Biodiversity: what is genetic diversity?

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Biodiversity, literally 'biological diversity', consists of three levels: ecosystem diversity, species diversity and genetic diversity. The first definition concerns the various types of habitats present on Earth, each with its own environmental conditions that support certain communities of living beings. The second definition refers to the variety of living organisms that inhabit a given ecosystem and is the definition one usually thinks of when talking about biodiversity. Lastly, genetic diversity is the variability of genes within a population or between populations of the same species: it is often underestimated, but it is a diversity that is fundamental in allowing species to adapt to environmental changes.

The genetic diversity can be defined as the variety of DNA sequences present in a population of a given species and refers to the amount of variants of the same genes present in the genes of the population. These variants are called alleles and determine variations in individuals: an example might be the colour of men's eyes or hair. At the population level, the greater the number of different alleles for a given gene, the greater the genetic diversity. This diversity is the basis on which natural selection operates: mutations that occur naturally in genes create new variants, and some variants can make individuals possessing them better adapted to the environment. Genetic diversity is thus fundamental to the adaptability and survival of species.

Genetic diversity is studied through a variety of molecular techniques, the best known of which is probably DNA sequencing or genetic marker analysis. Modern technologies such as next-generation sequencing (NGS) allow large amounts of genetic data to be analysed in a short time, thus enabling the identification of genetic variants in different populations. Other methods include the use of single nucleotide polymorphisms (SNP), a very useful method for studying genetic variability. These techniques allow genetic diversity to be assessed at the level of specific genes or whole genomes with relative ease, especially when compared to older techniques, and are used in fields ranging from wildlife conservation to human genetics.

The genetic variability of individual species is as important as the diversity of species within an ecosystem and allows populations to adapt to environmental changes. To understand this better, we must remember that evolution occurs through natural selection, a process in which the environment exerts selective pressure on a population, favouring some genetic variants (combinations of alleles) over others. Variants that confer a reproductive advantage tend to spread in subsequent generations. A population with a large variety of genes is more likely to contain individuals able to resist disease or extreme conditions.

Without genetic diversity, a species becomes more vulnerable and subject to major declines due to epidemic diseases or sudden environmental changes, because individuals capable of adapting and surviving would be lacking. When genetic diversity is lacking, a population or species becomes more susceptible to disease, environmental stresses and inbreeding. Inbreeding, or inbreeding, is the reproduction between closely related individuals and leads to an increase in genetic diseases, as harmful alleles tend to occur more frequently: an example is the dog breeds. Without genetic diversity, populations find it very difficult to respond to new selective pressures, so their chances of long-term survival will be greatly reduced. This can lead to extinction, especially in species that are already threatened or have small populations.

A well-known example is the development of pesticide resistance in insect populations. When only a few individuals possess genetic variants that confer resistance, these survive pesticide use and reproduce, leading to a resistant population. The bottleneck effect is another example of loss of genetic diversity, occurring in Lord Howe's stick insect populations (which we discussed here), a relative of the leaf insects whose natural reduction in numbers has drastically limited their genetic variability, compromising their adaptability and thus necessitating special strategies to enable their survival and repopulation. Very small populations are more vulnerable to genetic drift, which reduces the survivability of the population or the species as a whole.

The genetic drift occurs when, in a small population, the frequency of certain genetic variants changes in a manner unrelated to environmental selection, thereby reducing genetic diversity. It is therefore essential to maintain sufficiently large populations and to ensure turnover between sub-populations through ecological corridors or management plans suitable for promoting genetic exchange. Repopulation andreintroduction programmes, as well as active management of habitats, can help prevent genetic drift. Regular genetic monitoring also makes it possible to detect any signs of loss of genetic variability, allowing early intervention.

The conservation of genetic diversity requires strategies that promote variability within populations. Key measures include the maintenance and restoration of natural habitats to prevent fragmentation and thus allow species to live in optimal conditions and maintain natural gene flows. Captive breeding programmes must also consider genetic variability, carefully selecting breeding individuals and planning exchanges of breeding individuals between the facilities involved to avoid inbreeding. Furthermore, the conservation of seeds and genetic material (sperm and ova) in gene banks is an important resource to protect biodiversity and restore endangered populations.

Genetic diversity is invisible to the naked eye, but is fundamental to the well-being and survival of species. It allows populations to adapt to change, resist disease and thrive in challenging environments by having the necessary elements to survive changing selection pressures. The loss of genetic diversity can have serious consequences and even lead to the extinction of species and, consequently, the degradation of ecosystems. Safeguarding this invisible resource is crucial to maintaining the natural balance and ensuring a sustainable future for the earth's biodiversity.