Evolution in action

(photo by Rebecca A. Pyles)

Of course, evolution is always "in action." But to borrow a phrase, it's normally like a snapshot in time. Still, there are snapshots and there are snapshots. So I thought it was interesting to read two separate articles this morning where evolution is not just ongoing, but obvious.

The first is this fascinating story in the National Geographic about a species of Australian lizard where the evolution of live births, from egg-laying, has been "caught in the act." (Incidentally, the photo above shows pretty clearly the family relationship between lizards and snakes, wouldn't you say?)

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.

This would seem to be a pretty major difference, wouldn't you think?

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 fetus to the ovary 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.)

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

I'll let you read the article if you want to know how this problem is resolved. (Incidentally, you might want to click on that link to see four "extreme" animals in the womb. Neat stuff!)

But the other obvious question is... why?

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.

Keep in mind that these reptiles aren't making a conscious - or even an unconscious - decision about any of this. There's no planning involved. It's just that skinks in the warm coastal lowlands do better - leave more descendants - when they lay eggs, while those in harsher climates do better giving birth to live young.

This sort of change requires a mutation first, perhaps more than one. (But this change, in reptiles at least, is not as drastic as it might seem at first. Although most reptiles lay eggs, live birth is also pretty common. According to this article, about 20% of reptile species give birth to live young only. And if more than one mutation is required, they don't have to occur at the same time. A prior mutation can already be present in the population, for some other benefit, or perhaps at low level from mere chance.)

But if there's an advantage in live birth, those individuals with the mutation will tend to leave more offspring. In effect, they will out-compete egg-laying skinks, at least in particular environments. These two populations of yellow-bellied three-toed skinks might well be in the process of splitting into separate species.

Another article, at the BBC, describes how orcas - killer whales - are also evolving into two separate populations.

Researchers have discovered that two distinct types of orca, a large and a pygmy form, are rapidly diverging, evolving away from each other.

The scientists' study reveals each type of orca carries a unique gene mutation that benefits its particular lifestyle.

There's a size difference between these two populations, and there's also a difference in food preferences, with the larger orcas feeding mostly on seals and the dwarf orcas feeding mostly on fish. But most notably, each has a specific genetic mutation that appears to govern energy production and metabolism.

Each population has the gene complex that's most suited for its lifestyle. I don't know if there's still interbreeding between them, but given this difference, crosses between the two types would have a poorer success rate (at survival or leaving descendants), which would encourage further separation. Eventually, assuming no other changes, we'd expect to see two entirely separate species (which would continue to follow their own evolutionary paths and might well become quite different indeed).

Neat stuff, isn't it? Of course, if you're a birder, you'll know that spotting evolution in action isn't at all uncommon. Every time a new field guide to the birds is published, it seems like there are a different number of species - sometimes more, sometimes fewer. No, biologists aren't discovering new species this quickly, nor are that many going extinct (not quite this quickly, at least).

What's happening is that it's not always easy to tell when birds are a single species or two. It's not a clear dividing line, and there are many populations which are right on the edge. Well, "species" is a human concept. We like to categorize things. But it's not always so clear in nature.

Many species - of birds, at least (I'm no biologist) - seem to be right on the verge of splitting into two species. Some taxonomists might consider a population one species, maybe just different subspecies, while others argue that they are two separate species. And the consensus tends to change, depending on the current evidence.

Sometimes, birds that are nearly impossible to distinguish in the field (by sight, at least) are fully accepted as being of two separate species. Others, which are quite different in appearance, may be considered just different forms of the same species. It's very, very clear that species do split and evolve separately. Any birdwatcher should be fully aware of that.

Actually, any educated adult should be aware of that. Even if you didn't graduate from high school, this is just very, very basic stuff. Evolution is the foundation of modern biology. You should have learned that in elementary school, in fact, because it really is... elementary. Yet only a minority of Americans apparently "believe in" evolution. Heh, heh. Insane, isn't it? In biology, at least we are easily the most ignorant of all developed countries.

Here's a chart that should make you embarrassed to be an American. The question was whether "human beings, as we know them, developed from earlier species of animals."

Incredible, isn't it? Especially in the 21st Century, more than 150 years since Darwin first published On the Origin of Species? We rank down with Turkey on this issue. Look at the countries at the top of the list. What does that tell you?

This degree of ignorance is just astonishing. Frankly, it would be astonishing in any nation on Earth, let alone in the world's last remaining superpower. What has happened to America? What has happened to us?