are calledvariations. Due to variations some individuals would be better adjusted towards the surroundings than the others.
According to Darwin the variations are continuous and those which are helpful in the adaptations of an  organism towards its surroundings would be passed on to the next generation, while the others will disappear.

(4) Survival of the fittest or natural selection – The original idea of survival of fittest was proposed by Herbert Spencer.
According to Darwin most suitable and fit individuals are successful in struggle for existence. The individuals with most favourable adaptations are able to lead most successful life and are able to win over their mating partners.

Darwin called it Sexual Selection.
In the struggle for existence only those members survive which posses useful variations means nature selects fit individuals. This was calledNatural Selection.Fitness is the end result of the ability to adapt and get selected by nature.

(5) Origin of New Species – Darwin explained that variations appearing due to environmental changes are transmitted to the next generation.
So offspring become different from ancestors. In next generation process of Natural selection repeats so after many generation a new species is formed.

Criticism of Darwinism –

1. Darwin does not explain the development of vestigial organs.
2. This theory has no satisfactory explanation for the cause, origin and inheritance of variation.
3. Darwin is unable to explain why in a population only a few individuals develop useful variation and others have harmful variations.
4. Criticism of Darwinism was based on sexual selection. Why only female selects the male for mating why not vice versa.
5. Darwin was unable to differentiate somatic and germinal variations.
6. This theory was unable to explain over-specialization of some organs like tusk of elephants, antelers of deer.
7. This theory only explain the survival of fittest but unable to explain arrival of fittest.
8. The main drawback of Darwinism was lack of the knowledge of heredity.

Theory of Pangenesis According to this theory all organs of an individual produce Pangenes, which are minute particles carrying information about the organs. The pangenes travelling through the blood stream will ultimately reach the gametes, so that each gamete will have pangenes for each of the different organs. After zygote formation, the pangenes tend to form the same organs from which these pangenes were produced.

COMPARISON OF LAMARCKISM AND DARWINISM