This can occur when part of a population of a species becomes separated from the remainder, they may over time evolve different characteristics from the parent population.

When part of a population of a species gets separated from the remainder, it is termed as geographical isolation. It may evolve different characteristics due to genetic change over a period of time. If geographical barriers are removed by the humans, members of two populations will be unable to mate successfully each other and as a result new species will emerge. The formation of new species is termed as speciation. Speciation due to geographical isolation is called allopatric speciation

i think its the first one