Genetic chimerism is a condition in which a single organism is composed of cells with more than one distinct genotype. Elves inhabit this world, but do not possess magic in this reality. Instead, they are all what can be genetically considered chimeras because they have a strange way of reproducing. When partners decide to conceive, a male must add his genetic material at different intervals throughout several years to the female. Genetic material could come from a single male or various ones with no connection to each other. Through this method, 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 led to a lively "trade" in customized child production. Families and clans select and breed for distinctive traits that they consider valuable in political and economic agreements. However, this can be a danger to the Elven race. With clans selecting traits they themselves see as useful, this can lead to bottlenecks and a loss of genes within the species. It would also lead to specialization, which could be detrimental to their effectiveness and survival if their environment changes.
What mechanism can I include to prevent this from happening?