Yeah a fish main porpuse in life is to mate. The reason it will happen in a tank is because they will spend so long trying once they mature, to find a mate of their own species withoput success that they will just turn to anotherspecies (which will probably also be one of a kind in that tank). The difference here is in the wild a fish which cant find another one of its own kind to breed will be few an far between, then of course they have to find another different species of fish without a mate, which makes it even more unlikely where as in the tanks it is a lot more common. I'm no marine Bioligist (yet) but it seems a reasonable assumption.
very interesting thread this. It has inspired me to finally set my 90gal up for African. Hopefully will be started tomorrow.