Viviparity
[4] These modes are distinguished from viviparity, which covers all the modes that result in live birth: At least some transport of nutrients from mother to embryo appears to be common to all viviparous species, but those with fully developed placentas such as found in the Theria, some skinks, and some fish can rely on the placenta for transfer of all necessary nutrients to the offspring and for removal of all the metabolic wastes as well once it has been fully established during the early phases of a pregnancy.
In such species, there is direct, intimate contact between maternal and embryonic tissue, though there also is a placental barrier to control or prevent uncontrolled exchange and the transfer of pathogens.
The embryo then produces invasive chorionic tissues that grow between the cells of the uterine lining till they can absorb nutrients from maternal blood vessels.
The next evolutionary development would be incipient matrotrophy, in which yolk supplies are gradually reduced and are supplemented with nutrients from the mother's reproductive tract.
However, its numerous evolutionary origins imply that in some scenarios there must be worthwhile benefits to viviparous modes of reproduction; selective pressures have led to its convergent evolution more than 150 times among the vertebrates alone.
This offers powerful options for dealing with excessive changes in climate or when migration events expose populations to unfavourable temperatures or humidities.
The idea that the tendency to favour egg-retention selectively under cooler conditions arises from the thermoregulatory benefits, and that it consequently promotes the evolution of viviparity as an adaptation, is known as "the cold climate hypothesis".
[14] Through ancestral state reconstruction, scientists have shown that the evolution of viviparity to oviparity may have occurred a maximum of eight times in the genus Gerrhonotus of anguid lizards.
[17] Some of these modifications would be the redevelopment of uterine glands to synthesize and secrete shell fibers, the restoration of the careful timing of oviposition due to eggshell thickness, etc.
