Tuesday, October 14, 2008

Evolution 101 done right

Since I believe that sniping from the sidelines is bad form, here's my take on how Evolution 101 ought to be presented:

The theory of evolution is a scientific explanation of how the diversity of life on this planet arose. In a nutshell, that explanation consists of four crucial elements:

1. Parents pass genetic information to their children. That genetic information determines, to some extent, their physical characteristics.

2. Some of that genetic information is randomly changed in each generation.

3. Some of those random changes make the children who have them more likely to reproduce than others

4. Over very large periods of times (millions of years) the cumulative effect of all those random changes and reproductive selection can account for all the life on earth.

In science-speak, the genetic information passed from parent to child is called a genotype. The physical characteristics that a genotype produces is called a phenotype. Random changes in genetic information are called mutations. And the fact that some phenotypes reproduce better than others is called selection. Most evolution is due to natural selection, but some of it, especially in modern times, has been due to artificial selection.

Evolution is often criticized on the grounds that a "random" process can't possibly be responsible for the incredibly rich and complex variety of life that we observe. It would be like flipping a coin and having "heads" come up a billion times in a row. Such criticism misses the important point that while mutation is indeed random, selection is not. Also, the fourth element -- operating over long periods of time -- is crucial.

Evolution makes many predictions. Every one that has ever been put to the test has confirmed the predictions of evolution. This includes tens of thousands of experiments that have been conducted over the years, and not just in biology. Chemistry, geology, anthropology and even basic physics all provide support for the theory of evolution. The theory has been modified a little over the years as data has shown that some of Darwin's original ideas were not quite right. But the basic framework of evolution described above has so far withstood every experimental test it has ever been put to.

We now understand the mechanisms that drive evolution in rather excruciating detail. Some of these details are quite familiar. For example, we now know that the genotype is encoded in a molecule called deoxyribonucleic acid, more commonly known as DNA. We know the exact structure of this molecule, how genetic information is encoded, and how that information is transcribed into proteins, which form the basic building blocks of life. We know the familial relationships of nearly all species on earth. We understand many, though not all, of the complex and often surprising ways that phenotypes interact with their environment to produce reproductive fitness. We know how many of the complex structures of living organisms evolved, including the human eye.

The weight of the evidence for evolution is so staggering, so overwhelming, that I can only touch on a few highlights here. I'll focus on things that, in the main, a reader can independently verify if they choose to.

1. The geologic column. As you dig into the earth (or have nature do it for you you find that the earth has layers. Lots and lots of layers. And what you find if you start to pay attention to the structure of these layers is that they tend to be consistent all over the planet. In general, the closer you are to the surface of the earth, the younger the things you find. In the top layers you'll find things that were put there very recently. Very near the surface you will find plastic water bottles and old iPods. A little deeper you will find ancient pottery. As you go deeper you find fewer and fewer iPods, fewer and fewer pottery shards, and fewer and fewer bones that look like they came from animals that are alive today. You don't have to go very deep before you get to layers that have no artifacts at all, but do have bones from creatures that no longer exist like mammoths and giant sloths.

As you go even deeper you eventually get to a layer called the K-T boundary. The K-T boundary is a very distinctive layer. It exists nearly everywhere on earth if you dig deeply enough. It is distinguished by a very high concentration of the element iridium, which is how it can be unambiguously identified.

As you keep digging below the K-T boundary you find an even more remarkable thing: dinosaurs. You don't have to go very far. Almost immediately below the K-T boundary you will start to find dinosaur fossils, and you will keep finding them as you keep digging. As you go deeper the kinds of dinosaur fossils you find gradually change. They get smaller and smaller, they change shape, and they eventually just kind of peter out.

All of this is consistent with species gradually evolving over time, and their bones being fossilized in layers with newer layers sitting atop older ones. The K-T boundary is almost certainly the result of a giant meteor impact in the Yucatan peninsula 165 million years ago. The remains of the impact crater were discovered quite recently.

