How does the mechanism of 'natural selection' or an accumelation of mutations account for the complementary male and female reproductive organs?
In higher-order animals? It accounts for them because lower-order animals have them too. Sex isn't a recent development.
The idea is that gender starts at the genetic level, in single-cell or colonial organisms. For instance slime molds have some 50 different genders or so.
The difference in sexual morphology is just a natural development from a genetic gender. Also you might be interested to know that there's a species of ungendered snails that both have spiky penis-like structures. To mate two snails will "fence" until one of them is penetrated by the other's implement. At that point it's that snail that will bear the offspring (lay eggs or whatever.)