The mechanism that accounts for high frequencies of sickle cell hemoglobin in malarial environments is:

Selection -Selective advantage

Explanation:

Sickle cell hemoglobin result from the sickle shape of red blood cell instead of the ameboid shaped cell.

Malaria is a deadly disease that result from mosquito bites.

Sickle cell hemoglobin has a selective advantage because of the presence of AS genotype, the s allele in a sickle cell is an harmful recessive allele that remains in the gene pool it becomes expressed when the s allele is expressed in SS form after an AS individual have mated with either an AS or SS individual.

AS genotype has a high resistance against malaria, they have the ability to thrive making the allele s that is recessive for sickle cell thrive well in the environment of malaria in an AS individual.