The sickle-cell allele is pleiotropic (i. e., it affects more than one phenotypic trait). Specifically, this allele affects oxygen delivery to tissues and affects one's susceptibility to malaria. Under conditions of low atmospheric oxygen availability, individuals heterzygous for this allele can experience life threatening sickle cell "crises". Such individuals remain less susceptible to malaria. Thus, pleiotropic genes/alleles such as this can help explain why
A) new advantageous alleles do not rise on demand
B) evolution is limited by historical constraints.
C) adaptations are often compromises
D) chance events can affect the evolutionary history of populations.

1. true.2. true.3. true.4. false.5. true.the concept of darwin's theory of evolution involves the idea that there are limited resources in a given living environment and that the organisms which live within have variations in their genetic make up due to random mating. the ones with beneficial genetic variations can express certain traits which give them an upper hand in surviving and reproducing in the said environment compared to ones who may have a more disadvantageous variation in traits. over time, the organisms with an upper hand will survive and reproduce more frequently and pass on their beneficial genes while the others die. the result is evolution, where we observe (usually, but not always) eventually, phenotypic distinct changes.