Sources of variations comprise factors and processes that induce changes in thee organisms and are associated with the natural selection and adaptations of the organisms. As a result, these variations, or variabilities stimulate the process of the evolution of the organisms. Sources of variability are directly linked to genetic factors. Sources of genetic variations include mutations and formation of new gene alleles, random mating, random fertilization, recombination between homologous chromosomes, and genetic drift - alterations in allele frequencies due to various natural events. The expression of variability represents the strength with which a certain trait is expressed. In other words, expression of variability is a complex interaction between environment and genes that lead to the expression of certain favorable traits.