Genetic variability is either the presence of, or the generation of, genetic differences. It is defined as "the formation of individuals differing in genotype, or the presence of genotypically different individuals, in contrast to environmentally induced differences which, as a rule, cause only temporary, nonheritable changes of the phenotype."^[1] Genetic variability in a population promotes biodiversity, as it ensures that no two living things are exactly alike.^[2] While many factors can cause genetic variability, some factors can also decrease genetic variability.

Causes

There are many sources of genetic variability in a population:

• Homologous recombination is a significant source of variability. During meiosis in sexual organisms (sexual reproduction), two homologous chromosomes cross over one another and exchange genetic material. This random process of exchanging genetic material is known as recombination, and it is governed by its own set of genes.^[3] Being controlled by genes means that recombination is variable in frequency. Generally, it is more common in areas on the chromosome where there are a higher number of genes. It creates more combinations of genes.^[3] After this occurs, the chromosomes are split apart and are ready to contribute to forming an offspring.
• Immigration, emigration, and translocation – each of these is the movement of an individual into or out of a population. When an individual enters a new population after coming from a genetically isolated one, they will increase the genetic variability of the next generation, provided that they reproduce.^[4]
• Polyploidy – having more than two homologous chromosomes. This allows for more recombination during meiosis and more genetic variability in one's offspring. However, this phenomenon can also prove difficult for cell division.^[5]
• Diffuse centromeres – in asexual organisms where the offspring is an exact genetic copy of the parent, there are limited sources of genetic variability. One thing that increases variability, however, is having diffused centromeres instead of localized centromeres. Being diffused allows the chromatids to split apart in many different ways, which produces chromosome fragmentation and polyploidy.^[6]
• Genetic mutations – contribute to the genetic variability within a population and can have positive, negative, or neutral effects on a fitness.^[7] This variability can be easily propagated throughout a population by natural selection if the mutation increases the affected individual's fitness and its effects will be minimized/hidden if the mutation is deleterious. If the individual can survive with the mutations they have, those mutations will likely be passed down to offspring. However, the smaller a population and its genetic variability are, the more likely the recessive/hidden deleterious mutations will show up causing genetic drift.^[7]

DNA damages are very frequent, occurring more than 60,000 times a day per cell on average in humans. This is due to metabolic or hydrolytic processes as summarized in DNA damage (naturally occurring). Most DNA damages are accurately repaired by various natural DNA repair mechanisms. However, some DNA damages remain and give rise to mutations.

Additionally, not all types of mutations occur as much as others do. Some mutations might have a huge impact on the human body, and some might not. It depends on what combination of base pairs is changed.^[8]

Most spontaneously arising mutations result from error prone replication (translesion synthesis) past a DNA damage in the template strand. For example, in yeast more than 60% of spontaneous single-base pair substitutions and deletions are likely caused by translesion synthesis.^[9] Another significant source of mutation is an inaccurate DNA repair process, non-homologous end joining, that is often employed in repair of DNA double-strand breaks.^[10] (Also see Mutation.) Thus, it seems that DNA damages are the underlying cause of most spontaneous mutations, either because of error-prone replication past damages or error-prone repair of damages.

Factors that decrease genetic variability

There are many sources that decrease genetic variability in a population:

• Habitat loss, including:
  • Habitat fragmentation produces discontinuity in an organism's habitat, so that interbreeding is limited. Fragmentation can be caused by many factors, including geological processes or a human-caused events. Fragmentation may further allow genetic drift to lower local genetic diversity.
  • Climate change is a drastic and enduring change in weather patterns. By driving species out of their fundamental niche, climate change can lower population size and consequently lower genetic variation.
• The founder effect, which occurs when a population is founded by few individuals.