Your answer will be Organ.
The simple answer is mucous cells. the stomach is lined with gastric mucosa and the intestine is lined with intestinal mucosa.
populations were prevented from interbreeding by geographic isolation
Living in herd is a structural adaptation.
There are three different kinds of adaptations: behavioural, physiological and structural.
Adaptations help animals survive and their ecosystem.
The second sentence does not sound correct but it seems correct to me (just not in grammar).
Hope it helped!
