Genetic variation is the diversity of genetic information among individuals within a population, between populations of the same species or, more broadly, across species and lineages. It can involve differences in DNA sequence, chromosome structure, gene copy number, regulatory regions and other aspects of the genome. At a particular gene, alternative forms are called alleles. Genetic variation means that members of a population are not genetically identical, even though individuals of the same species share much of their DNA.
Variation can affect observable traits, biochemical processes, development, behaviour, disease susceptibility and reproductive success. It is fundamental to evolution because natural selection and other evolutionary processes change the frequencies of genetic variants across generations. Without heritable genetic variation, natural selection would have much less raw material on which to act.
Genetic variation has several important sources. Mutation creates new DNA sequence variants, including substitutions, insertions, deletions and larger genomic changes. Recombination during meiosis produces new combinations of existing alleles by exchanging genetic material between homologous chromosomes. Independent assortment causes chromosomes of different parental origin to be distributed into gametes in many possible combinations. Sexual reproduction then combines genetic material from two parents, further reshuffling existing variants. In many organisms, gene duplication and chromosomal rearrangements can also generate important genomic differences. Horizontal gene transfer contributes substantially to variation in some microorganisms. Consequently, genetic diversity is not produced by a single mechanism but by a network of processes that generate, rearrange and redistribute genetic information.
Genetic variation can occur at different biological scales. A single nucleotide polymorphism involves variation at one DNA base position, while small insertions or deletions alter short sequences. Structural variants can involve larger deletions, duplications, inversions or translocations, and copy number variation can change the number of copies of particular genomic regions. Variation can also occur in mitochondrial or chloroplast genomes in organisms that possess these organelles. At the chromosome level, individuals may differ in chromosome number or chromosome structure. Some variation has little or no observable effect, some alters traits, and some can have major consequences for fitness. Modern genome sequencing has revealed that genetic variation is extensive even among individuals who appear very similar phenotypically.
Not all genetic variation is expressed visibly. A DNA variant may occur in a noncoding region, alter a gene only under particular environmental conditions, or produce a synonymous change that does not alter the encoded amino acid. Other variants can have regulatory effects that change when, where or how strongly a gene is expressed. Genetic variation can therefore contribute to differences that are biochemical or physiological rather than obvious externally.
Even when a variant affects a phenotype, its effect may depend on other genes and environmental conditions. Gene by environment interactions occur when the effect of a genotype differs according to environmental circumstances. This means that the relationship between DNA variation and phenotype is often probabilistic and context dependent rather than a simple one gene to one trait relationship.
Genetic variation is essential for populations facing changing environments because it can provide variants that selection may favour when conditions change. For example, a population containing multiple alleles influencing resistance to a pathogen may contain individuals with different levels of susceptibility. If the pathogen imposes strong selection, variants associated with increased resistance can become more common. However, variation does not guarantee adaptation. A population may lack appropriate variants, experience demographic decline before selection can act, or lose useful diversity through genetic drift. The amount and distribution of variation therefore influence evolutionary potential, but evolutionary outcomes also depend on population size, mutation rate, reproductive system, migration, selection and environmental stability.
Genetic variation can be measured in several ways. Researchers may measure nucleotide diversity, heterozygosity, allele frequencies, haplotype diversity or genetic differentiation among populations. Heterozygosity describes the proportion of genetic loci at which individuals carry different alleles, while nucleotide diversity measures average sequence differences among sampled DNA sequences. Population geneticists often examine allele frequency distributions to study selection, drift and migration. Conservation biologists may monitor genetic diversity because small isolated populations can lose variation through inbreeding and genetic drift. Maintaining genetic diversity can increase the ability of populations to respond evolutionarily to disease, environmental change and other pressures, although the appropriate measure of diversity depends on the organism and research question.
Genetic variation should be distinguished from phenotypic variation. Phenotypic variation refers to differences in observable characteristics, whereas genetic variation specifically concerns differences in genetic material. Phenotypic differences can arise entirely or partly from environmental effects, so observing different appearances does not automatically demonstrate genetic differences. Conversely, genetically different individuals may look similar because the variants have little phenotypic effect under the current conditions. Genetic variation should also be distinguished from genetic diversity, although the terms are closely related. Genetic diversity is often used to describe the amount and distribution of genetic variation within populations or species, particularly in conservation contexts. These distinctions are important when interpreting experiments, medical genetics studies and evolutionary evidence.
Genetic variation underlies many important biological phenomena, including adaptation, disease resistance, domestication and speciation. Artificial selection by humans can rapidly change allele frequencies when breeders preferentially reproduce individuals with desired traits. Natural selection can similarly change allele frequencies when environmental conditions favour particular inherited characteristics. Genetic drift can remove variation randomly, especially after population bottlenecks or founder events, while gene flow can introduce variants from other populations. Mutation continually provides potential new variants, although most newly arising mutations are neutral or have effects too small to be strongly selected. The interaction among mutation, recombination, selection, drift and migration determines how genetic diversity is generated, maintained, distributed and lost over evolutionary time.
