Genetic variation in a population Arise due to reshuffling of genes caused by recombination and mutations. Recombination occurs due to formation of synapsis during crossing over in cell division, while mutation are sudden heritable changes in the sequence of genome. Reproductive isolation and selection do not cause variation, as due to this, selective genome cause inheritance of similar traits in offspring. Related Theory According to Hugo de Vries mutation theory. new species arise suddenly showing abrupt deviations in characters from the normal forms. Thus, evolution is not a slow and gradual process but a sudden discontinuous and jerky process. The mutation theory differs from Darwin's theory in the nature of the variations, whereas continuous variations are the raw materials for the formation of new species in Darwin's theory of natural selection; but in De Vries theory. discontinuous variation or mutations are the raw materials.