Here's why:

* Isolation promotes genetic divergence: When populations become isolated, they experience reduced gene flow. This means that mutations and genetic changes that occur in one population are less likely to spread to the other. Over time, these genetic differences can accumulate, leading to significant divergence between the isolated populations.

* Different environmental pressures: Isolated populations often face different environmental pressures (climate, food sources, predators, etc.). This can drive natural selection in different directions, further accelerating genetic divergence and leading to distinct adaptations.

* Reproductive isolation: As genetic differences accumulate, isolated populations may eventually become reproductively isolated. This means that they can no longer interbreed and produce fertile offspring. At this point, they are considered separate species.

Examples:

* Darwin's finches on the Galapagos Islands: Different islands had different food sources, leading to the evolution of distinct beak shapes and feeding habits, eventually resulting in 13 separate species.

* Cichlid fishes in Lake Victoria: This large lake has many isolated areas, each with its own unique environment. This isolation has led to the evolution of over 500 species of cichlids, each adapted to a specific niche.

In summary: Isolation is a key driver of speciation, as it allows for genetic divergence, adaptation to different environments, and ultimately reproductive isolation.