Mutation and Evolution
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[edit] Definition
A mutation is a change the genetic material of a cell, or of a virus. There are two kinds of mutation, somatic mutation and germ-line mutation.
Somatic mutation affects non-reproductive cells. It is usually harmless, except in the case of mutations which cause uncontrolled reproduction of the affected cell --- that is, cancer. Besides that, there is very little that one cell, however strangely mutated, can do to harm your body.
Germ-line mutation affects reproductive cells --- in animals, sperm or ova --- and so will be present in every cell of the body of an animal formed from the union of such a sperm or such an ovum.
Germ-line mutations are of great importance in biology because the theory of evolution accounts for evolution by the cumulative effect of a sequence of small random germ-line mutations each of which, if favored by natural selection, becomes prevalent in the gene-pool in which the mutation takes place.
For this reason, we shall use "mutation" to mean "germ-line mutation" throughout the rest of this article, unless we specifically say otherwise.
[edit] Discussion
There are a number of ways in which creationists attempt to confuse this issue. This is depressing: any decent biology textbook makes the issue quite clear, so either they are deliberately dishonest or, and this goes for the greater number of them, they are not getting their definition of mutation from biology textbooks, but from tatty creationist pamphlets.
In order to refute their nonsense, it is only necessary to explain, point by point, that when we write “the theory of evolution accounts for evolution by the cumulative effect of a sequence of small random germ-line mutations each of which, if favored by natural selection, becomes prevalent in the gene-pool in which the mutation takes place”, we mean exactly what we say, and not some other unscientific nonsense which creationists have made up.
[edit] Yes, we said cumulative
One of the strangest creationist obfuscations is to pretend that the change involved is sudden --- say, a bird hatching out of a dinosaur egg. Indeed, this is a favorite scene in creationist cartoons. This has never happened: the theory of evolution states that it is impossible, since this would require many large coordinated simultaneous mutations, rather than on the small random cumulative mutations required by the theory of evolution.
[edit] Yes, we said small
The chances of a large mutation being useful are minute, because organisms have coordinated parts. Imagine a mutation which doubles the length of the hindlimbs of some animal. It would fall over, unless another simultaneous mutation took place to double the length of the forelimbs. Not very likely, is it? But suppose for a moment the creature got lucky --- then there is still the problem of musculature to move the thing about on its new stilts. It would have to get lucky again. And then there’s the question of the robustness of the bones. The weight of the animal has increased, so the bones need to be thicker --- which requires another lucky break. Ooops, but now the leg bones don’t fit into the pelvis.
We needn’t continue this little fable. It is clear that a large mutation, in order to be beneficial, would require a massively improbable coincidence of other mutations.
On the other hand, consider what the theory of evolution teaches: evolution by the cumulative effect of small mutations.
Suppose, for example, a species with short legs in an environment where there is a selective pressure to run fast --- say, the presence of predators. Suppose a germ-line mutation to affect some individual to make the back legs slightly longer, and that this has a net benefit, in that the better leverage in the back legs more than compensates for the slight increased likelihood of going head-over heels. Then this mutation, since it has net benefit, will favour the individuals that possess it, and so will spread through the gene pool.
But now there is a benefit to be gained by having slightly longer forelimbs, for better balance, and any mutation causing such a slight increase will be beneficial and will itself spread through the gene pool.
And plainly we could continue this narrative by pointing out that this process, if continued, leads to a net benefit for a mutation making the bones slightly more robust, and so forth.
[edit] Yes, we said random
We are sometimes asked questions along the lines of "how do the animals know which mutations to have". Of course, they don’t know --- they have no say in the matter --- and mutations are not invariably good. But it is only those mutations which are beneficial which are favored by natural selection.
In the example in the previous section, there is nothing at all to prevent some individual being affected by a mutation which makes its legs shorter. But that animal and its descendants, if any, will run slower than the rest of their species, and so be more vulnerable to predators, and so this gene would tend to be quickly rubbed out of the gene pool of the species by natural selection.
[edit] Yes, we said germ-line mutations
Much creationist rhetoric depends on confusing the mutations of interest to evolutionary biology (i.e. germ-line mutations) with the sort of “mutations” depicted in comic books. We are unable to say whether this is a deliberately dishonest attempt to confuse the issue, or whether creationists do, in fact, get all their "biology" out of comic books.
The mutations in comic books, if they happened, would be large, not small, coordinated, not random, simultaneous, rather than cumulative, and somatic rather than germ-line.
We sometimes wonder if there is a competition among creationists to see how wrong they can be. In order to create a “theory of evolution” which can be rebutted, it is, after all, only necessary to get one detail of the real theory wrong. To be this wrong seems excessive and unnecessary.
[edit] Yes, a mutation only spreads through a gene pool if it’s beneficial
We have been much amused by creationists claiming that one mistake in the evolutionary process would cause the extinction of the species, as in this hilarious example, which also makes the mistake of requiring the mutations involved to be large and simultaneous, and of supposing that there is no intermediate stage between permanently immersed in water and bone-dry:
- "One of the inconsistencies in the amphibian-reptile evolution scenario is the structure of the eggs. Amphibian eggs, which develop in water, have a jelly-like structure and a porous membrane, whereas reptile eggs, as shown in the reconstruction of a dinosaur egg on the right, are hard and impermeable, in order to conform to conditions on land. In order for an amphibian to become a reptile, its eggs would have to have coincidentally turned into perfect reptile eggs, and yet the slightest error in such a process would lead to the extinction of the species." [1]
The name of the website is darwinismrefuted.com, which would appear to be false advertising. We are not absolutely certain --- the author does not say --- whether he supposes that a mutation happens simultaneously to all members of a species, or whether he supposes that a mutation which interferes with reproduction can spread through a gene pool. Either way, he’s wrong.
A germ-line mutation affects only one individual to begin with. Any such mutation which causes sterility, as in the example above, or death, will die with that particular individual. Even a mutation which only carries a small disadvantage will be rubbed out by natural selection over time.
[edit] Yes, there are beneficial mutations
It is common for creationists to deny the possibility or existence of beneficial mutations. It is very easy to find examples of beneficial mutations both in the wild and in the laboratory, and as presenting even a small selection of such examples runs to several thousand words, we have dealt with this question in a separate article on Beneficial Mutations.
