Biological Evolution: Darwinian Theory & Natural Selection | Biology Class 12 - NEET PDF Download

DARWINISM 

Charles Robert Darwin was born on 12^th feb, 1809 in England. Darwin traveled 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 Charles 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 indefinitely from original type".

There was striking similarity between the view 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 by both 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 : (High rate of Reproduction)

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 year a single pair would produce about 19 million elephant. Thus some organisms produce more offspring and other produce fewer offspring This is called differential reproduction.

(2)  Struggle for existence :

Every individual competes with other of the same and other species for basic necessities like. Space, shelter and food. It is called struggle for existence and it continues for the whole life from zygote stage to its natural death.

(3)  Variations and heredity :

Except identical twins no two individuals are similar and their requirements are also not same.It mean there are differences among the individuals. These differences are called  variations.

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 adaptation 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 called Natural 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 nest 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.   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 between 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 traveling 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.

NEO DARWINISM : 

Neo Darwinism is a modified form of Darwinism along with recent researches of Weismann, Mendel, DeVries, Huxley, Gates, Stabbins ets. They performed many experiments to remove the objections against Darwin's theory.

The salient features of neo darwinism are as follows–

1.   Rapid multiplication : All organism multiply in geometrical ratio.

2.   Limited food and space : Food and space are limited.

3.   Struggle for existence : It is of three types. Intra-specific, Inter-specific and environmental.

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.

4.   Genetic Variations : They are inheritable variation which can occur due to the following reasons.

(a)  Mutation : They are discontinuous variations which develop due to permanent changes in genotype. Mutations are of three types –

–  Genomatic mutations : Change in number of chromosome.

–  Chromosomal Aberrations : Changes in number of chromosome.

–   Gene Mutation : Change in nucleotide.

(b)  Gene recombination : They are new combination of genes which are usually caused by crossing over.

(c)  Hybridization and gene migration : It is crossing of organisms which are genetically different in one or more traits.

(d)  Random Genetic drift : It is the elimination or addition of the genes of certain certain characters when some animals in population migrate or died or immigrate. It changes the gene frequency of remain population. Genetic drift operates only in small population.

(Changes in frequency of genes in a gene pool is called drift)

Founder Effect :    Gene pool is the sum total of all the genes found in a population.

Change in the frequency of gene in a gene pool is called genetic drift.

Genetic drift always operates in small population.

By genetic drift often the phenotype of this small population quickly become different from the parental population and some time form a new species. Such an effect is called Founder Effect.

Bottleneck Effect : Death of several members of population due to natural calamities (Earthquake , Storm, Flood) also leads to genetic drift.

The original size of population is then restored by mating among the survivor.

The new population may lack the genes of certain.

This may produce a new species after some time.

The loss of a section of population by death an after sometime a new species is formed that effect is known as Bottleneck effect.

5. Natural Selection :

If differential reproduction (some individuals produce more, some only few and still others none) continue for many generations, genes of the individuals which produce more offspring will become predominant in the gene pool of the population.

Thus natural selection occurs through differential reproduction in successive generations. 

6.   Isolation :

Isolation is a segregation of populations by some barriers which prevent interbreeding.  The reproductive isolation between the populations due to certain barriers leads to the formation of new species.

Example of Natural Selection –

(1)  Industrial Melanism : 

This phenomenon was studied by Barnard kettlewell. 

Before industrial revolution, the dull grey forms of prepared moth-Biston betularia – were dominant; the Carbonaria form (Black) was rare because it was susceptible to predation by birds.

The industrial revolution, resulted in large scale smoke which got deposited on tree trunks tuning them Black. Now grey varieties became susceptible – the black forms flourished.

Replacement of coal by oil and Electricity reduced production of black moth so the frequency of grey moths increased again.

(2)  Drug resistance :

The drugs which eliminate pathogens become ineffective in the course of time because those individuals of pathogenic species which can tolerate them, survive, flourish to produce tolerant population.

(3)  Sickle cell Anaemia and Malaria : 

Individuals homozygous for sickle cell Anaemia die at an early age.

In heterozygous individuals, the cells containing abnormal haemoglobin Sickle shaped.

In fact, When an RBC becomes sickle-shaped, it kills Malarial parasite effectively so that these individual area able to cope with malaria infection much better than normal persons.

The processes of natural selection thus maintains the abnormal form of haemoglobin along with the normal form in a region where Malaria is common.

