Genetic diversity refers to the total range of inherited traits within a species. It encompasses the variations in genes that make each individual unique, influencing characteristics like physical appearance, behavior, and physiological responses. A species with high genetic diversity possesses a wide variety of these traits, providing a broader “toolkit” of adaptations. Conversely, low genetic diversity means a species has a limited range of these characteristics, making it more vulnerable to various threats.
How Low Diversity Increases Vulnerability
Low genetic diversity significantly compromises a species’ ability to survive. It primarily weakens defenses against disease, limits adaptation to environmental shifts, and increases inbreeding risks. A lack of variation in immune system genes means that if a single pathogen emerges that can overcome existing immune responses, the entire population becomes susceptible. This uniformity allows diseases to spread rapidly and devastate a population.
Beyond disease, a narrow genetic toolkit hinders a species’ ability to cope with environmental changes such as shifts in climate, habitat, or food sources. Natural selection acts on existing genetic variation, favoring individuals with traits that enhance their survival and reproduction in altered environments. If a population lacks the necessary genetic variations, it cannot evolve quickly enough to keep pace with these changes, potentially leading to decline or extinction. For example, a population exposed to a new drought might lack individuals with genes for deeper root systems or greater water use efficiency, leaving them unable to adapt.
Small, genetically uniform populations face increased risks from inbreeding depression. This occurs when closely related individuals mate, increasing the likelihood of their offspring inheriting two copies of harmful recessive alleles. Recessive genes can remain “hidden” in the population, expressed only when an individual inherits two copies. Inbreeding brings these hidden harmful alleles to the forefront, leading to reduced fitness, lower fertility, and decreased survival rates in the offspring.
Species Illustrating Low Diversity’s Impact
Several species demonstrate the consequences of low genetic diversity. Cheetahs (Acinonyx jubatus) are a prominent example, exhibiting extremely low genetic variation, possibly due to a severe population bottleneck thousands of years ago. This genetic uniformity makes them highly susceptible to diseases. For instance, a coronavirus-associated feline infectious peritonitis outbreak in an Oregon breeding colony in 1983 decimated about 50% of the cheetah population, while lions, with greater genetic variation, showed no symptoms. Cheetahs also experience difficulties in captive breeding, high juvenile mortality, and frequent sperm abnormalities, all linked to their limited genetic diversity.
The Florida panther (Puma concolor coryi) also illustrates the impact of low diversity, primarily due to inbreeding depression. By the early 1990s, the population was reduced to a mere 20-30 individuals, leading to health problems including heart defects, undescended testicles, kinked tails, and poor sperm quality. These issues were directly attributed to their limited genetic pool and the resulting inbreeding. Conservation efforts in 1995 introduced eight female pumas from Texas, a genetically more diverse subspecies. This significantly increased genetic variation and reversed many health problems, leading to a population increase.
Certain crop species also highlight this vulnerability. The Irish potato famine of the 1840s is a historical example where reliance on a single, genetically uniform potato variety led to disaster. When the protist Phytophthora infestans caused potato blight, nearly all potatoes were susceptible due to their shared genetic makeup, resulting in widespread crop failure and mass starvation. Similarly, the Southern corn leaf blight in the US in 1970 caused significant losses because most US corn was planted with a single, genetically similar type, making it highly vulnerable to a new fungal pathogen, Bipolaris maydis.
Drivers of Reduced Genetic Diversity
Several factors contribute to the reduction of genetic diversity within species. Population bottlenecks occur when a species undergoes a drastic reduction in its numbers, often due to environmental catastrophes or human activities. The small group of survivors carries only a fraction of the original population’s genetic variation. Even if the population rebounds in size, its genetic diversity remains low for many generations, leaving the population with a diminished gene pool and less potential for future adaptation.
Habitat loss and fragmentation are significant drivers of reduced genetic diversity. When habitats are destroyed or broken into isolated patches by human development, populations become separated. This isolation prevents gene flow between groups, leading to smaller, disconnected populations that lose genetic variation more rapidly through genetic drift. Over time, these isolated populations become less diverse and more susceptible to inbreeding and environmental challenges.
Selective breeding, particularly in domestic species, intentionally reduces genetic diversity to enhance specific desirable traits. Breeders choose individuals with particular characteristics to reproduce, which can lead to a uniform gene pool. This process can inadvertently eliminate other genetic variations, making the entire population susceptible to new diseases or environmental shifts. Modern crop varieties, for example, are often genetically uniform, replacing diverse traditional landraces.
The founder effect occurs when a new population is established by a very small number of individuals from a larger original population. This small founding group carries only a subset of the genetic diversity present in the source population, meaning the new colony will have inherently lower genetic variation. This leads to a distinct genetic composition that may be less robust than the original, more diverse population.