The complete set of genes within a living organism's cell is called genotype. On the other hand, the complete set of physical characteristics that make up the structure of an organism is called phenotype. The limitations of these factors will inhibit the reproduction process, which of course results differently from one species to another because of the differences in characteristics that are inherited and environment influences. This variation is also the result of a process called mutation, which are caused by radiation, viruses and errors during DNA replication. Genes that arise by mutation and found at the same place on a chromosome is known as allele. Thus, it can be said that evolution is the result of two opposing forces.
According to Hardy-Weinberg's Theory, the genotype and allele frequencies of a population will often remain in equilibrium, unless disturbing influences, such as mutation, is present. The equilibrium state is only possible when the genotype and allele frequencies do not change. This happens when a population has a large size, which prevents the occurrence of genetic drift, bottlenecks and founder effects.