Which type of speciation occurs when a physical barrier creates large, separated populations that can no longer interbreed?

The correct answer is vicariant speciation, which occurs when a physical barrier—such as a mountain range, river, or other geographical feature—splits an initially continuous population into two or more isolated groups. As the separated populations adapt to their individual environments over time, they may undergo divergent evolution, which can lead to the development of distinct species. The essential characteristic of vicariant speciation is that the separation happens due to the emergence of a barrier that prevents gene flow, rather than active movement of individuals to different locations.

It's important to note that while peripatric speciation also involves the formation of new species following a separation, it specifically refers to a scenario where a small group from a population becomes isolated from the main group. In contrast, vicariant speciation involves large populations being divided by a barrier, affecting a broader segment of the original population. Allopatric speciation is a more general term that encompasses any form of speciation occurring due to geographic isolation, including both vicariant and peripatric processes. This distinction highlights why vicariant is the most precise answer in this context.