Darwinism :
(A) General
Charls Darwin (1809 to 1882) was an English naturalist. He made world tour on a British fighter ship, “Beagle”. He collected various animals from different islands & countries. He studied Finches birds on Galopagos island of South America. These birds are now called as Darwin,s Finches.
A. R. Wallace (1823 to 1913) simultaneous to Darwin, but separately published view s similar to the Darwin. In 1858, the views of both Darwin & Wallace were published in a joint paper which were based on the views of Malthus.
In 1859, Darwin published a book “Origin of species by natural selection. Darwinism is also called as “Theory of natural selection”.
(B) Six fundamental propositions of Darwinism.
1. Enormous fertility or over production in organisms. All animals & plants have an tendency of over production.
Examples –
A rabbit reproduces 3 to 4 times in a year and each time gives birth to 4 to 6 young ones. An oyster lays about 6 crore eggs in a year. Similarly, an ascaris lays about 2.5 crore eggs in a life spawn. If the entire progeny survives and multiplies, it will be eight times the size of the earth in a year.
Elephant is a slowest breeder, matures at the age of 30 years. it’s life spawn is of 100 years and a single female gives birth to six young ones in its life spawn. It is calculated that in 750 years a single pair of elephant give rise to 1.9 crore of elephants. Thus all animals multiply in geometrical ratio but still their number is maintained at a constant level.
2. Struggle for existence
Irrespective of over production, the animal population is maintained at constant level. Because the food & space are limited which results in struggle for existence. The struggle for existence is of three types-
(a) Intra specific struggle
(b) Inter specific struggle
(c) Extra specific struggle
The animals which are not successful in the struggle, do not survive.
3. Variation & Heredity
Variations occur commonly in the organisms and no two individuals are alike. The fittest which survive have advantageous variations.
Variations give rise to new characters and heredity passes them on to the next generation.
There are three sets of contrasting variations-
(a) Meristic and substantive variations
(b) Continuous and discontinuous variations
(c) Somatic and blastogenic variations
4. Survival of the fittest or Natural selection
In the struggle for existence only for existence only those individuals survive which are fitted biologically to the new conditions, as they possess advantageous variations. The animals which are unfit get perished Darwin called it “Natural selection “ and the Spancer termed it “ survival of fittest”.
5. Adaptation to the environment
Environment of any region is never constant. The environment changes are in food supply, climatic conditions (temperature, lightning, drought, food) and geographical conditions. The variations which best suited in a given environment may prove unfavorable in the changed environment. Therefore, in order to survive an organism must be capable of changing in response to the changing environment.
For example in Mesozoic era, the environment was suitable for the dinosaurs, They ruled the Earth & occupied land, water and air. But later on in the Mesozoic are glaciations led to changes in the temperature and the vegetation became rare. In these conditions, the dinosaurs could not cope with the environment & soon became extinct.
6. Origin of new species
In the due course of time, the fittest are automatically selected. They survived & adapted to the natural environment. The adaptations of the survivors may result in the formation of new structures which are called as variations. The useful and favorable variations were transmitted from generation to generations and led to the formation of new species.
(C) Criticism of Darwinism
Initially, Darwinism was accepted enthusiastically, but later several objections were pointed by the scientists which are as follows-
• It describes the survival of fittest but not the “origin of fittes”
• It’s natural selection does not explain the presence of the connection links.
• It’s natural selection fails to explain super specialization of some organ, as a result some species get extincted.
• Darwin was ignorant about heritable & non-heritable variations. It is called as agnostic period of Darwinism.
• Darwin described micro variations in the evolution, but he failed to explain the initial origin of an organ.
• Darwinism failed to explain variations found in somatic & germ cells.
• Darwinism fails to explain the presence of vestigial organs.
• Darwinism failed to explain that in man sometimes most unfit but luckiest are protected and the fittest are handicapped. It fails to explain toe origin of terrestrial animals from, the aquatic animals.
• Darwinism failed to explain that how the characters are acquired & modified.
(D) Neo-Darwinism
In the beginning of 20th century, Darwinism had been modified on the basis of modem discoveries which is now called as New-Darwinism.
Neo-Darwinism was strongly supported by Wallace, Davenport, Waldane etc New-Darwinism includes following 5 series
1. Gene mutation
2. Changes in the number of structure of chromosomes
3. Genetic re combinations
4. Natural selection &
5. Reproductive isolation
(1) Gene mutations
Change in the number of sequence of nucleotides in the DNA of gene is called as gene mutation.
Most of the mutations are detrimental but few may be beneficial .
The gene mutations are caused due changes in the environment. There is no specific rate of mutations Normally, gene seldomly mutates but the large number of genes & large number of organisms provide sufficient chances of variations.
(2) Changes in the structure & number of chromosomes
Changes in the configuration & number of nucleotides & genes led to changes in the structure of the chromosome. These change are caused as a result of delition, duplication, translocation & inversion.
Changes in the structure of the chromosomes causes variations in the animals which finally led to origin of species.
(3) Genetic recombination
Game to genesis involves meiosis in which crossing over results in the development of new genetic re combinations. As a result variation are generated in the organisms.
(4) Natural selection
Natural selection facilitates beneficial gene recombination’s to flow into the off springs.
(5) Reproduction isolation
When the different populations of a species get isolated by the barriers, after some times they fail to interbreed. It is due to the acclimated changes appeared in the gene structure as a result of mutations, chromosomal aberrations & polyploidy. These changes lead to the variations, as result the populations evolved into a new species.
Geographical barriers like mountains, sea, rivers etc and physiological barriers prevent intraspecies reproduction
The species which are developed as a result of geographical barriers are called as allopatric species. The species which are developed as result of reproductive (physiological) isolation in the same geographical environment are called as sympatric species.