The K-T boundary is one of evolution's smoking guns. It's a sharp demarkation line in the history of the world as recorded in the geologic column. No dinosaur fossil has ever been found above the K-T boundary, and no hominid fossil has ever been found below it (or even anywhere near it). If you ever find either of these things (and can verify that it's not a hoax) you will surely secure a prominent place in scientific history.

2. The structure of DNA. We now understand DNA in astonishing detail. We know how it encodes genetic information. We know how it makes copies of itself. We have countless examples of mutations that provide increased reproductive fitness (i.e. beneficial mutations). Even in humans we have at least two such examples: lactose tolerance, which allows people to digest milk and enabled them to survive in the colder climates of northern europe, and the sickle-cell mutation, which provides a defense against malaria. We understand DNA so well that we can even engineer it directly for our own ends.

The structure of DNA also provides another bit of "smoking gun" evidence for evolution. Because we are able to sequence DNA, we know that we share about 98% of our DNA sequence with chimpanzees. And yet, humans have 23 pairs of chromosomes while chimpanzees (and indeed all the great apes) have 24. This would seem to be evidence that we are not related. However, it turns out that one of our chromosomes is the result of taking two ape chromosomes and sticking them together end-to-end. The evidence for this is that the ends of our chromosomes have a unique DNA sequence called a telomere. This telomere exists only at the ends of chromosomes -- and in one other place: the middle of one human chromosome, that just happens to have the exact same basic structure as two ape chromosomes stuck end-to-end. (It also has the remnants of an extra centromere.) If humans were produced by an intelligent designer, he took great pains to make it appear on very close examination that we are related to chimps.

3. The age of the earth. I've written about this before so I won't belabor it here.

Note that evolution says nothing at all about how life actually first arose, except that it almost certainly happened only once (which is to say, we are all descended from a single common ancestor). We don't yet know how life was actually created. All we know is that once it was created, no supernatural processes are needed to explain how life became so rich and diverse. The same basic process that created mastiffs and chihuahuas from wolves also created us -- and every other living thing -- from a common ancestor, probably a blue-green algae, about four billion years ago.

3 comments:

ineluki said...

Very well put. Even if you hadn't gone to the trouble to write this, others have, and there's a lot of places where evolution is well explained, without sacrificing rigor.

I can't understand why a leading University is not among them. It's not like they don't care - after all, they HAVE put an Evolution 101 page. And I don't think they can't do better. I'm sure they can. So why?

At the beginning of the creationist debate, one of the problems was the perceived lack of charisma and glamour of the scientific community. The first debates were easily won for the creationist cause. They were putting on a show. Their spokesmen were, well, showmen. They were focused on selling their point. While the poor scientists on the other side were trying to explain as if they were speaking to other scientists (which they weren't). They didn't want to sell anything, they only cared for the truth.

It's different today. Scientist are much better as presenting their point. They still care for the truth - but now they understand they have to put on a fight so truth isn't drowned by drama. They still are open - but they are ready to show the kind of evidence they need, and why the creationist arguments are bogus.

This page by Berkeley has somehow reminded me of those early days. Well meaning, but naïve.

Don Geddis said...

Good summary, Ron.

One little point at the end: evolution says nothing at all about how life actually first arose.

Well, yes, evolution doesn't address this, but science as a whole does. Abiogenesis is closely related to evolution.

We don't yet know how life was actually created.

I'll grant that abiogenesis is not nearly as well understood as evolution. But your words suggest the connotation that nobody knows anything.

May I recommend this video. I don't mean to suggest that it offers "the" answer. But it shows a significant amount of legitimate research about abiogenesis, and the beginnings of some answers.

(The science starts around 2m40s. The early part is all anti-creationism -- you may enjoy that too! But that isn't what I was referring to.)

Ron said...

> Good summary, Ron.

Thanks!

> But your words suggest the connotation that nobody knows anything.

Yes, that was quite deliberate (including the "suggest" and "connotation" part).

> May I recommend this video.

Thanks. That gives me an idea for my next blog post.