The american flying squirrel and the flying phalanger of australia live in similar environments and look very similar. however, they are not closely related. their resemblance is most likely an example of

I believe the answer is: convergent evolution

convergent evolution refers to the process when organisms that are not closely related  (such as squirrel and phalanger) developed a very similar characteristic as an evolutionary effort to survive in the same environment.
Other examples of convergent evolution would be the similar shape of fins between sharks and dolphins (one of them is a fish, one of them is a mammal)