The volcano is currently dormant,but in a hypothetical future scenario,satellite cones at the base of Mt.Kilimanjaro spew sulphurous gases and lava,destroying all life located between the base and 1830 metres above sea level.As a result of this catastrophe,how should the frequency of the sickle-cell allele change in the remnant human population that survives above 1830 metres,and which phenomenon accounts for this change in allele frequency?