mutations, among other things, can cause phenotypical variations. The dna can change, which changes genotype, and therefore phenotype. If the altered phenotype can help the organism survive (eg. bigger muscles) then survival of the fittests insues, causing natural selection
The first response above is of course correst, in a very general way, but it is by no means the whole story, and might even be a little misleading. The first thing to say is that all organisms other than bacteria etc have 2 copies of each gene. Mutation can only affect one of these at a time, and this gives rise to what is called an allelic pair i.e. 2 different versions of the same gene. Now in their expression in the phenotype, a pair of alleles interact in one of 2 main ways - one is dominant over the other (the other here is then said to be reccessive) or both alleles contribute equally to the phenotype. This is called co-dominance. The majority of new mutations, especially those that don't give pathological phentotypes, are reccessive. In other words, the new mutation is not expressed in the first individual that carries it, because reccessive alleles can only be expressed when they are in "like" pairs. Here's an example. Original "wild-type" alleles AA. Mutation →Aa, no new phenotype. aa = new phenotype. In other words the new allele a has to "find" another a in the population. That is the only way in which phenotypic variation can be observed. What immediately follows from this, of course, is that there is an enormous lot more genetic variation in a population than there is phenotypic variation. What should also be apparent is that genetic vaiation can only be expressed in diploid organisms which reproduce sexually. And this raises another issue (can you be bothered?) During the process by which gametes are formed, chromosome pairs, each carrying their own set of alleles, participate in segmental reciprocal exchange which results in new combinations of alleles. (that's why only monozygotic twins can be identical). Moreover, as many phenotypic characteristics are determined by combinations of alleles this by far and away the most important, or rather the commonest, source of phenotypc variation in sexually reproducing populations. As to selection - when it comes to new mutations (i.e. only one such allele in a population initially) that is a bit of a puzzle still which deserves a whole new post so I'll leave it. But in general remember that selectiononly works on the phenotype. By doing so, of course, it eventually changes the frequency of the alleles determining it, but from what I've just said, you will see (I hope) that it is going to be a bit more complicated than you might think.
QH, I have a question about mutation process and from reading your post you, if anyone can, answer this question I am sure. When a mutation ocurs because of a mutated, or altered gene the offspring (2nd generation) organism, is affected as observed. OK, no problem so far, but how does a gene become mutated to the extent that it gets passed on to the offspring? How many trillion of chromosomes are there? I am assuming that the process of a DNA gene altered by radiation for instance is relatively a local event. I mean if a swath of radiation passes through the organism and an H gene located in the toe regulating toenail smoothness and hence a better food grabbing potential is established. I surmise that most probably the radiation was not gene selective and that the same gene on the chromosome located in the nose would not necessarily be be affected, nor would a large number of the same gene be mutated alike. Now I see a problem (for me) in passing the trait of the toenail on to offspring. The question is in three parts parts. In order to pass on the gene toenail the gene must, exist in sufficient numbers and located in the proper volume space where inserted into the sperm is likely and Then of course, the sperm has to win the great race to impregnation. SO QH, How do the numbers of mutated genes gets sufficiently high in the proper location? I was gazing through my anatomy book a couple of days ago and was wondering about the navigation complexiy for a gene, a chromosome, to migrate from a toe to wherever sperm are assembled. If there were 1 gene I don't think there's a roadmap available that is going to select out the toe gene and guide it through the convoluted and hypercomplex channel system of the organism. So how does it do it? The DNA mutation getting into the impregnated female? It seems to me a matter of chance to the nth degree unlikely that any mutated trait gets passed on to the offspring. And that if any mutatation at all occurs, it will most probably be harmful with respect to being beneficial to the offspring. Geistkiesel
We distinguish two sorts of mutation. Somatic and germ line. Somatic means in the body - something like if you lie in the sun too long you might get a mutation in a skin cell that causes it to divide uncontrolably and give you skin cancer. There is no way - none - that this can be passed to your children. Then there are germ line mutations. They happen..er..well, halfway between your belt and your knees, if you get me. Never mind for now how they are caused, but these are the guys that get passed to the next generation. So to answer your Q: the number of genes gets sufficiently high because the mutation is already present in sperm/egg which then contributes to the entire genotype of the next generation. Genes don't "migrate". Germ line mutations are passed to the entire genotype of the offspring, but where the affected gene is expressed in only one "location" (your toenail example) that's the only place it will show in the phenotype
Mutations naturally occur as a result of random chance effects, or as the result of an external factor such as radiation poisoning, drug abuse, etc. Phenotype variation can occur as a result of all of the above postings, but also as a result of variation in the development of the embryo. Another words, you start off with the genotype (DNA composition), two individuals mate (if your talking about people only?) then the egg is fertilized and it undergoes embryological development. From that point on, the embryo is subject to environmental forces of all sorts that determine its eventual phenotype for the rest of its life, even after it is born.
