Genetic variation arises from mutations, from natural selection, migration between populations (gene flow) and from the reshuffling of genes through sexual reproduction. Mutations lead to a change in the DNA structure, as the order of the bases are rearranged. Resultantly, different polypeptide proteins are coded. . Some mutations may be positive and can help the individual survive more effectively in their environment. Variation is counteracted by natural selection and by genetic drift; note too the founder effect, when a small number of initial founders establish a population which hence starts with a correspondingly small degree of genetic variation. Epigenetic inheritance involves heritable changes in phenotype (appearance) or gene expression caused by mechanisms other than changes in the DNA sequence.
Human phenotypes are highly polygenic (dependent on interaction by many genes) and are influenced by environment as well as by genetics.
Nucleotide diversity is based on single mutations, single nucleotide polymorphisms (SNPs). The nucleotide diversity between humans is about 0.1 percent (one difference per one thousand nucleotides between two humans chosen at random). This amounts to approximately three million SNPs (since the human genome has about three billion nucleotides). There are an estimated ten million SNPs in the human population.
Research has shown that non-SNP (structural) variation accounts for more human genetic variation than single nucleotide diversity. Structural variation includes copy-number variation and results from deletions, inversions, insertions and duplications. It is estimated that approximately 0.4 percent of the genomes of unrelated people differ, apart from copy number. When copy-number variation is included, human-to-human genetic variation is estimated to be at least 0.5 percent.