14 September 2010
Evolution in action?
Last month National Geographic carried an interesting item which may well be an example of evolution in action Scientists are decoding how a species of Australian lizard is abandoning egg-laying in favour of live birth.
Along the warm coastal lowlands of New South Wales (map), the yellow-bellied three-toed skink lays eggs to reproduce. But individuals of the same species living in the state's higher, colder mountains are almost all giving birth to live young. Only two other modern reptiles use both types of reproduction.
Evolutionary records shows that nearly a hundred reptile lineages have independently made the transition from egg-laying to live birth in the past, and today about 20 percent of all living snakes and lizards give birth to live young only.
One of the mysteries of how reptiles switch from eggs to live babies is how the young get their nourishment before birth. In mammals a highly specialized placenta connects the foetus to the ovary wall, allowing the baby to take up oxygen and nutrients from the mother's blood and pass back waste.
In egg-laying species, the embryo gets nourishment from the yolk. The mother forms eggs, but then retains them inside her body until the very last stages of embryonic development. The shells of these eggs thin dramatically so that the embryos can breathe, until live babies are born covered with only thin membranes—all that remains of the shells.
Both birthing styles come with evolutionary tradeoffs: Eggs are more vulnerable to external threats, such as extreme weather and predators, but internal fetuses can be more taxing for the mother. For the skinks, moms in balmier climates may opt to conserve their own bodies' resources by depositing eggs on the ground for the final week or so of development. Moms in harsh mountain climates, by contrast, might find that it's more efficient to protect their young by keeping them longer inside their bodies.