Answer:
They are exposed to different environments.
Explanation:
The isolated organism should have the same gene pool. This gene pool should be maintained if there is no other factor that disrupts it.
But the condition of the habitat might be different and this will create a natural selection. The environment will give advantages to certain traits while giving disadvantages to other traits. But a different environment will support different traits, so this can create variation.
Because! They are exposed to different environments.
Species mutate to fit their environment, like a hummingbird's beak getting longer because its new environment only has deep set flowers (found that out on some animal documentary).