Darwinian Theory on a Molecular Level
JENNIFER LUDDEN, host:
This is ALL THINGS CONSIDERED from NPR News. I'm Jennifer Ludden.
For the past academic year, we've been reporting on debates raging in school boards across the country on whether and how to teach the theory of evolution. It's been nearly 150 years since Darwin first published on "The Origin of Species." Since then, biologists have been observing the process of natural selection that Darwin laid out on an ever smaller scale.
To find out how modern-day molecular biology and genetics have shed light on the theory of evolution, we turn to Lynn Helena Caporale. She's a biochemist at Columbia University and the author of "Darwin in the Genome: Molecular Strategies in Biological Evolution."
Hello, Dr. Caporale.
Dr. LYNN HELENA CAPORALE (Columbia University; Author, "Darwin in the Genome"): Hello.
LUDDEN: Darwin never knew there was such a thing as a gene. So when he spoke of evolution by natural selection, what did he mean?
Dr. CAPORALE: Darwin meant that--he used the word natural to say that nature does what farmers were doing at the time, which is that they were breeding cows to make more milk, that were better milk producers, for example, and other--pigeon fanciers were breeding pigeons with strange characteristics. So they were selecting from the variation among the population of, say, cows the best milk producers to breed. And so Darwin suggested that that happened in nature, too.
LUDDEN: Today, we know so much more about why different species are varied and different. We've mapped the human genome, as well as that of other species. Has the information that's come out in this past century and a half, has it re-enforced Darwin?
Dr. CAPORALE: Yes, I think it actually gives us a much deeper appreciation for what--for the power of natural selection. I feel that Darwin was really very perceptive. When people think of Darwin, they often think of apes or something, but actually thought of a process by which there would be variation. This variation could be inherited, although he didn't know how, but he knew, for example, that parents and children resemble each other somewhat and that there would be selection and that would cycle back and forth. And so that was a process.
And since then we've learned that there are genes. He didn't know that there were genes. He didn't know that there were mutations. He didn't know how this process of variation worked, but he laid out a scenario which we can now reinterpret in terms of DNA and, in fact, looking at the genome.
LUDDEN: So take us back to science class for a minute. What happens in a cell when there's a mutation?
Dr. CAPORALE: Well, the DNA is made up of A's, T's, G's and C's--these letters which actually are chemicals--short for chemical structure. And our own DNA is three billion of these in our genome. And so an A might change to a G, for example.
LUDDEN: How do they change? What determines if this pattern changes?
Dr. CAPORALE: There's a few ways they can change. One this is the DNA can be hit with ultraviolet light, for example, or other kinds of damage. But it's not always a bad thing. Our own immune system during our own lifetime has a very impressive infrastructure for generating variation of several different kinds, patching things together, changing specific pieces of the DNA, to generate variation so that it can attempt to bind, to attack any pathogen, even--we can generate far more antibodies, distinct antibodies than we actually have genes in our DNA through this very efficient variation generating mechanism.
LUDDEN: You talk about snails, a certain kind of snail.
Dr. CAPORALE: Yes. The snails--actually, the beautiful cone snails, and if you go to the Pacific beaches and pick them up, be very careful because they are full of toxins and that enables snails to, for example, kill fish. And this little region of the snail genome that's involved in generating these toxins has an unusually high mutation rate.
LUDDEN: I mean, from a survival of the fittest perspective, what's happening with that cone snail as its toxins mutate?
Dr. CAPORALE: Well, it's making it able to explore the possibility of having new prey. So in evolution, of course, we're living in a context. The bacteria and us are trying to deal with each other; the cone snails and the prey. So if there were just one toxin, probably some prey had a mutation that allowed it to not be attacked by that toxin and then--and the cone snails wouldn't have any--their trick wouldn't work. You know, they wouldn't...
LUDDEN: The cone snail would go hungry.
Dr. CAPORALE: Right. Exactly. So by generating all these toxins, they're exploring new potential forms of prey.
LUDDEN: Can you give me a sense of the time involved? I mean, over what kind of time can scientists observe mutations?
Dr. CAPORALE: Very quickly. I mean, mutations can happen from generation to generation. They...
LUDDEN: So quickly is like 30 years or...
Dr. CAPORALE: Well, yeah.
LUDDEN: Are you talking minutes, weeks, years?
Dr. CAPORALE: Well, there's different kinds of mutations. I mean, if you're talking about a mutation that's been selected, that's another story. I mean, a mutation is really a very quick chemical event and what happens after that. So, for example, our own immune system is constantly generating mutations. Within, you know, milliseconds or seconds, the variable regions of our antibodies, the parts that bind the pathogens in it, if you're looking in a cell that's at the stage of its development when it's generating new antibodies, that's got--it's just generating them very rapidly, generating mutations.
LUDDEN: You say some mutations can happen in milliseconds. But what kind of a time frame would there be, for example, for, you know, a species to evolve into another species?
Dr. CAPORALE: That's a very challenging question. There's--there you have to go to the fossil records for history, but--and there's three million years or, you know, millions of years between species. But that's really something that we can't say yet at a molecular level what was happening then.
LUDDEN: You write about how looking at the--what we know of the genome today, we can see connections between different organisms. Tell me about that.
Dr. CAPORALE: Oh, when you look at genomes, for example, the genome of any two people--we're always focused on the differences between us--is more than 99.97 percent the same. So there's only some tiny, little fraction of a percent difference in our genome. If we look at chimpanzees, we see just a couple of percent difference in our genome. And you can say, `Well, gee, I'm really different from a chimpanzee.' But if you think of from the point of view of a fertilized egg, just one egg that has to get from being a single egg to being either a chimpanzee or us, most of what it has to do is the same. And you can go way back like that.
For example, myosis, this complicated, double reduction cell division we all learn about in high school biology, we need all the enzymes that control that and so on and so does yeast, which undergoes myosis. So we share a lot of biochemistry and sequences in our genome even with yeast.
LUDDEN: So are you saying that modern science has discovered even perhaps greater links between humans and animals, if not plants, than Darwin imagined?
Dr. CAPORALE: Yes. I think that one of the most beautiful things that I found by looking at DNA sequences is the tremendous sense of connection with all life on Earth.
LUDDEN: Dr. Lynn Helena Caporale is the author of "Darwin in the Genome: Molecular Strategies in Biological Evolution." She's a biochemist at Columbia University.
Thanks so much.
Dr. CAPORALE: Thank you.
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