the gazelle with shorter legs in this period has more offspring, because of the environment they had a better chance of hiding in tall grass.
Survival of the Fittest
Mutation caused the change of variation which was shorter legs, this was the heritable trait and was passed off as their offspring. They adapted to the environment with them because it gives them a better advantage of hiding in tall grass.
Shift in reproduction
Even if it gave them a better hiding advantage, it gives the gazelle a harder chance to survive, because they are slower due to the length of legs and gives lions a better chance, and gives a few short-leg gazelles to reproduce.
Changes in gene pool
environments with longer grass give shorter leg gazelles a better chance because their shorter legs help them hide in the grass better, their offering survived and they passed their trait on.
Grass gets shorter over time due to the environment. This gives the gazelle no advantage to survival, and longer leg gazelle get a better chance due to faster speed. The shorter leg gazelle was still passing on their allele, but not as much.
The fitness for this animal is high because longer leg gazelles have the favorable trait and live, and reproduce while the gazelles with shorter legs start dying and not reproducing as much because of environmental change, and natural selection.