It is a unique combination of Darwins Theory of natural selection nad post-Darwininan theory of gentic variability in populations.
In 1965, Mendel showed the poroblem of gentic variability by his laws of Heretdity. But nobody realised it unitll 1900. In 1900 correns in Germany, de Vries in Holland, Tschemark in Austria rediscovered Mendelism. De Vries gave mutation theory in this regard and stated that new species arise by sudden changes or steps called mutations rather than by gradual processes.
Wagner suggested the role of geographical isolation in the formation of every species and subspecies of animal nad plants on nearth.
Hardey weinberg stated taht the relative frequencies of genes will remain constatn from one generation to the next if
- the population is large
-if there is no selection for or against any specific gene
-if no mutation occurs
-if there is no migration
This brought out the fact that a population undergoing evolution experiences change in environmental conditions, All these developments had given scientists to identigy th raw material for the operation of natural selection.
In 1930s, some important scientific developments took place. Dobzhasky (1937) in his book 'Genetivs and the Origin of Species' emphasized the role of genetic changes in populations in the process of evolution. Julian Huchsley 1924 and earnest Mayr 1942 explained the mechanism of variation in higher animals and Stebbins 1942 in higher plants.
Several biometricists synthesized the research results pertainng to different fields of meodern biology and gave shape to synthetic theory.
According to Synthetic theory of evolution, the genetiv variability in population is produced by three major processes namely
- Genetic mutation
-changes in the structure and number of chromosomes
-gene re-combination nad 3 accessory processes: Migration, Hybridization, Genetic Drift
This genetic variability in poulation provides raw material for the operation of natural selection and re-productie isolation.
All popualion of sexually reproducing organisms contain a large gene pool of genetic variability. The gene pool of a population maintains a dynamic equilibrium between in flow and out flow of genes and may become large or smaller depending on various external and internal factors. Genes may be added to pool by migraiton or mutation and removed by natural selection and genetic drift. Genetic recombination for acceptance of rejection of natural selection. Thus, natural selection guides population of organisms into adaptive channels.
Thus in short synthetic theory concieves evolution as a process of functional adaptations of organisms by continuous production of variation and natural selection operating on these variations. The fact is that evolution is occuring and is reflected through the change in gene frequency at the population level.
Postulates of sunthetic theory-
& postulates were identified. of these 1 to 4 are concerned with the origins of genetic variability, from 5 to 6 are concerned with origin of species and 7th one highlights how evolution is a continuous process.
Postulates-
1) At least in higher animals and plats, evolution proceeds principally as a result of interation between 4 indispensable processes
-mutation
-gene-recombination
-isolation
-natural selection
2) Mutation neither directs evoution as the earlier evolutionists believed, nor even serves as an immediate sources of variability upon which selection may act. It is rather a reserve or potential resource of variaility, which serves to replenish the gene pool as it becomes depleted through the action of selection.
3) The mutation which are most likely to be accepted by selection and so to form the basis of new types of organisms are those which individually have relatively slight effect on the pheno type and collectively form the basis of multi factorial inheritance.
4) The role of natural selection is much more than the purely negative one of eleminating unfit types. By greatly increasing the frequency of gene conbination which otherwise have a very low chance fo appearing. selection has an essentially creative and progressive effect.
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5)The continued separation of new adaptive lines of evolution from related lines with different adaptations require the origin of barriers of reproductive isolation, preventing or greatly restricting gene flow between them. this separation is essential for maintaining the diversity fo adaptation which exists in any one habitat and so should be regarded as the basis of species formation.
6) The origin of reproductive isolation like that of new adaptive types, requires the establishment of nmany new genetic changes, including structural alternatives of the chromosome and cytoplasmicc changes as well as gene mutaiton.
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7) The origin of genera and other higher catergories, as well as the long time trends, which give rise to increasingly complez and highly organised formss of life. This results from teh continuation into geology spans of time fo the processes responsible for evolution of racial and species level. The only new elemet which mush be considered is the increasingly eminen extinction of populatoins intermediates between these successful lines.
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