This sort of thing is discussed all the time in the lab -- examples of convergent evolution, where a particular type of gene mutation is so beneficial that it arises (or very similar genes arise in separate unrelated species.A gene in Asian monkeys that may have evolved as protection against a group of viruses that includes HIV has been identified by Harvard Medical School researchers, who add that their finding suggests the current AIDS epidemic is not a new kind of scourge.
....This is the second time a TRIM5-CypA hybrid gene has been identified in monkeys. The other one -- TRIMCyp -- was found in South American owl monkeys in 2004. But it's not likely that these two gene combinations arose from a single common ancestor, the Harvard researchers said...
The eye is a similar process... where many eyes as we know them are wired in a backwards manner (the optic nerve runs into the eyeball on its way out, rather than just being affixed to the outside)... but a few unconnected marine species exist where the eye is wired in a "sensible way" -- suggesting that the eye structure evolved concurrently on parallel tracks, and the variation between attributable to the original random mutations.