ABSTRACT
The human genome consists of 3 billion nucleotides which are 99.5% to 99.8% similar
between any two individuals (1). This 0.2% to 0.5% of the genome (or 6-15 million
nucleotides) that varies is believed to cause the wide interindividual variation in normal
and disease risk alleles. Most genetic variation is between individuals within a specific
population as opposed to between populations on different continents (2-4). Genetic
variation within and between populations results from natural selection, mutation, genetic
drift, and admixture. These evolutionary forces, along with inbreeding and nonrandom
mating, have predictable effects on the levels of variation among and within populations.
A summary of whether or not each of these components of evolution increases or
decreases variation within and between populations is given in Table 1. Given that genetic
drift can be highly variable in terms of its effects on various genomic regions, and that we
have limited knowledge on the action of natural selection and the demographic and
migration histories of human populations, it is important that we have empirical data on
genetic variation across the world’s populations and genomic regions. Projects such as the
International HapMap (5) have played a key role in addressing these needs and can serve
as starting points in discussions of which of these evolutionary forces best explains
observed patterns of human genetic variation.
