Jean-François
* "How much variation is there in the DNA sequence? I was quite surprised to learn of the enormous variance between humans. One out of every thousand males has an extra X chromosome, and a similar number (one out of a thousand) of males have an extra Y chromosome. About one out of every thousand females only have one X-chromosome, instead of two. If you were to have a look at the DNA sequence for any two human beings, there would be more than a MILLION base pair (bp) difference between them! Many humans carry major chromosome translocations, where part of the arm of one chromosome has broken off and been attached to another chromosome. Think about this - there are roughly 3,000,000,000 bp in the human genome. Even if you were to copy this at an error rate of only one in a million, which is pretty close to what happens in the real world, then you will still get 1000 bp difference EVERY time the cell divides. All humans started out as a single cell, which divides many times over - hence the DNA sequence is quite diverse, even within different cells from the same person! How can we possibly survive all this variation? Most (about 98%) of the DNA in humans does not code for proteins, so we can take quite a bit of deterioration in sequence and still live. Furthermore, the mutations (changes in DNA sequence) are not in fact completely random, but somewhat localized to regions that are not as essential for life.
There are many well documented cases of new species arising from large scale changes in the DNA sequences. For example, in simple cases, the chromosomes get duplicated. This is the likely origin of trout fish - if you examine the trout cells, they contain FOUR copies of each chromosome of a smaller related fish, rather than the expected "normal" two copies. This extra set of genetic material not only make the trout larger than their ancestors, but gives them extra space to change and adapt with. This happens very often with plants, although complete polyploidy is much less common in animals. This is how many plants have obtained such large genomes - some more than a thousand times the size of the human genome!
The smaller changes, such as gene duplications, are also well documented. Transposable genetic elements can splice themselves in and out of genomes, creating duplicate copies of genes, and also causing problems for the genomic organization and stability of organisms. At the level of bacteria, it is possible to look at complete genomes, and see which regions have come from bacterial viruses. A bacterial virus is really quite simple: a small piece of DNA and a protein coat. That's all. Pretty simple. Yet this is a very powerful mechanism of introducing genetic change in bacteria. Similarly, eukaryotic viruses can bring foreign DNA sequences (e.g., from other organisms) into human cells. So the genetic information is varied and constantly changing."
(David Ussery, http://www.cbs.dtu.dk/dave/Behe1.html )