(4)  Malaria and G-6-PD deficiency :

Glucose 6- Phosphate dehydrogenase deficiency  is a common abnormality in Negroids. Haemoglobin gets denatured and is deposited on cell membrane.

The disease is called favism. In these RBC, the malarial parasite cannot complete It's cycle. Such persons get protection from Malaria.

Artificial Selection – 

Man has been taking the advantage of genetic variations for improving the qualities of domesticated plants and animals.

He selects the individuals with desired characters and separates them from those which do not have such characters. The selected individuals are interbred.

This process is termed as Artificial Selection. This process is man made.

If it is represented for many generations it produces a new breed with desired characters.

By artificial selection animal breeders are able to produce improved varieties of domestic animals like dogs, horse, pigeons,, poultry, cow, goats, sheep and pigs from their wild ancestors. Similarly the plan breeders have obtained improved varieties of useful plants like wheat, rice, sugarcane, cotton, pulses vegetables fruits etc.

Artificial Selection is similar to natural selection except that the role of nature is taken over by man and the character selected are of human use.

The breeders have successfully produced the toy-like Shetland pony, the Dane dog, the sleek Arabian race horse by selection.

Many crop plants like broccoli, kale cabbage, Cauliflower, Brussels sprouts and kohirabi have been produced through selective breeding.

The various breeds of fowl ranging from the ceremonial cocks (the Japanese onago-dori) to the broiler. leghorns are all derived from a single jungle fowl Gallus gallus.

Reproductive Isolation – 

Reproductive isolation is the prevention of inter breeding between the population of two different or closely related species.

It maintains the characters of the species but can lead to the origin of new species.

The mechanism the characters of the species but can lead to the origin of new species.

The mechanism of reproductive isolation is explained by Stebbins in his book 'Process of Organic Evolution'.

Old NCERT Syllabus

Two main sub-types of reproductive isolation are– 

1.  Premating or prezytotic isolation : 

Prevent matting or formation of zygote.

(1)  Ecological isolation : Two species live in different habitats and do not meet. (One may be living in fresh water and the other in the sea).           

(2)  Temporal isolation : Breeding seasons or flowering time may be different in the two species.         

(3)  Behavioural isolation :  The males of one animal species are unable to recognize the females of another species as potential mates.

(4)  Mechanical isolation : The structural differences in genitalia of individuals belongings to different animal species interfere with mating.

(5)  Gametic isolation : The sperms and ova of different species of animals are unable to fuse. In plants, the pollen coming from a different species may be rejected by the stigma. 

2.   Postzygotic Isolation  : A hybrid zygote is formed but it may not develop into a viable fertile adult.

(1) Hybrid inviability : Hybrid zygotes fail to develop. In plants, embryos arising from inter-specific crosses abort.

(2)  Hybrid sterlity : Hybrid adults do not produce functional gametes. (Mules and henny are common example) in mammals. Several hybrid ornamental plants are sterile.

3. Hybrid breakdown  : Sometime inter specific mating produce a hybrid, which give rise to next hybrid by back cross but they have reduced vigour or fertility or both.

GENETIC BASIS OF ADAPTATIONS :

Joshua Lederberg  & Esther Lederberg  shown genetic basis of adaptations by experimenting on bacteria. This experiment is known as Ledeberg's Replica plate experiment.

1. Lederberg cultured the bacterial cells on agar plate.
2. Many bacterial colonies or groups grew on this agar plate.
3. In this every colony is formed  by the division of bacterial cells.
4. Therefore its all cells were of same genetic structure.
5. This type of group of cells is known as clone.
6. This multi colony agar plate is known as master plate.
7. On this master plate one sterile velvet plate was pressed slightly so that some bacteria got stuck on velvet plate. In this way this becomes replica of master plate. 
8. Now efforts of preparing replica had been made on those agar plates whose agar contains an antibiotic penicillin. It was seen that some bacteria failed to grow on penicillin agar plate while some bacteria were able to grow and developed new colony.

It was concluded that these bacterial colonies were penicillin resistant.

These bacteria have penicillin resistant mutant gene.

 Lamarckian view : Penicillin induced a change in some bacterial cells enabling them to grow in medium containing penicillin (wrong concept).

According to Darwin some bacterias were penicillin resistant in bacterial suspension. In penicillin medium normal bacteria did not survive while mutant bacterias survived, as they are adapted, and form colony.