Pre-Darwinian The theory of evolution put forwarded by Jean Baptiste Lamarck commonly known as Lamarckism, mainly centers around the inheritance of acquired characters.characters are acquired by use and disuse of different organs and evolution is the result of adaptation of organisms to the environment. he wrote, “….so does nature by means of heat , light, electricity and humidity, achieve the spontaneous or direct generation of organisms which exist in the beginning of both kingdoms, animal and vegetal, where their most simple forms are to be found.this indicates that he was in the favor of spontaneous generation of life. Lamarck’s theory can be summarized as: a) as per need under different environmental conditions organism as a whole or a part of it tend to change due to some internal forces. b) organs are modified or new rudimentary organs are formed because of varied nature of use and disuse of the same. continued use helps the organ to develop fully while disused part start diminishing until it finally disappears. c) the modified charcters thus acquired during the life time of an individual are inherited by the offspring. Example. Originally the modern giraffe had short necks. they lived on grass and leaves of short trees. but when that type of food became scarce, the giraffe had to stretch their necks to reach leaves of tall trees. with the gradual stretch the neck were modified and became longer and longer which helped them adapt to the environment for their survival. Limitations There are some insects like worker bees, ants and wasps which do not procreate , because they are neuter and hence cannot transmit changes.but they adopt themselves to their environment and living condition.again the exo-skeleton of some higher insects are so rigidly formed that it is impossible for environment or living conditions to change that external structure.a certain variety of fly didn’t go blind after living in dark for 60 years. Neo-lamarckism The Neo lamarkians argue that time was not sufficient to produce effect of external factors.the twentieth century science of genetics made a rapid progress and the finding do not give evidence to support Lamark or Neo-Lamarckism. Uniformitarianism Charles Lylel by studying rock layers and geological processes, arrived at the conclusion that since the beginning of time some forces were operating to shape and reshape the earth. Darwinism The theory of evolution proposed by Charles Darwin is commonly known as Darwinism. Darwin theory of natural selection is based on the following five major points: a) all organisms show a tendency to increase in numbers. they produce more number of offspring than that which can survive to reproduce. b) the number of individuals in a given species remains more or less constant. higher death rate must be the reason for this. c) the individuals exhibit variation in respect of physiological, structural and behavioural traits.that means they are not identical. d) the individuals having selective advantage or suited to the environment survive, and they pass on their advantageous advantage to the future generations. e) the individuals of the future generations continue to modify the adaptations thereby gradually bring changes in the species. eventually a new species is evolved. The organisms are to struggle for existence. the nature select those who are more suited. the fittest one survive. The descent of man-the book, in which Darwin said that man descended from minute organisms; the simplest form of the distant past. from simple form evolved the complex from through various stages. the most complex form is represented by humans. Background During his voyage around the world he not only observed events but also collected materials of varied natures on the basis of which he developed his theory of evolution. In Brazil he marked that certain animal species of the southern part were slightly different from those of the southern part. the differences were so small that they did not form different species, but could be called local varients.in argentina he found fossils of huge creatures, the features of which resembled those of the living armadillos of that region.thus he concluded new species evolved from older species and therefore, both had some common characteristics. Limitations At time when Darwin first proposed his theory there were very little fossil evidence on the basis of which the ancestral formof humans could be described. this one of the two major weaknesses of his theory of evolution.the second flaw in his theory was that that he could not explain how traits are passed fromone generation to another;though he gave his pangenesis theory but till then mendel’s theory was published. Mendel’s theory Mendel used the term Factor. In 1900 this was replaced by the term Gene. accordingly this newly developed science is known as Genetics. Germ Plasm Theory August Wismann said that there are two kinds of cells, one is called the Germ Plasm, which is transmitted from one generation to other and therefore, it may be regarded as potentially immortal, while the other is termed as somatic cell. two germ Plasm cells carrying certain determinants meet to form a fertilized egg, which develops to embryo. Mutation Theory A new characteristics may suddenly evolve spontaneously without passing through an intermediate form.the new one is evolved from an earlier form. the new characteristic immediately becomes permanent.in this way any new form may develop from the old one.this phenomenon is called mutation.the new trait passes from one generation to another.as a result a new type of organism may evolve. NEO-DARWINIAN or Synthetic Theory. A population shares a common gene pool. biologist with the help of modern statistical methods and experiments started working on such a population to understand the evolutionary processes involved in that population and the effects of those processes. on the basis of their findings population geneticist developed the idea that evolution is a change in gene frequency in the gene pool of a population over certain time period.through the process of natural selection the frequency of advantageous gene increases and these survive in a given environment to reproduce.it could be said that at present this could be the modern concept of evolution.this theory is called the synthetic theory of evolution, because in this theory the basic concept of natural selection theory, Mendelism, mutation theory and population geneticist has been synthesized.this is also called the Neo-Darwinism theory as the theory of natural selection of Darwin has been developed giving it a new dimension incorporating the basic ideas of the other theories, more particularly that of population genetics.