What is a polymorphic trait? provide two examples of polymorphic traits in modern humans, and give one example of a monomorphic trait. why is the incidence of sickle-cell anemia an excellent example of a "balanced polymorphism," in which two or more alleles are maintained by natural selection in a population? why is the peppered moth an excellent example of directional selection

Polymorphic traits are varying forms of a phenotype in a population. Sexual dimorphism, blood group and hemoglobin in humans are examples of polymorphic traits.  Monomorphic traits, on the other hand, are those are invariably expressed across a population. An example is two-leggedness in humans.

When the different forms of polymorphic traits occur in balance in a population, the traits balanced polymorphism. Sickle cells anemia is an example because the form in individuals with heterozygous for Sickle cell allele confers defense against malaria.  

When, in a population, one extreme phenotype is preferred over others forms by natural selection, the phenomenon is referred to as directional selection. During the industrial revolution, the resultant polluted environment enabled the peppered moth more capable of camouflage hence were preyed on less by their predators. Natural selection, therefore, favored the peppered form of the moth in the population in this environment.