Evolution in progress

03november2010
Source: nationalgeographic.com (text and photo)
An Australian lizard species seems to be in the middle of the evolutionary transition from egg-laying to live birth. The population that lives on the coast still lays eggs, while another population in the mountains give birth to live young.
The species at hand is the yellow-bellied three toed skink, a skink with a yellow belly and three toes (Saiphos equalis) that lives in the state New South Wales (Australia). The skinks in the warmer coastal area lay eggs, while those in the cooler, higher, mountain areas give birth to live young. There have been nearly a hundred reptile lineages in the past that – independently – made the transition from egg-laying to live birth. Now about 20 percent of snakes and lizards reproduce by live birth.

The Saiphos equalis offers scientists a chance to see the evolutionary process in action. One of the questions around the reptiles’ switch to live birth is how the young obtain their nourishment. In mammals the baby receives oxygen and nutrients via the placenta. Animals that develop in an egg obtain their nutrients from the yolk and the shell is an important source of calcium. In ovoviviparous animals the young develop in an egg inside the mother’s body. The egg’s shell is usually very thin so that the embryos can breathe through it. The question is then how the young receive enough calcium. Scientists now discovered that in the Saiphos equalis the uterus wall secrets calcium, which is then absorbed by the embryo. This can be called an early stage of the development of a placenta in reptiles.

Both birth styles have their tradeoffs: eggs run the risk of being taken by predators and they can suffer from bad weather conditions, but a pregnancy on the other hand can take a toll on the mother.
The scientists note that while the transition form egg-laying to live birth seems like a very complicated process, this does not appear to be the case. Historically, the transition is quite common in reptiles. This research was published in the Journal of Morphology.