A green rat snake that lives in the grass and a brown rat snake that lives in the desert is a form of what ?

A geographically distant species is a green gopher snake that dwells in the grass and a brown rat snake that dwells in the desert.

These species don't have any viable progeny.

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A green rat snake that lives in the grass and a brown rat snake that lives in the desert is a form of geographically separated species.

Explanation:

The habitats of the green rat snake  and brown rat snake shows that they are geographically separated species.

The two rat snakes are different species because of their distinct habitat and morphology.

When two species get separated by habitat their breeding method changes either by morphology or breeding pattern.

Such species do not produce viable offspring.

Thus a green rat snake and a brown rat snake have very different habitats they are now two different species.

Such species are said to be reproductively isolated species. Two species having genetic divergence undergo natural selection to adapt to the environment.