Evolutionary constraints hypothesis of maintenance of sexuality
The existence of evolutionary constraints could be the cause of the preservation of sexual reproduction. Although it could provide a sort of evolutionary advantage for the organism, the transition from sexual reproduction to asexual reproduction in such a complicated organism as a mammal or bird would require such fundamental and extensive changes in the physiology and anatomy that the probability of their occurrence is negligibly small. A less drastic variant of this hypothesis points out the fact that, while the transition to parthenogenesis is, in principle, possible, this would be such a drastic intervention in the physiology of the organism that the parthenogenetic individuals would necessarily have substantially reduced viability and fertility compared to the other members of the population for a great many generations (Uyenoyama 1984).
The existence of a great many parthenogenetically reproducing species in such complicated organisms as reptiles, amphibians or fish, however, indicates that evolutionary constraints will probably not be the main reason for preservation of sexuality in the vast majority of species.