A mutation is a permanent change in a gene (or more precisely in the DNA of the gene). We know a lot more about these now than they did back in the 1920s. This shouldn't surprise anyone.
The early geneticists thought all mutations were harmful. They studied these "errors" in genes in the hope that they would help them understand the way genes normally work. Remember that they had no knowledge of what genes were made of, or how they worked chemically. It turns out that they were wrong. Most mutations are silent (cause no real change) or neutral (cause a change that doesn't make any real difference); of those that do make a difference, most are harmful (at least in the organism's current circumstances), but a small percentage simply cause an alteration in function, or may even provide an advantage. Also, whether a mutation is harmful or not is sometimes situational a change which is harmful in some situations may actually be beneficial in others.
We now know that genes are made of DNA, a magnificently simple/complex molecule which actually encodes a language. It carries information just as a book does. The language has 4 letters which form 64 three letter words.
The actual job that a gene does is to tell a cell how to build a particular protein. Thus, there is a gene with the instructions for making the protein insulin, and another for making the protein myoglobin, etc. Some proteins require more than one gene for instance, hemoglobin, which requires at least two different genes plus some others to do associated building tasks. It's wonderfully complicated and marvelous.
Proteins are made by chaining together small molecules called amino acids. There are 20 different amino acids coded for by the genetic code. To make a particular protein, you need to string together a specific sequence of amino acids. An average protein has about 150 amino acids, and what makes the protein do what it's supposed to do is having the correct amino acids in the correct order. That's the information that a gene carries it's a string of about 150 (average) of those three-letter words, specifying the order of amino acids for that protein. One word for each amino acid.
If you look at these numbers, you can begin to see why a lot of mutations are silent. There are 64 DNA words, but only 20 amino acids. That means that a lot of DNA words "mean" the same thing, so if you change one into another, you don't change what the molecule says to do.
Another thing to understand is that not all parts of a protein are vital to its function. You can make some changes in the less vital parts without ruining what the protein is supposed to do. Sometimes you get a change in the protein's function that doesn't make much real difference. And sometimes you get a change which changes the conditions under which the protein functions.
For instance, the Siamese cat's color pattern is the result of this kind of mutation. The dark legs, tail and face of this cat are due to a change in one of the proteins necessary to manufacture pigment for the fur. Or more accurately, the light colored body is the result of that change.
All proteins are temperature sensitive in other words, you can "melt" them if you get them too hot. Normally, the melting point of a protein is significantly higher than body temperature. However, the color producing protein in Siamese cats is produced by a mutated gene. It's got a change in its chain of amino acids which makes it "melt" at a much lower than usual temperature right around the body temperature of the cat. Therefore, the cooler parts of the body get lots of color, and the warmer part gets very little. This is hardly a life threatening change, but it is the result of a mutation.
The question of whether a mutation is good or bad isn't as simple as it sounds. Yes, many mutations are devastating, but some are actually helpful. A mutation that broadens a parasite's host range is certainly helpful for the parasite though I doubt the new host would agree. Mutations which create diversity can also be helpful. For instance, a mutation which slightly increases the size and robustness of a bird's beak may allow the bird to expand its potential food sources.
Then there's the question of circumstance. Whether a mutation is harmful, neutral or helpful can be very situational. An example from human evolution illustrates this very clearly.
The evidence clearly indicates that our ancestors came originally from Africa. All Australopithecus species were entirely confined to Africa, as was Homo habilis. In most of Africa sunlight, which contains potentially harmful ultraviolet radiation, is very direct and intense. UV causes sunburn, but it also causes skin cancer. Our skin pigmentation (melanin) absorbs UV. Thus, human ancestors living outdoors in Africa evolved to have very dark skin lots of melanin to absorb enough UV to prevent them from getting skin cancer. Selective pressure in a situation like this strongly favors dark skin and makes any mutation for lighter colored skin harmful, since it reduces UV protection. So in this situation, mutations which decrease the amount of pigmentation in the skin are harmful. (Note: skin color in humans is a relatively complex trait genetically, involving several genes working together to determine the range of melanin the individual can make.)
However, eventually our ancestors meandered into other parts of the globe. When Homo erectus moved into Europe and Asia, the environmental conditions were quite different from those in Africa. In these temperate regions, sunlight was less intense, thus decreasing the selective pressure favoring dark skin, and allowing mutations for lighter skin color to become fixed in the population (in other words, increasing the diversity of skin pigmentation genes). And as the migration spread further and further to the north, a brand new selection issue began to influence skin color.
You know about vitamins or at least you know that there are a number of them that we need to have for our bodies to function properly. One of those essential vitamins is vitamin D; deficiency in this vitamin causes rickets. Our bodies can make vitamin D for themselves, but there's a catch. The final step in the manufacture of this vitamin occurs just under the skin, and is catalyzed by ultraviolet radiation. In other words, to make vitamin D, you have to absorb enough UV to cause this final step to happen. In the intense, direct sunlight of Africa, this was no problem, since even with the darkest melanin protection in the skin, enough UV penetrates to make plenty of vitamin D. But the further north (or south) you go, the less intense the sunlight is, and eventually you reach a latitude in which the selective pressure for skin color reverses and it becomes an advantage to have pale skin and a disadvantage to have darkly pigmented skin.
So here's a clear demonstration that there is often no clear "good" or "bad" about a mutation it may all depend upon the situation. These two contrary selective pressures are largely responsible for the magnificent diversity in modern human skin pigmentation.
Incidentally, lest anyone be concerned about the need to absorb UV to make vitamin D, this problem was ameliorated by the development of sun-curing techniques for meat, particularly fish. If you cure your fish in the sun, it will make vitamin D, and you'll be able to get as much as you need from your food. Of course, many people simply take exogenous vitamins these days, so it matters even less.
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Updated 25 September 2004