what role will nature selection most likely have in the frequency of the alleles responsible for the presence of tissue pockets in the shrimp population over time

Through natural selection, those organisms which have better adaptations to survive in an environment are able to live and pass on their alleles to their offsprings. Hence, any trait which is beneficial to an organism will be favoured by natural selection.

As tissue pockets is a trait which is required by the shrimps, hence with the passage of time shrimp population having tissue pockets will increase and will be favoured by nature.