Evolution in Action: Lizard Moving From Eggs to Live Birth

By Brian Handwerk, National Geographic
September 1, 2010
http://news.nationalgeographic.com/news/2010/09/100901-science-animals-evolution-australia-lizard-skink-live-birth-eggs/

Summary
This article provides an interesting insight into evolutionary changes in organisms as a consequence of adaptation. The Australian yellow-bellied, three-toed skink has provided the scientific community with a demonstration of how it adapts and evolves. The skinks that live in the warmer coastal lowlands of New South Wales lay eggs as their method of reproduction. However, majority of the same species of skinks that live in the cooler higher mountains, have abandoned egg-laying for live birth. Evolutionary records show that nearly a hundred reptile lineages have made the switch from laying eggs to giving live birth in the past but now only 20 percent of reptiles, specifically snakes and lizards, give birth to live young only. This a rare opportunity for scientists to study evolutionary changes in action and observe the skinks' transition of birthing methods. This can be done by studying the differences among populations that are in the different stages of this process and seeing the transition that follows it. A question that is raised in this evolutionary process is how the babies get nourishment before birth. In mammals, the fetus get their nourishment from the specialized placenta which is connected to the uterus so that they can take up oxygen and nutrients from the mother's blood. For eggs, the embryo gets nourishment from the yolk and calcium is absorbed from the shell. Organisms that use this mix of birthing styles, form the eggs but keep them inside the body until the last stages of the embryonic development. For this method, the shell formed is thin so that the embryo can breathe and it then reduces to a thin membrane in later stages. However, the shells are so thin that it is not an adequate source of calcium. These embryos get the calcium from the uterus.


Connection
This connects to climate change because laid eggs are more vulnerable to extreme weather and so skinks in cooler mountain climates find it easier to protect its young by keeping it inside their body for a longer time than the skinks that live in warmer climates. Another connection it makes to evolution is that we can observe how easy it is for skinks to switch from one birthing method to another because of the affect of climate change and possibly even predators. What makes this reptile's adaptation to giving live birth so successful is that in order to prevent the embryo from resulting in calcium defeciency, the skink's uterus secretes calcium that forms into the embryo. Scientists refer to this, as the early stages of the evolution of a placenta in reptiles.