Final answer:
Recessive lethal alleles cannot be completely eliminated through selection because they are only lethal in the homozygous form. In the heterozygous form, individuals may have a normal or altered non-lethal phenotype, allowing the recessive lethal allele to persist.
Step-by-step explanation:
The frequency of recessive lethal alleles cannot be reduced to zero through selection because these alleles are only lethal in the homozygous form, meaning that individuals with two copies of the recessive lethal allele will not survive. However, in the heterozygous form (one copy of the recessive lethal allele), individuals may have a normal or altered non-lethal phenotype. This allows the recessive lethal allele to persist in the population even though it is not directly lethal.