How is lactase an example of fitness in humans?

Evolutionary advantages. Lactase persistence is a text-book example of natural selection in humans: it has been reported to present stronger selection pressure than any other known human gene. However, the specific reasons as to why lactase persistence confers a selective advantage "remain open to speculation".

Lactase breaks down the lactose in food so your body can absorb it. People who are lactose intolerant have unpleasant symptoms after eating or drinking milk or milk products. These symptoms include bloating, diarrhea, and gas. Lactose intolerance is not the same thing as having a food allergy to milk.

In pastoralist populations, individuals with the lactase-persistence mutations were more likely to survive, reproduce, and pass on their alleles than people without the mutations, especially in times of famine.

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you must remain stopped until the stop signal is retracted and the red lights are turned off. approaching traffic in the opposite roadway of a divided highway shall not be required to stop, even if the school bus has the stop-arm extended and the alternately flashing warning signal lamps on.

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.