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.

C) adaptations are often compromises

Explanation:

Adaptations are normally intended to make an individual better suited to there enviroment.

It occurs as a result of natural selection and the altered genes or alleles are heritable.

Some adaptations are often compromises. For example, people living in malaria prone regions have high cases of sickle cell anemia to to an adaptation against malaria. Sickle cell is a point mutation in the Beta-globin. The red blood cells will be abnormally shaped hence giving the individual some resistance to malaria.

Due to this adaptation, it compromises there body's intake of oxygen.

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.

the dali lama

explanation:

he is quite fat and then we can talk about the details