One of the assumptions of the Hardy-Weinberg Law is that the population is closed. But for many populations this is not the case. Human populations clearly are not closed. Migration will change gene frequencies by bringing in more copies of an allele already in the population or by bringing in a new allele that has arisen by mutation. Because mutations do not occur in every population, migration will be required for that allele to spread throughout that species. Migration, in a sociological context, implies the movement of individuals into new populations. In a genetic context, though, migration requires that this movement be coupled with the introduction of new alleles into the population. This will only occur after the migrant has successfully mated with an individual in the population. The term that is used to described this introduction of new alleles is gene flow.The two effects of migration are to increase variability within a population and at the same time prevent a population of that species from diverging to the extent that it becomes a new species. The first effect is important because it provides the variability that a population will require to survive if the environment changes drastically. As migration continues over a period of time, the new mutation will be shared between populations. This blending effect helps stabilize the similarities between the population and prevent more isolated populations form evolving reproductive barriers that may lead to speciation.