146 I The Process of Evolution 



higher in adults than in young children, indicating that this genotype 

 is at a selective advantage. The advantage of the heterozygotes is 

 about of the magnitude theoretically necessary to maintain the fre- 

 quency of the sickle-cell gene (about .20). This advantage appears 

 to be responsible for the maintenance of a balanced polymorphism 

 at the sickle-cell locus throughout much of Africa, in spite of the 

 very low viability of the homozygous recessives. 



Glass and his coworkers studied the frequencies of genes con- 

 trolling a number of characteristics (blood groups, mid-digital hair 

 types, etc.) in a "Dunker" religious community in Pennsylvania 

 where the population was less than 100 individuals. The gene fre- 

 quencies at several loci deviated strongly from those found in the 

 surrounding population and in the population from which the group 

 was originally derived (western Germany). Drift is tentatively con- 

 sidered to be responsible for these deviations. Interestingly, there 

 was little or no deviation at difiFerent loci known to be under rather 

 strong selection (e.g., Rh). 



The one known example of selection against heterozygotes in- 

 volves the Rh locus in man. When an Rh negative mother (double 

 recessive) is fertilized by an Rh positive sperm, the resultant hetero- 

 zygous fetus runs a high risk of death due to antigenic incompatibil- 

 ities between it and the mother. This selection is apparently com- 

 pensated to some degree by a tendency of families with Rh problems 

 to have repeated pregnancies until a number of children are raised 

 successfully. This, however, is not sufficient in itself to account for 

 the continued presence of the polymorphism in the human popula- 

 tion. The gene-frequency equilibrium point, although no longer at 

 .5 ( see Chap. 6, selection against heterozygotes ) , is still unstable. It 

 is possible that migration has helped to prevent fixation in the human 

 population as a whole. Fixation of the Rh positive genes may be 

 approached in some subpopulations, and fixation of Rh negative in 

 others. Intermixing of these populations may then reestablish the 

 polymorphism. 



Pasture Plants 



Unfortunately botanists have found few situations in nature that 

 are amenable to the sort of analysis discussed in the foregoing cases. 

 Kemp in southern Maryland studied pasture seeded with a grass- 

 legume mixture and subsequently partitioned. One half was pro- 

 tected from livestock, while the other was used for grazing. Three 

 years later, plants of bluegrass (Poa pratensis), orchard grass 

 {Dacfylis glomerata), and white clover (Trifolium repens) from 

 each half of the pasture were dug up and transplanted to an experi- 



