The Example of the Sickle-Cell Allele Demonstrates a Key Aspect

The example of the sickle-cell allele demonstrates a key aspect of evolution through natural selection, in that

A) human populations in the tropics are the most susceptible to random changes caused by natural selection.
B) adaptation and fitness are in relation to specific environments; traits are not universally adaptive or maladaptive.
C) adaptation and fitness are in relation to the individual organism, not the general population.
D) although natural selection usually acts upon the phenotype, it can sometimes act upon the genotype.
E) natural selection increases the variety in a population upon which subsequent natural selective processes can act.