What is the term describing the process that occurs when a species evolves into a new species without a physical barrier separating populations?

Sympatric speciation should be correct.

Explanation:

Hope this helps.

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.