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Darwinism or Darwin's theory of evolution /postulates of Darwin || definition , natural selection ,speciation , example. - science info
after collection a vast amount of information, Darwin began to realised that under the intention competition of members in a population, any variation, which favour survival in a particular environment would increase the individual ability to reproduce and leave fertile offspring. a less favourable variation would be at disadvantage and organism possessing them would, therefore, have decreased chance of their successful reproduction.
this data provide Darwin with the framework to formulate, a theory of evolution by natural selection in 1839. he did not publish his finding at that time. indeed, Darwin greatest contribution to science was not so much to show that evolution occurs but how it might occur .
in the meantime, another naturalist Alford Russel Wallace, came to the same conclusion as Darwin regarding natural selection . in 1858, Wallace wrote a 20 pages essay outlining his theory and send it Darwin. Darwin and Wallace presented paper on their ideas at a meeting of the Linnea Society in London
in November 1859 Darwin publish his thought on evolution in 'Origin of Species ' by the name of natural Selection.
POSTULATE OF DARWINSM :
the main postulate, which formed the basics of Darwin theory of natural selection are as follows
overproduction (rapid multiplication)
All organism possess enormous fertility. they multiply in a geometric proportion with some organism producing very large number of species. Despite of this high rate of reproduction of a species, its number remain constant under fairly stable environment. the production of more offspring by some organism and fewer by other is termed as differential reproduction.
limited resource:
the resource like food, space, etc. remain limited inspite of rapid multiplication of the individual of all the species.
struggle for existence :
the limited resource present in an environment are the main cause of struggle for existence, which may occy at following three levels.
intraspecific struggle :
it is the struggle among the individual of same species for their common requirement like food, shelter, mate, breeding, places, etc. intraspecific struggle is very severe, e.g. young tree in a forest, cannibalism, (eating individual of its own species), human wars, etc.
interspecific struggle : it is the struggle between the individual of different species for their similar requirement like food and space. for example, a frog feed on insect and it prayed upon by snake while kite feed on both as well as snakes.
environmental struggle : it is the struggle of living from against the environment condition like extreme heat, cold, drought, earthquakes, storms, disease, volcanic eruption, etc.
4 appearanceof variation :
all individuals are dissimilar in some of their characters except the identical twins. this dissimilarity is mainly due to the variation .
those are the small or large difference among the individual . variation allow some individual to better adjust with their environment.
variation can be categorised into following type
somatic variation : these variation affect the somatic cell of an organism. they are also called modifications or acquired characters because they are caused by various environment factors, use and disuse of organs and conscious efforts, etc.
germinal variation : these are inheritable variation recognised by Darwin . they are formed mostly in germinal cell . they are further two type
(a) continuous variation : these are fluctuation variation, which oscillate due to race, variety and species .
(b)discontinuous variation : these appear suddenly and show no spots gradation . this variation were termed as' spots ' by Darwin and Mutation by Hugo de Vries . Darwin regarded continuous variation to be more important because the discontinuous variation being mostly harmful would not be selected again.
the organism which are provide with favourable variation would survive, which the unfit are destroyed. originally it was an idea of Herbert spencer ( 1820-1903) who used the phrase ' survival of the fittest ' first time Darwin named it natural selection.
survival of the fittest refers to the idea that the natural selection tents to favore those organism that are most fit to survive with the reproductive age in a particular environment and to produce offspring.
for example, according to the theory of Darwin , the long necked giraffe evolved due to natural selection.
he started that initially both long neck and legs and short neck and leg containing giraffes were present , but due to the scarcity of grass on land ,they had to feed on the leaves of tall trees.
Now giraffe with long neck and longer legs could get food more easily and better change of survival . as a result, long neck and legs containing giraffes become abundant and giraffes with short neck legs become extinct .
the giraffes which short necked and limb soon died due to starvation when climate change due to their natural selection .
evidences of natural selection :
it has the following evidence
(1) rate of reproductive , limitation of resources , competition and struggle for existence for existence can be seen in natural normally.
(2) abundance of variation is evidenced by the absence of two similar individual except monozygotic twins , which also show some environmental variation
(3) preproduction of new varieties of plant and animals through artificial selection . these show evidence that the natural having vast resources can easily that the natural having vast resources can easily produced new species through the natural selection .
(4) pedigrees of some animals (horse, camel, elephant), mimicry and protective colouration also supports natural selection.
formation of new species (speciation)
Darwin considered that as a result of struggle for existence, variability and inheritance, species become better adapted to their environment . this beneficial adaptation are preserved and accumulation in the individual of species generation after generation . this result into the origin of species or speciation.
