Evolution in Action

National Geographic News has a story that shows exactly what evolution is capable of accomplishing. The story, by Brian Handwerk, is about the Yellow-Bellied Three-Toed Skink. He writes:

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.

Along the warm coastal lowlands of New South Wales, 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—another skink species and a European lizard—use both types of reproduction.

This has shown that the transition from eggs to live young happens much more quickly than had originally been thought and presents no barriers for evolutionary theory.

Over at Uncommon Descent, however, Denyse O'Leary wonders what all the hubbub is about. Of this "supposed live birth" she writes:

I don’t get how this shows that live birth is evolving.

Animals that produce their young in eggs have way different systems from those that produce them via placentas, as do mammals.

With eggs, live birth is just a technicality.

Some snakes lay eggs, and others give live birth.

In other words, the eggs hatch indoors instead of outdoors.

Maybe some snakes or skinks are in between, but so?

To me, a more significant interest is the reptiles that protect their eggs or young, like crocodiles, alligators and cobras. that suggests that the theory of the unfeeling reptilian brain is not true.

It is hard to respond to this in a cogent fashion since it seems to have been written without much thought in the first place. If one species can be caught in the act of switching from one birthing process to another, they plainly are not "way different systems." Egg-laying is the primitive way of having offspring. This is a means of reproduction used by amphibians and the vast majority of reptiles. As you move up the evolutionary ladder, placental development becomes more and more common to the point where the only mammals to lay eggs are the monotremes, which are universally understood to be the most primitive mammals in existence.

How is egg-laying selectively advantageous? If you live in a colder climate, it is harder to keep eggs warm and mobility is restricted: you have to create a nest and then stay with it once the eggs are laid. If your fetuses are within you, you can move around and escape predation. When you go look for food, your children-to-be go with you. As the article notes, the evidence from the skink presents an evolutionary snapshot. Over time, given the changing environment, the live-bearing skinks will likely out-compete the egg-laying ones.

----------------
Now playing: Steve Hackett - Walking Away from Rainbows
via FoxyTunes