Synthetic theory
During the last few decades, it was realized, through some important work, that mutations and natural selection both are equally important for organic evolution which neither Darwin, nor de Vries could recognize. Important workers who contributed to the synthetic theory included Th. Dobzhansky, R.A. Fisher, J.B.S. Haldane, Sewall Wright, Ernst Mayr and G.L. Stebbins. In his book, 'Processes of Organic Evolution', Stebbins discussed the synthetic theory and recognized the following factors : (i) gene mutations, (ii) changes in chromosomal structure and number (iii) genetic recombination, (iv) natural selection, and (v) reproductive isolation, with the first three providing the genetic variability and the last two giving a direction to the evolutionary process. There are two accessory factors also, migration of individuals from one population to another and hybridization between races, species or even related genera, increasing the genetic variability available to the populations undergoing the process of evolution. It appears, therefore, that mutations, genetic recombination and natural selection are most important in all classes of individuals and no distinction can be made between them with respect to their relative importance. The genetic recombination is a very important factor, since it is also responsible for providing the genotypes with adaptability to different environmental conditions.