NATURAL SELECTION: I 465 



Hence "the types of food predominant in a given region may be important 

 in determining the chromosomal composition of the Drosophila popula- 

 tions which inhabit this region" (da Cunha, 1955). Spiess and others have 

 studied the physiological properties of homozygotes and heterozygotes for 

 different gene arrangements, investigating the effect of the latter on such 

 things as egg-laying capacity, longevity, wing-beat frequency, and wing 

 dimensions. These investisations and a multitude of others were sum- 

 marized by da Cunha (1955). 



In this connection we should note that the chromosomal structures 

 investigated constitute another example of balanced polymorphism. The 

 polymorphism discussed earlier (pp. 366-376) dealt with structures visible 

 to the unaided eye; the present polymorphism is revealed only with the aid 

 of a microscope. But it is nonetheless real. Previously we have noted cases 

 of polymorphism connected with action of predators (e.g., in industrial 

 melanism). The present polymorphism is maintained by virtue of hetero- 

 zygote superiority (which indeed, as we noted, may also be involved in 

 industrial melanism). Just what is superior about a heterozygote? In most 

 cases the necessary investigations have not been made to answer the 

 question. But interestingly enough man himself provides an example of 

 balanced polymorphism maintained in this manner, a case in which the 

 question of what is superior has been determined. Persons suffering from 

 a severe disease known as sickle-cell anemia are homozygous for the gene 

 concerned. Heterozygotes for the gene are not diseased but are detectable 

 by virtue of the fact that their red blood cells will "sickle" (assume 

 unusual shapes when deprived of oxygen ) . Only a small proportion of 

 sickle-cell homozygotes live to reproduce and pass on the gene. Yet in some 

 sections of Africa the gene remains at a fairly high level of frequency in 

 the gene pool. Why does not the gene disappear? Evidently, as in the 

 experiments described above, it would not disappear if heterozygotes 

 enjoyed some advantage. In this case Allison was able to show what the 

 advantage is. "The sickle-cell heterozygote is relatively resistant to 

 malignant tertian malaria and has as much as a 25 per cent better chance 

 of attaining adulthood than the normal homozygote in parts of Africa 

 where malaria is hyperendemic" (Allison, 1959; see also Allison, 1955). 

 Thus the gene remains in a balanced state, the point of equilibrium being 

 established by the tendency of the genes to be lost through low viability 

 of homozygotes, counteracted by the tendency of the genes to increase in 

 number by virtue of the fact that heterozygotes survive better, and thus 

 presumably reproduce more, than do persons lacking the gene entirely 



