Genetic diversity is the variety of inherited genetic differences among individuals and populations, usually considered within a species. It includes differences in DNA sequences, the presence and frequencies of alternative alleles, and combinations of variants across the genome. Alongside species diversity and ecosystem diversity, it forms a major component of biodiversity. Genetic diversity supplies variation for evolutionary change and is important to the persistence of wild populations and the improvement of domesticated organisms. (genome.gov)
Forms and levels of variation
Genetic differences range from changes at a single nucleotide to insertions, deletions, duplications, and rearrangements involving larger DNA segments. Variation occurs both within genes and in noncoding regions. Some variants affect biological function, while many have little or no detectable effect. The amount of sequence variation therefore does not directly indicate how much organisms differ in appearance or performance. A phenotype reflects genetic influences together with environmental conditions. (genome.gov)
Diversity can be assessed within individuals, within populations, or among populations. A diploid individual may carry two different alleles at a particular locus; a population may contain numerous alleles distributed among its members. Geographically separated populations can differ in allele frequencies or possess variants absent elsewhere. Consequently, preserving one genetically diverse population does not necessarily preserve all the genetic variation of a species. Both within-population variation and differences among populations are relevant to conservation. (pmc.ncbi.nlm.nih.gov)
Origins and evolutionary processes
Mutation is the ultimate source of new alleles. During sexual reproduction, meiosis and chromosomal crossover reshuffle existing genetic material, producing new combinations rather than necessarily creating new alleles. Gene flow, through successful reproduction by migrants or the movement of reproductive material such as pollen, can introduce alleles into a population and reduce differences between populations. (openstax.org)
Natural selection changes variant frequencies when inherited differences influence reproductive success. Depending on conditions, selection can remove variants or maintain alternatives. Genetic drift changes frequencies through chance sampling between generations and can eliminate alleles regardless of their effects. Its influence is especially strong in populations with small effective population size, which describes their genetic behavior relative to an idealized population rather than simply counting living individuals. Unequal reproductive success and population fluctuations can make effective size substantially smaller than census size. (openstax.org)
A population bottleneck occurs when population size is sharply reduced, potentially removing much of its variation. A founder effect arises when a new population is established by a small subset of a larger one. Both processes can leave descendants with allele frequencies and genetic diversity different from those of the original population. (openstax.org)
Measurement
Population genetics uses several complementary measures:
- Allelic richness: the number of distinct alleles at sampled loci, commonly adjusted for sample size.
- Observed heterozygosity: the proportion of sampled individuals carrying different alleles at a locus.
- Expected heterozygosity: the probability that two randomly sampled allele copies differ. For allele frequencies , it is ; under Hardy–Weinberg equilibrium, this also gives the expected heterozygous proportion.
- Nucleotide diversity: the average number of nucleotide differences per site between sampled sequences. (pmc.ncbi.nlm.nih.gov)
These measures describe different aspects of variation and need not rank populations identically. Allelic richness records the presence of rare alleles, whereas heterozygosity weights allele frequencies. Comparisons also depend on sampling, the genomic regions examined, and the mutation rates of the markers used. DNA sequencing permits genome-wide assessments, including analyses of long homozygous regions associated with shared ancestry and inbreeding. (pmc.ncbi.nlm.nih.gov)
Biological significance
Genetic diversity provides the inherited variation on which evolution acts. When environments change, some existing variants may contribute to greater survival or reproduction, enabling adaptation over generations. However, high diversity does not guarantee persistence: the relevant variants must exist, and demographic or environmental pressures may overwhelm adaptive responses. Measures of broadly neutral sequence variation are not interchangeable with direct measurements of variation in ecologically important traits. (openstax.org)
Inbreeding, or reproduction between relatives, increases the probability that offspring inherit allele copies from common ancestors. It can expose harmful recessive variants and reduce reproductive performance, a phenomenon known as inbreeding depression. Inbreeding changes genotype proportions and is not identical to allele loss, although both commonly accompany small, isolated populations. Genetic diversity and inbreeding measurements therefore provide related but distinct information. (openstax.org)
Conservation and agriculture
In conservation biology, maintaining genetic diversity involves protecting populations across a species’ range as well as sustaining variation within them. Population reductions, exploitation, and habitat alteration can erode genetic resources. Conservation can occur in natural habitats or outside them, including managed breeding populations and stored biological material. Target 4 of the Kunming–Montreal Global Biodiversity Framework explicitly addresses maintaining and restoring diversity within and between populations of native wild and domesticated species. (cbd.int)
Agriculture uses genetic variation in crop varieties, livestock breeds, and wild relatives as material for breeding. Different genetic resources may contribute traits useful under changing growing conditions. Gene banks preserve seeds, living collections, or tissues and make material available for research and breeding. Conservation in the wild and on farms complements stored collections by allowing reproduction, selection, and continuing evolutionary change. (fao.org)