Allopatric speciation takes place when physically separated populations of a single species gradually diverge genetically to the point of becoming unable to interbreed. A. Allopatric speciation is common because the gene flow between subpopulations is generally limited and the barriers that completely separate subpopulations can arise in a variety of ways. B. During past ice ages, some, but not all, subpopulations separated by glaciers evolved into distinct species. C. Speciation does not need to take place through allopatry because subpopulations will form distinct species whenever there are adaptive advantage to not. D. Physical barriers from glaciers and the movement of tectonic plates form so slowly that the subpopulations on either side of the barriers usually do not form distinct species. E. Graves's study of fish populations separately by the Isthmus of Panama may well provide a picture of the beginning stages of speciation. F. Graves's study of physically separated fish populations show that there must be large differences between the environments of the isolated populations if allopatric speciation is to take place.