Genetic diversity

Genetic diversity is the total number of genetic characteristics within the genetic makeup of a species. It is a fundamental concept in biology that describes how much variation exists among individuals. This diversity can be measured across different scales, ranging from the number of different species to small differences within a single species. It is distinct from genetic variability, which refers to the tendency of those characteristics to vary. Understanding this diversity is vital because it is closely linked to how well a species can survive over long periods of time.

Diversity acts as a tool for populations to adapt to changing environments. This process relies on the presence of different alleles, which are different versions of a gene. When an environment changes, a population with high variation is more likely to have some individuals with alleles suited for the new conditions. These specific individuals are more likely to survive and produce offspring. Because those offspring carry the beneficial allele, the trait spreads through the population over many generations. This mechanism allows a species to evolve in response to its surroundings.

Scientists categorize genetic diversity into two main types: neutral and adaptive. Neutral genetic diversity consists of genes that do not increase an organism's fitness or help it adapt. Natural selection does not act on these neutral genes. In contrast, adaptive genetic diversity involves genes that increase fitness and allow for adaptation to environmental changes. These adaptive genes control ecological, morphological, and behavioral traits. Because natural selection acts on these genes, the rate of evolution is often greater for adaptive genes than for neutral ones. Identifying these specific genes is a major goal in conservation science.

Several hypotheses explain how this diversity arises and changes. The neutral theory of evolution suggests that diversity comes from the accumulation of neutral substitutions. Diversifying selection occurs when two subpopulations of a species live in different environments. These different settings select for different alleles at a particular locus, or gene location. This often happens if a species has a large range but individuals do not move very far. Another idea is frequency-dependent selection. In this model, an allele becomes more vulnerable as it becomes more common. This is often seen in host-pathogen interactions, where a pathogen can spread more easily if a defensive allele is very frequent in the host population.

Genetic diversity is also deeply connected to the broader health of ecosystems. A 2007 study by the National Science Foundation found that within-species diversity and biodiversity are dependent upon each other. The lead researcher, Dr. Richard Lankau, noted that diversity within a species is necessary to maintain diversity among species. If one type is removed from the system, the entire cycle can break down. This can lead to a community being dominated by only a single species.

This delicate interdependence means that losing genetic variety can eventually lead to a loss of entire biological systems.

Small populations face unique risks that can rapidly reduce their genetic diversity. Large populations are generally better at maintaining genetic material. Small populations are more likely to lose diversity through genetic drift, which is the loss of traits due to random chance. When an allele drifts to fixation, it means that the specific version of the gene has become the only one left, and all other versions are lost. Small groups are also more prone to inbreeding. This is when individuals with similar genetic makeups mate, which perpetuates common alleles and decreases overall variety. A genetic bottleneck can also occur if a population's numbers drop suddenly, leaving a very limited gene pool.

Human activities, particularly in agriculture, significantly impact these patterns. Through selective breeding, humans have created monocultures, which are fields of nearly identical plants. While this can be efficient, it makes crops extremely vulnerable to disease. If a bacterium evolves to attack one specific genetic variation, it can wipe out an entire harvest. A famous example is the Great Famine in Ireland during the 1840s. Much of the population relied on the "lumper" potato, which lacked diversity because the plants were essentially clones. When a rot-causing organism called Phytophthora infestans arrived, it destroyed the crop and caused one million deaths.

Livestock diversity is also under pressure from economic globalization and expanding markets. Maintaining diverse animal breeds allows for better animal husbandry in different environments. However, many breeds are disappearing. According to the Food and Agriculture Organization of the United Nations, as of June 2014, 17 percent of recorded domestic animal breeds were at risk of extinction. Another 7 percent were already extinct. To help manage these resources, a Global Plan of Action for Animal Genetic Resources was developed in 2007. Protecting this diversity is essential for ensuring that food systems can adapt to future changes.