Evolution has been caught in the act: lizard moving from egg to live birth

http://news.nationalgeographic.com/...-skink-live-birth-eggs

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Evolution has been caught in the act, according to scientists who are decoding how a species of Australian lizard is abandoning egg-laying in favor of live birth.

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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.

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Only two other modern reptilesanother skink species and a European lizarduse both types of reproduction. (Related: "Virgin Birth Expected at ChristmasBy Komodo Dragon.")

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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.

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(See "Oldest Live-Birth Fossil Found; Fish Had Umbilical Cord.")

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But modern reptiles that have live young provide only a single snapshot on a long evolutionary time line, said study co-author James Stewart, a biologist at East Tennessee State University. The dual behavior of the yellow-bellied three-toed skink therefore offers scientists a rare opportunity.

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"By studying differences among populations that are in different stages of this process, you can begin to put together what looks like the transition from one [birth style] to the other."
Eggs-to-Baby Switch Creates Nutrient Problem

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One of the mysteries of how reptiles switch from eggs to live babies is how the young get their nourishment before birth.

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In mammals a highly specialized placenta connects the fetus to the uterus wall, allowing the baby to take up oxygen and nutrients from the mother's blood and pass back waste. (See related pictures of "extreme" animals in the womb.)

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In egg-laying species, the embryo gets nourishment from the yolk, but calcium absorbed from the porous shell is also an important nutrient source.

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Some fish and reptiles, meanwhile, use a mix of both birthing styles. The mother forms eggs, but then retains them inside her body until the very last stages of embryonic development. (Related: "Dinosaur Eggs Discovered Inside MotherA First.")

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The shells of these eggs thin dramatically so that the embryos can breathe, until live babies are born covered with only thin membranesall that remains of the shells.

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This adaptation presents a potential nourishment problem: A thinner shell has less calcium, which could cause deficiencies for the young reptiles.

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Stewart and colleagues, who have studied skinks for years, decided to look for clues to the nutrient problem in the structure and chemistry of the yellow-bellied three-toed skink's uterus.

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"Now we can see that the uterus secretes calcium that becomes incorporated into the embryoit's basically the early stages of the evolution of a placenta in reptiles," Stewart explained.
Evolutionary Transition Surprisingly Simple

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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.

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(Related: "Human Sperm Gene Traced to Dawn of Animal Evolution.")

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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.

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In general, the results suggest the move from egg-laying to live birth in reptiles is fairly commonat least in historic termsbecause it's relatively easy to make the switch, Stewart said.

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"We tend to think of this as a very complex transition," he said, "but it's looking like it might be much simpler in some cases than we thought."

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Actually, I disagree with this.Suspend your reality for a moment and try to imagine this. Suppose I'm in the bank and I'm very sloooooowwwwllllllyyyyyy robbing the bank. Starting at the door, I am moving veeeerrrrrrryyyyyy slloooooooowwwwwwlllllllllllyyyyyyyy toward the clerk. By slow, I mean I'm moving 1 inch every 60 minutes. It's all one continuous motion, but I'm just moving very very very slowly. Think of me as 1000 times slower than a snail.After how long will people start to think "hey, this guy is here to rob a bank.... very slowly."?The problem with noticing evolution is that it just moves too slowly by our standard. People are impatient. If a dog didn't give birth to a cat, then they can't understand it. People have a lot of trobule understanding gradualism.