The exact causes for the evolution of natal philopatry are unknown. Two major hypotheses have been proposed. Shields (1982) suggested that philopatry was a way of ensuring inbreeding, in a hypothesis known as the optimal-inbreeding hypothesis. He argued that, since philopatry leads to the concentration of related individuals in their birth areas, and thus reduced genetic diversity, there must be some advantage to inbreeding – otherwise the process would have been evolutionary detrimental and would not be so prevalent. The major beneficial outcome under this hypothesis is the protection of a local gene complex that is finely adapted to the local environment. Another proposed benefit is the reduction of the cost of meiosis and recombination events. Under this hypothesis, non-philopatric individuals would be maladapted and over multi-generational time, philopatry within a species could become fixed. Evidence for the optimal-inbreeding hypothesis is found in outbreeding depression. Outbreeding depression involves reduced fitness as a result of random mating, which occurs due to the breakdown of coadapted gene complexes by combining allele that do not cross well with those from a different subpopulation. However, it is important to note that outbreeding depression becomes more detrimental the longer (temporally) that subpopulations have been separated, and that this does hypothesis does not provide an initial mechanism for the evolution of natal philopatry.