Genetic variation refers to the differences in DNA sequences among individuals within a species. These distinctions influence unique traits like hair and eye color. Every gene can have different versions, known as alleles, which are passed from parents to offspring. A population with many different alleles for its genes is considered to have high genetic variation.
When these differences are few across a population, it results in low genetic variation, meaning the genetic makeup of individuals is very similar. This uniformity can present significant challenges for the long-term survival of a species.
Drivers of Reduced Genetic Diversity
A primary driver of diminished genetic variation is a population bottleneck. This event occurs when a population’s size is drastically reduced for at least one generation, often due to an environmental catastrophe. The small number of survivors can only carry a fraction of the original population’s genetic diversity into the next generation, creating a smaller gene pool.
A specific type of bottleneck is the founder effect. This happens when a small number of individuals break away from a larger population to establish a new colony. This new, smaller population will have a less diverse gene pool than the original source population. The genetic makeup of all subsequent generations will be determined by the genes of these founders.
Genetic drift also contributes to the loss of genetic diversity. It is the random fluctuation of allele frequency from one generation to the next due to chance events. Certain alleles can disappear completely, reducing the population’s genetic variability. This process has a more pronounced impact in smaller populations.
Population Vulnerabilities
A significant consequence of low genetic variation is an increased susceptibility to disease. When individuals in a population are genetically similar, a single pathogen to which the population has no inherent resistance can spread rapidly. This can potentially wipe out the entire group. The population lacks a “genetic toolkit” with alleles that might confer resistance and allow for survival.
Reduced genetic variation also limits a population’s capacity to adapt to environmental changes. Genetic differences provide the raw material upon which natural selection acts. Without a diverse array of traits, a population cannot effectively respond to new pressures such as a changing climate, the arrival of a new predator, or shifts in food availability. The population may not have the traits to survive under new conditions.
Real-World Case Studies
The cheetah is a classic example of a species impacted by low genetic variation. Having survived at least two population bottleneck events in its history, the species is now characterized by extreme genetic similarity among its members. This lack of diversity is linked to several survival challenges, including poor reproductive success rates and a heightened vulnerability to infectious diseases.
Tasmanian devils illustrate how low genetic diversity can facilitate disease spread. Devil Facial Tumour Disease (DFTD) is a contagious cancer that has decimated wild populations. The disease is transmitted when devils bite each other, a common behavior. Because the devils are so genetically alike, their immune systems often fail to recognize cancer cells as foreign, allowing the disease to spread efficiently.
The banana industry provides a lesson from agriculture. For decades, the export market was dominated by a single banana variety, the Gros Michel. Because these bananas were genetic clones grown from cuttings, they were all susceptible to a soil-borne fungus called Panama disease. When the fungus spread in the mid-20th century, it nearly wiped out commercial production, forcing the industry to switch to a resistant variety.
Human-Driven Genetic Homogeneity and Conservation
Human activities, through artificial selection, have intentionally reduced genetic variation in many species to favor specific traits. In agriculture, this has led to high-yield crops that are genetically uniform, making them vulnerable to widespread disease. Similarly, creating purebred dogs by selecting for desired traits narrows the gene pool, often leading to an increased prevalence of inherited health problems, such as hip dysplasia in German Shepherds or breathing difficulties in brachycephalic breeds.
In response to this threat, conservation scientists actively manage the genetics of endangered species. This field, known as conservation genetics, uses strategies to increase gene flow and variability. These efforts may include creating “genetic corridors” that allow animals to move between isolated populations or translocating individuals to introduce new genes into a depleted gene pool. These interventions are part of modern species management plans.