When elves decide to conceive, a male must add his genetic material at different intervals over several years to the female. Genetic material could come from a single male or various ones with no connection to one another. The developing child is slowly "built" with the genetic material of one father, or the makeup of several different fathers. After enough "material" has been collected, the embryo becomes fully formed and the gestation process can begin. This method allows elves to selectively breed for some distinctive and valuable traits amongst themselves.
I would like this to ultimately lead to specialization within the elven race, with various subraces being created in the process ( wood elves, aquatic elves, drow, etc). These subspecies would be perfectly adapted for their own environment. The eugenics movement failed when humans tried to manipulate their own evolution, and was ultimately discredited. With these parameters, would this method be able to allow this to work for elves?