For example, sequenced DNA fragments from two people, AAGCCTA to AAGCTTA, is different in a single nucleotide. In this case we say that there are two "alleles". Almost all common SNPs have only two alleles. SNPs occur most often in regions of the DNA which do not affect the survival of the organism: otherwise they would be weeded out by natural selection. Other factors, like genetic recombination and mutation rate, can also affect SNP density. There are variations between human populations, so a SNP allele that is common in one geographical or ethnic group may be much rarer in another.