Lets start this off by stating that all creatures in Tamriel have clearly developed from just one single celled ancestor, like us on Earth. You can tell this by just looking at various of the races - alot of them are humanoid, which suggests that orcs, elfs, humans, khajit and whatnot all came from 1 common ancestor.
Anyhow, I think I am right in saying that the Orcs come from a mountainous, rocky region do they not? I think its the dragontail mountains or something. The point I am trying to get at is WHY are they green then? Why would orcs adapt to have green skin, its not like its an advantage or anything? And the only reason things in nature will adapt is for their own benefit. Anyone have any ideas why?
Almost correct.
Evolution as on Earth does not exist on Tamriel, there hasnt been enough time. Only about 6500 years have passed since the Dawn era.
In The beginning there were the Ehlnofey and the Hist.
The Ehlnofey wandered and changed, while the Hist are immobile and unchanging.
Everything on Nirn is descendant from the Ehlnofey, men, mer, akaviri, giants, plants etc.
Except for the Hist, and the Argonians, as the Argonians are of the Hist.
However this process is called subgradiation and not evolution. In the Dawn time many spirits had to become smaller and more numerous, to begat children and become mortal, in order to survive.
After the Dawn time many of the changes the lifeforms on Nirn were subject to were magical and/ or divine in nature.
Take for instance the Orsimer, who used to be a subsect of Aldmer who worshiped their god-ancestor Trinimac.
When Trinimac was defeated by Boethiah (some say he was eaten and passed) he turned into Malacath and his followers turned into Orcs.
Or take the Dunmer, who used to be Chimer (an Aldmer subsect who left the Summurset isles due to religous differences) until they were cursed by Azura.
Nirn is a magical and mythical place, not one of physics and mathematics.
http://www.imperial-library.info/content/monomyth
http://www.imperial-library.info/content/ages-man