This can occur when part of a population of a species becomes separated from the remainder

Geographic isolation

Explanation:

Geographical isolation can occur as a result of either an accident or a coincident due to which the a part of the population of a species gets separated from the reminder of the population. As a result of this separation, the separated population is unable to exchange its genetic material with the remainder of the population and thus, with time accumulates novel mutations and this may lead to the formation of a new species and this mechanism is known as allopatric or geographic speciation. Geographic isolation occurs when a geographic barrier occurs due to events such as volcanic eruptions and the populations gets separated.