What term describes the formation of new species when populations are geographically separated?

The formation of new species when populations are geographically separated is called allopatric speciation.

In allopatric speciation, a physical barrier divides one population into isolated groups. Mountains, rivers, glaciers, newly formed islands, continental movement, and habitat fragmentation can all interrupt gene flow. Once separated, the populations accumulate different mutations and experience different environmental pressures, natural selection, and genetic drift.

Over many generations, the isolated populations may become reproductively incompatible. Even if they later meet again, they may no longer successfully mate or produce fertile offspring. Geographic isolation is therefore the starting condition, while genetic divergence and reproductive isolation complete the speciation process.

Allopatric speciation is often divided into vicariant speciation, in which a widespread population is split by a new barrier, and peripatric speciation, in which a small peripheral population becomes isolated. The main alternatives in the answer choices describe different patterns: sympatric speciation occurs in the same geographic area, parapatric speciation across neighboring ranges, and adaptive radiation is the rapid diversification of one lineage into multiple ecological niches.