Genetic diversity is the total range of variation found within the genetic material of all organisms in a population or species. This variation is the raw material upon which evolutionary forces act, allowing a population to adapt to environmental changes over time. A greater pool of varied genes provides the resilience necessary for a species to survive disease outbreaks or shifts in climate. Conversely, a reduction in this diversity lessens a population’s capacity for long-term survival.
Understanding the Concept of Genetic Diversity
The foundation of genetic diversity lies in the presence of different versions of a gene, known as alleles, across the individuals in a population. A population is considered genetically diverse when there is a high degree of polymorphism, which means many gene locations have more than one common allele. The most direct way scientists measure this variation is through calculating heterozygosity.
Heterozygosity refers to the probability that an individual possesses two different alleles at a specific location on their chromosomes. When an individual has two identical alleles, they are homozygous for that gene. A high rate of heterozygosity across a population indicates a large and varied gene pool.
Researchers estimate genetic diversity by examining allele frequencies at many different genomic locations, often using molecular markers. Expected heterozygosity compares the observed level of variation to what would be expected in a randomly mating population. A significant difference between the observed and expected values can signal non-random mating, such as inbreeding, which results in a shift toward homozygosity.
Mechanisms That Reduce Genetic Variation
The reduction of genetic variation is often a consequence of several biological and human-driven processes. One such process is genetic drift, which describes the random fluctuation of allele frequencies from one generation to the next. Genetic drift is particularly influential in small populations, where a random event can cause an allele to be lost entirely, permanently reducing the gene pool.
A population bottleneck is a specific event where a population’s size is drastically reduced by a non-selective event, like a natural disaster. The small group of survivors may not carry the full spectrum of the original genetic diversity, and their limited genetic profile becomes the foundation for all future generations. The Founder Effect is a similar process, occurring when a small subset of a population separates to establish a new, isolated group.
In human-managed populations, artificial selection and inbreeding are the primary mechanisms for reducing diversity. Artificial selection involves choosing individuals with specific, desired traits to breed, which concentrates those genes while discarding others. Inbreeding, the mating of closely related individuals, rapidly increases the rate of homozygosity across the entire genome, accelerating the loss of variation.
The English Bulldog: A Case Study in Extreme Selection
The English Bulldog represents an example of how extreme artificial selection can narrow a breed’s genetic foundation. The breed’s history began with its use in the sport of bull-baiting, where dogs were selected for tenacity and specific physical traits. After the sport was banned in 1835, the breed underwent an intense genetic bottleneck, with a small number of surviving individuals forming the basis for the modern dog.
Breeders then selected heavily for the exaggerated physical traits that define the modern Bulldog, such as the short muzzle, heavy-set body, and characteristic underbite. Scientific analysis of the breed’s DNA confirms that this selection has led to low genetic diversity. One study showed that the breed’s gene pool is dominated by only four paternal haplotypes, with a single one being present in 93% of the dogs analyzed.
Researchers found that the Bulldog genome contains numerous large regions of “runs of homozygosity,” where both copies of the chromosome are identical. The study concluded that the breed has so little genetic variation remaining that there is limited capacity for breeders to make significant genetic improvements to health from within the existing population. This fixity means that genes for desirable physical traits are inextricably linked with genes for many health issues.
Health Consequences of Limited Gene Pools
The lack of genetic diversity results in a high prevalence of fixed, deleterious recessive traits throughout the breed. While harmful recessive alleles are often masked by a dominant, healthy allele in diverse populations, the high homozygosity in Bulldogs means these traits are frequently expressed. This genetic homogeneity directly contributes to the breed’s characteristic health problems, which are reflected in their median lifespan of only 8.4 years.
One of the most widely recognized issues is Brachycephalic Obstructive Airway Syndrome (BOAS), a condition resulting from the breed’s shortened skull structure that causes severe breathing difficulties. The breed is also highly susceptible to chondrodysplasia, a skeletal disorder that results in conditions like hip and elbow dysplasia, contributing to mobility issues.
Furthermore, the Bulldog’s immune system is compromised due to the lack of variation in the Dog Leukocyte Antigen (DLA) complex genes. The DLA complex is responsible for regulating immune responses, and studies show that the Bulldog population has low DLA diversity compared to other dogs. This lack of immune repertoire leaves the dogs vulnerable to autoimmune diseases, allergies, and chronic dermatological infections in their characteristic skin folds.