quote:Almost all (99.9%) nucleotide bases are exactly the same in all people.
Of course they are....we're all human! The human nucleotide bases are adenine (A), cytosine (C), guanine (G) and thymine (T). It's not the base pairs responsible for distinct populations, however, but the sequence of the base pairs. I'm not sure how versed you are in population genetics, so this is widly simplified: DNA conatians our genes. DNA is composed of, in part, these four base pairs. The genes depend on the sequence of the base pairs, which actually code for proteins. If we were to pick an arbitrary gene, the basic protein would fundamentally be the same, but variations in the protein would difference. So for the gooblyglock gene, which has the basic structure ATGCATGCATGC----ATCGATCG
the blank area can be any combination of base pairs. Different combination of base pairs would then make different versions of the genes, called an allele. Some populations have alleles which are endemic to themselves in a much higher frequency than found in other populations, whichwhen taken into account with the frequency of their other alleles gives them a set of traits elatively distinct within their group, although individually these traits may be found in others. The easiest example I can think of is blue eyes in Northern Europeans. While some other populations may exhibit blue eyes on occassion(such as Australian aborigines and the Nigerian Igbo), the chances of this are very small, and the overwhelming majority of these populations never will. Of course, injury, disease, and congenital deformities may also produce blue eyed individuals, but in these cases blue eyes are an abnormality and not a set heritable characteristic. With the above rarities, blue eyes are almost only found in the "Caucasian" category, and then more than likely in the subgroup known as Northern Europeans whites. In some countries such as Finalnd, fully 90% of the population is naturally blue eyes. The frequency of blue eyes in Europe are highest in Scandinavia and the Baltic area, and decreases the further south you go (Spain, Portugal).
Now all populations who have lived in contact with one another and actively interbred will over time start to exhibt characteristics which have a high (or low) frequency within their group. There of course will be overlap with other groups, but when all traits are taken together, they are genetically distinct from other populations. Of course, the smaller the and more isolated the groups bein compared, the more distinct they will be. Does that make any sense?