DNA has to be changed (a mutation) before the individual carrying that DNA is changed. The individual is changed before the population (cultural group) is changed. If a constant rate of mutation is assumed (not sure this can be assumed), then there has been 1 mutation per 25,000 - 50,000 years that has survived to be past down to us today! It is a good thing that most mutations do not seem to affect the functioning of the individual!
The causes of mutations fall into several categories. The most ancient was probably radiation since the nitrogen molecule is the most resistant to radiation. Also the double helix hold the codons (genes) in their matrix that is most resistant to radiation.
The second most likely is the errors that occur during the duplication (replication) of the DNA. You can imagine that each cell replicates hundreds of times, and some probably thousands of times. Somewhere along the line, there is bound to be an error made.
The third cause seems to be viruses. Some viruses have only a protein coat and a single strand of DNA/RNA inside. If this virus gets control of the cell (which it does when it invades the cell), then it can induce changes by its very action.
Chemicals in the environment called "mutagenic chemicals" have been shown to change the DNA pattern. This is especially true if the embryo is exposed at a very early date.
A last category is called "transposions". This describes the changes that happen when one nucleotide is shifted (transposed) with another. It being "moved around" in the codon line so to speak.
When these mutations occur, and they are pasted on to the next generation, and enough of the next generations carry these changes, the changes probably happened in a common ancestor. Bingo...a halogroup.
Today, these changes (mutations) have been identified for the various cultural groups in the land. The changes have been labeled on the Y-DNA (passed down through the males), and mtDNA (mitochrondial DNA) passed down through the female. When enough of a single population group carried a mutation, this become their halogroup marker. Having your DNA done, will provide you with a halogroup marker. This is the first step in using DNA for genealogy research.