Back in the middle of last month, I had a few things to say about Casey Luskin (DI flak) and his understanding of so-called junk DNA. It’s now the middle of the month again, and Casey is again talking a lot – and understanding very little – about “junk” DNA. Larry Moran has a post up where he tries to educate Casey about the fact that a hell of a lot of DNA is still, at least as far as we know, junk. I’m going to take a look at something a little bit different – one of the methods scientists use to identify areas of “junk” DNA that have important functions. It’s a pretty cool way of doing things, but it’s not one that Casey likes to talk about – because it’s really one of the finest examples of how our understanding of Darwinian evolution has lead to new discoveries about living things.
First, I should probably take a minute or so to talk about what Casey means when he uses the phrase “junk” DNA. In Casey’s world, there are two classes of DNA: coding regions, and “junk”. That because, a long time ago, scientists thought that the only important DNA was the DNA in coding regions (that’s the DNA that’s used to make proteins) and that everything else was unimportant. Someone used the phrase “evolutionary junk” to describe the non-coding regions, and Casey (and other anti-evolutionists) somehow or another decided that the “neo-Darwinian paradigm” demanded that non-coding DNA was junk, and interfered with research that might indicate otherwise.
That’s the story in the world of the Discovery Institute. In Realityworldland, things are just a tad bit different.
To begin with, scientists began to realize that at least some non-coding regions had important regulatory functions fairly early on – without them, the DNA can’t be used to make proteins. In other cases, stretches of DNA seem to function as spacers. In these cases, the DNA sequence doesn’t matter much (if at all). Instead, the important thing is that the spacer ensures that, for example, a regulatory region sits the necessary distance away from the coding region of the gene. Casey lumps all of those types of DNA together with DNA that we don’t know the function of and DNA that we are pretty sure has no function, labels all of it junk, and claims victory for Intelligent Design anytime any of it turns out to do something important. That’s more or less the level of integrity that we’ve come to expect from Casey, of course, but it’s still somewhat disappointing that he can’t muster the strength of character to argue honestly.
But, as Arlo said, that’s not what I came to tell you about.
Came to talk about the junk – and how to tell when it really isn’t.
There are some stretches of DNA that do something important, but we don’t know what. Scientists are working on figuring it out, but right now all we know is that they’re important. We know this because we understand how evolution works, how natural selection works, and can – do – use that understanding to investigate DNA.
We know that mutations happen. DNA does not copy itself perfectly, so segments of DNA tend to change over time within populations, unless natural selection acts to prevent that. When a segment DNA does something that’s very important – so important that the organism can’t reproduce well without it – that segment of DNA doesn’t evolve much. Exactly how much it changes depends on just how important its function is, and on just how much change the segment can tolerate while still working well enough.
We are now able to sequence not just small stretches of DNA, but the entire genome of an organism. It’s not easy yet, and usually involves a lot of scientists working very hard for a long time, but we can – do – do it. We’ve sequenced the entire human genome, the entire chimp genome, a rat, a mouse, chicken, -the list goes on, and keeps growing. Last week, the genome for an anemone was published.
We are able to compare the genomes that are sequenced. This is also a bit challenging, and can require a lot of computer power and a lot of time, but it’s done routinely at this point. We can take newly-sequenced genomes, compare them to other sequenced genomes, and look at the similarities and differences. In some cases, scientists have discovered that some non-coding regions are very, very similar between species that are very different. That can mean only one thing: those non-coding regions must be doing something important.
There is simply no other possibility. Natural selection is as much a force of nature as gravity, and if selection was not acting to “purify” those regions – to keep them unchanged – they would not be so similar between different species. Knowing this, scientists can begin to work to discover the functions for these areas.
In some cases, our understanding of evolutionary relationships can help narrow down the possible functions for the DNA in question. For example, there are regions of DNA with unknown functions that are very similar in mice and men, chimps and chickens, dogs and dogfish – but are not known at all in flies and worms. We don’t know just what they do, but we can safely predict that it’s something that’s important to vertebrates and irrelevant to inverts.
That’s how evolutionary science drives the discovery of functions for some of the so-called “junk” DNA. I’m still waiting to hear what Intelligent Design could add to that. I’ll be waiting for a while, I expect.