Many species of finches, tortoises, cacti, and iguanas are found on the Galápagos Islands and nowhere else in the world. Which physical attribute of these islands has not played a role in permitting such unique life forms to evolve?

The island are isolated from the rest of the world and human interaction rate it's lower than other places

Explanation:

Those species are endemic from the Galapagos Islands, just like almost every island in the world, human interaction becomes harder due to hard access to the island and the competition between species is unaltered because of the lack of humans in the area.

Natural selection is still working there like the moment when Darwin visited those islands. Another example of islands with that characteristics are Australia and Madagascar.