Describe how natural selection can sometimes lead to the persistence of harmful or even lethal alleles. include an example.

Heterozygosity of genes sometimes allows natural selection to maintain deleterious alleles. Sickle cell anemia is only exhibited by people who carry both alleles as recessive.
The recessive/recessive allele can become advantageous in a different environment or time. Sickle cell recessive allele protects their carrier against acquiring malaria.

It should be cryosphere and hydrosphere