Well, true to a degree. All evolution happens by circumstance and chance--it certainly isn't a guided or forceful (and by no means conscious) thing. A mutation chanced along that happened to be advantageous for the tang juvenile. Because of that, more survived and passed on that similar allele. They even found similar benefits from mimicking other angels, as well. This is obviously dependent the area the angel and tang are found in and coincides with angel populations. They don't do it on purpose, but it was obviously advantageous so the trait sort of "stuck." It is possible that the coloration developed separately, but it is unlikely given that these fishes' habitat now overlaps. There is far too much evidence to the contrary, especially given the information we have on other batesian mimics.
