Genetic variability refers to the potential for a given characteristic or genotype to vary within a population when faced with a particular influence. As the genetic variability of a population increases, so does its resistance to environmental and genetic influences and ultimately extinction. Consequently, genetic variability is directly tied to biodiversity and evolution.
In terms of evolutionary biology, if a population lacks sufficient genetic variability, it also lacks the potential to evolve and adapt. Biodiversity is like an insurance policy because it acts as a buffer to minimize damage from catastrophes that might otherwise wipe out an entire population. In terms of genetics, variability among population genotypes can explain why different people have different responses to various treatments, infections and drugs.
Climate change, disease and natural disasters all have the potential to influence genetic variability and drive evolution. Without sufficient variability, a population loses its potential to withstand these influences. A biological system that has heritable genetic variability and is vulnerable to natural selection is considered evolvable. In other words, in order for a population to evolve, it must have the ability to do so through natural selection.
Increased variability also increases fitness. This is a concept that is promoted by natural selection. Evolutionary adaptations in nature are the result of a species’ genetic variability paired with environmental and genetic influences.
The evolutionary adaptations actually observed in nature are described in terms of variation rather than variability. These two terms are often interchanged in scientific literature despite the subtle differences between the two terms. For a given gene, variation describes the observable differences, and variability is limited to the potential for differences between individuals within a population.
A primary genetic cause of variability is the pure randomness of homologous recombination and polyploidy in sexual organisms. The frequency and location at which recombination occurs is completely random, so it follows that increased recombination leads to increased variability and fitness. Polyploidy occurs when there are multiple homologous chromosomes, thereby increasing recombination in offspring.
In asexual organisms, sources of variability are limited because the genetic code is the same for the parent and offspring. A similar limitation occurs when inbreeding is allowed, because the genetic material from the parents is less variable to begin with. The lack of variability within a population can lead to genetic problems such as mutation and drift.
Other causes of genetic variability are related to the environment. Environmental factors such as loss of habitat and climate change can create circumstances that leave individuals isolated from the population. If a new individual joins the population, then the potential for variation increases.