Q.If the variation occurs in a population by chance alone and not through natural selection, and it brings about a change in the frequency of an allele, what is it called?

The modern synthetic theory of evolution draws together genetics, ecology, comparative anatomy, biogeography and palaeontology into a single, fuller account of how evolution works, giving due weight to both mutation and natural selection rather than favouring one over the other. Its principal contributors include R. Fisher, J. B. S. Haldane, T. Dobzhansky, J. Huxley, E. Mayr, G. G. Simpson, G. L. Stebbins, Sewall Wright, Mendel and T. H. Morgan; Stebbins organised the theory around five key factors — gene mutation, mutation in chromosome structure/number, genetic recombination, natural selection and reproductive isolation — usually grouped into three broad concepts: genetic variation, natural selection and isolation. Genetic variation itself, meaning any change in genes or their frequencies within a population (the gene pool), arises through several mechanisms: gene (point) mutation, which alters a single gene and its phenotype; genetic recombination (crossing over) during gamete formation, which creates new allele combinations; gene flow, the movement of genes into or out of a population via migration, pollen dispersal or DNA transfer; genetic drift, a random, chance-driven fluctuation in allele frequency that is especially significant in small populations, including the special cases of the founder effect (a few individuals founding a new population with different allele frequencies) and the bottleneck effect (a disa …