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.

Interesting stuff!

17 comments:

Andrew Scott said...

And a weird looking little critter too (although it may be thinking the big critter holding it looks exceedingly weird). It's not going to be much of a boxer with those little arms, but its descendants might be in a billion years or so.

susan said...

I thought that was a newt but I'm not sure I've ever seen one. Hey, live birth, little hands.. given some time they may be driving Newt-mobiles and playing the market.

jams o donnell said...

Well it does lack a bit in reach, Andrew, but it has heart and no glass jaw... Also it has a greater intellect of an EDL member so it's well on its way!

I wonder if they will herd us Susan!

CherryPie said...

Fascinating.

jams o donnell said...

It really is eh?

Liz said...

Amazing.

jams o donnell said...

It really is Liz!

Sean Jeating said...

What came first? The egg or live birth.
'None', not only according to a mastermind that of Irish science.

More serious: Nature is amazing, isn't it, Jams?

jams o donnell said...

Bloody hell Sean! What an utter dick!

Nature is truly amazing

Sean Jeating said...

The Irish are living once again in interesting times, hm?
Biffo babbling as if (?) drunk, the Lenihans of finance and science, Ha(i)rney for health, etc. etc. etc.
It's time that Sergeant Pluck withdraws a bunch of bicycles from circulation.

jams o donnell said...

Perhaps De Selby should release the DMP!

Sean Jeating said...

Haha. That would be an august September for Augustinus.

jams o donnell said...

Perhaps James Joyce cold write it up for the CTS!

Sean Jeating said...

CTS = Catholic Truth Society?

If so: ha ha ha ...

jams o donnell said...

It is indeed Sean. Shame that the Dalkey Archive was relatively poor stuff. It had some wonderful ideas, even if some of them were rehashes of the Third Policeman

SnoopyTheGoon said...

I submit, Jams, that an Aussie critter cannot serve as proof of anything.

Next they will produce a kangaroo that lays eggs. So what?

jams o donnell said...

Haha true, Aussie fauna is a bit of a headfuck!