The field of evo-devo has started showing that big evolutionary changes can happen with only limited genetic changes. Changes in genes that control the expression of other genes can have dramatic effects on an organism. A small change in the expression of one protein can create a much larger and different beak in a bird, for example. So evolution does not require invention of entirely new genes -- just tinkering with expression of the ones already in existence.
Question: Is there any indication that these control genes might be unusually susceptible to mutation? Or maybe there are lots more versions of these control genes, creating more places for selection to act on? If so, this would help explain why evolution sometimes occurs very rapidly.
-A curious adult from England
November 30, 2007
What an interesting question! Sometimes a small change can lead to a big difference. This happens when the small change causes lots of other changes throughout the organism. Changes in control genes are one way to get a lot of bang for your buck.
Genes contain the instructions for making an organism. And as you might guess, control genes, well, control other genes. Often they control lots of other genes.
What this means is that when one control gene is changed, lots of genes actually get changed all at once. Because a single DNA change alters how this whole set of genes works.
There isn't any evidence that control genes build up small changes called "mutations" any more quickly than other regions of DNA. And they don't have to. The changes in these genes are significant enough that even an occasional change can have profound effects.
We should be clear though, that evolution can sometimes create new genes. It does so through a process where a gene is duplicated and the extra gene builds up mutations. But the extra gene doesn't mutate any faster; mutations are just better tolerated there. The end result is a gene so different that it is very nearly new.
That was the quick answer. Let's go over all of this in a bit more detail to better understand what scientists think is going on.
As I said, small changes happen occasionally in DNA over time. These mutations can happen in people, animals, and plants. When you add up lots of mutations, you can get some pretty big differences. That's how we see differences between humans and chimpanzees, in Chihuahuas and Great Danes, and even in the size of bird beaks!
Mutations can happen anywhere, but they are relatively rare. If a mutation pops up in a gene, sometimes it can tinker with the way the gene originally worked. This tinkering process could make the original gene different enough to become an entirely new gene.
Sometimes mutations get passed down from one generation to the next or from a parent to a child. Many, many generations down the line, more mutations can happen and get passed down too.
This is a big part of how evolution works. Evolution is the change in traits of organisms, or living things, over time. It occurs so slowly that it can take hundreds or thousands or even millions of years to really see a difference. Eventually, all these differences make one organism very different from another and it becomes a new species.
Sometimes though, a single change can have lots of effects all at once. How? If the mutation affects a gene that affects lots of other genes. Let's delve a bit deeper into genes to see how this might work.
Genes and Control Genes
Genes have the instructions to make a protein. And proteins do all of the work in a cell. They help us breathe, see, think, develop, etc.
Changes in genes can have all sorts of effects at the protein level. Sometimes a mutation makes a gene stop working so it can't make a protein. Sometimes the changes tell a gene to make more or less protein. Sometimes a mutation makes a gene that has instructions for making a protein that works differently. Sometimes a change in a gene does absolutely nothing that is different in the end.
Proteins interact with each other just like people do! Some proteins may be friends with other proteins and hang out with each other. Some proteins might have only a few friends while others are popular and have many friends.
Some genes make proteins that might even be like the leader of a club. The leader might have many protein friends to work together on a project. If the leader of the club changes, even just a little bit, it could change the way the whole club works.
The leaders of a club are like the "control genes" we've been talking about. A control gene is one gene or a small number of genes that can control or tell the other genes what to do. They can even tell others when and where to do something.
Changing the Beak
Let's look at a specific change that has happened in one of these control genes. We'll focus on the beaks of the finches that Darwin studied in the Galapagos Islands. The different shapes and sizes of these beaks are caused by a DNA change in a single gene.
Last year some scientists discovered that there is a change in one control gene called "calmodulin." This change made calmodulin work a lot more than before and then that affected other genes by making them work more too.
Guess what? This created big differences in the size and shape of the beaks (see image below). More calmodulin actually made the beaks become long and pointed which is perfect for finches that eat cactus.
This is very different from the ground finches that crack and eat seeds with their deep, wide beaks. It's also very different from some of the tree finches that probe for insects with their short, pointed beaks. Calmodulin in ground finches doesn't work as hard as in cactus finches.
So a single DNA change has a huge effect on how the beak is formed. In other words, to change a cactus finch into a ground finch only takes changes in a single gene.
Of course, single changes don't happen that often. But if a change leads to an advantage, then you can get a finch with the change replacing the ones without it.
In evolution terms, we would say this happened pretty rapidly. But, it really took a few million years to create all the different types of beaks in finches. This is rapid if life formed on Earth about 3 or 4 billion years ago!
But, even when you see the big differences, there is no evidence that mutations happen in control genes more than in other genes. There also are not more versions of control genes than other genes for selection to act on.
It seems that all we have here are mutations tinkering around a bit on a few control genes. And we end up with some pretty amazing differences!
A Little Deeper: El NiÃ±o and Natural Selection
Of course, once the mutation has happened, natural selection is then free to work on it. Under some conditions, a mutation might be very useful. In others it wouldn't and the mutation might disappear.
Let's look at the Galapagos Islands and see how changing weather can select for different beak sizes when the calmodulin mutation is already in the population. The Galapagos go through periods when there is a lot of rain (called El NiÃ±o years) and periods of little or no rain (called La NiÃ±a years). Different weather patterns favor different kinds of beaks.
Dry weather gives hard dry seeds that are hard to crack open. This favors thick, strong beaks. Rainy weather gives lots of little soft seeds that are easy to crack open. This favors little beaks.
Amazingly, changing weather can result in different populations of bird beaks in just a few years. For example, in 1977 no rain fell so there were only hard-to-open seeds available. Most birds couldn't crack open these hard seeds, so about 80% of the finches died. But the birds with the bigger beaks survived and dominated the population. For a few years anyway.
In 1983, there was a lot of rain. And lots of small, soft seeds that birds with little beaks could take advantage of. And they did. This time, the small-beaked birds were better at eating small seeds and survived much better than the other birds.
If it had been dry for a long period in the 1800's, then Darwin would have found more birds with bigger beaks. If it had been wet, he'd have found more birds with smaller beaks.
It is important to note that these beak changes are small -- less than a millimeter. But processes like these can change a species over a long period of time. Add to this new mutations that can occasionally pop up and you can begin to appreciate how, over an extended time, you could end up with the thirteen different finches that are on the islands today.