EVOLUTION, GENETICS, ANTHROPOLOGY — MOURANT 513 



While, however, natural selection has almost certainly been the pre- 

 ponderant influence in determining blood-group frequencies in differ- 

 ent populations, accidental fluctuations have undoubtedly affected the 

 frequencies found in small isolated communities, and in some cases 

 such accidentally determined frequencies may have become stabilized 

 when, in an improved but still isolated enviroimient, the numbers of 

 a population have undergone a large increase. The extent to which 

 such a process may have affected the blood-group frequencies now 

 found in large population gi'oups is difficult to estimate. For light 

 upon this problem we must look on the one hand to experimental 

 studies of animal population genetics, and on the other to such work 

 as that initiated by Vogel, Pettenkofer, and Helmbold, and to the ex- 

 amination of many more small and intermediate human population 

 groups. 



It is tempting to regard the various genetical systems which have 

 been discussed as providing us with a series of probes reaching vary- 

 ing distances into the past, the hemoglobins some hundreds of years, 

 the ABO blood groups one or two thousand years, the Eh and MNSs 

 blood groups and perhaps most of the others several thousand years. 

 If this situation represents the truth, then we are even better provided 

 with information than if all genetical frequencies were highly stable, 

 for then we should have no genetical clues to events taking place 

 within any of the major population groups since conditions became 

 stable. 



Empiricall}^, by calling written history to witness, we can show 

 that the temporal hierarchy of genetical systems just suggested does 

 at least in part account for the present genetical constitution of popu- 

 lations. However, until far more is known than at present of the 

 conditions determining the frequencies of the genes we are studying, 

 we must be content to feel our way gradually from one established 

 fact to the next. For instance, though blood-group frequencies, in- 

 cluding ABO frequencies, are similar in populations known to be 

 closely related, this may be the result not, as we have tended to sup- 

 pose, of the absence of selective influences since separation, but of the 

 presence of a number of strong selective forces causing gene fre- 

 quencies to remain balanced at particular levels, levels determined by 

 some condition, whether wholly external like climate, or cultural like 

 food preferences, which the two populations, though separated, have 

 continued to share. Alternatively, even in the absence of any con- 

 tinuing similarities in the external conditions responsible for natural 

 selection, the frequencies of a particular set of allelomorphic genes 

 may have been maintained at or near particular levels by the sta- 

 bilizing influence of the gene pool as a whole controlling the operation 

 of natural selection. 



