Final answer:
A volcanic eruption is the most likely event to cause geographic isolation as it can create significant, long-lasting barriers such as mountains and lava flows that prevent species from interbreeding. This can lead to allopatric speciation where populations evolve differently due to being separated.
Step-by-step explanation:
The phenomenon of geographic isolation happens when populations of the same species become separated by a physical barrier and cannot interbreed. Among the options provided, a volcanic eruption (volcanic eruptions) is most likely to cause geographic isolation due to its capability to significantly alter the landscape. Such an eruption can create mountains, lava flows, or ash deposits that form barriers to the movement of organisms.
Ecological succession and biogeography also relate to geographic isolation as they involve changes in an ecosystem over time and the distribution of species across different geographic locations, which can lead to the institution of barriers that result in isolation. For example, new lakes form and erode to separate populations, leading to allopatric speciation.
While other events like the widening of a lake or a forest fire could also cause separation of populations, they are usually less permanent than a volcanic eruption. Therefore, considering the permanence and scale of impact, volcanic eruptions are the most likely cause of geographic isolation.