Question

Complete the analogy.
Stabilizing selection:sickle cell anaemia:
Directional selection:__________
(A) Elongation of neck of giraffes
(B) Australian marsupials

Answer 1

• Stabilizing selection is the type of selection that balances the two traits, also known as balancing selection. E.g.; Sickle cell anemia, in these two alleles, there is one allele that is lethal and causes death this is eliminated by selection while another allele is a heterozygote that protects the individual from malaria this allele is favored by the selection, thus balances both the traits.
  
• Directional selection is the type of selection in which the traits are only favored by the selection and the selection moves only in one direction and reaches to the extreme. E.g.; Long neck of giraffe, in this it was found that earlier giraffes were having short necks due to which they were unable to eat leaves which are higher on the plants, due to continuous process of evolution in one direction, slowly the neck of the giraffe became long and help them in eating leaves from higher positions of plants.

So, the correct option is 'Elongation of the neck of giraffes'.