ABSTRACT
Most of the mutations that people were aware of before the 1960s, in particular in humans and Drosophila had strong phenotypic effects and many were congenital defects. In human history, migration has been a major cause of genetic changes in many countries, via movements of armed forces during wars, occupation, transfers of slaves and prisoners-of-war or marriages. There are some important differences between the effects of an immigrant and that of a mutation on the recipient population. A mutation changes the allele frequencies at a single locus in a single individual. By contrast, a single immigrant carries with it an entire genome, and can potentially incorporate many new alleles in the gene pool of the recipient population. Rare alleles which appear in many subunits are evidence for gene flow – because independent mutations in different populations are unlikely. On the other hand, rare alleles with limited distribution but high conditional frequency indicate isolation and restricted gene flow.
