Genetic polymorphism occurs when multiple distinct forms of a gene, or alleles, exist within a population at frequencies too high to be explained by new mutation alone. When natural selection actively maintains these different forms in a stable ratio across generations, the result is known as balanced polymorphism. This state represents a dynamic equilibrium where no single allele is driven to complete loss or fixation. The different genetic versions persist indefinitely within the gene pool because selection pressures favor genetic variation, establishing permanent diversity.
How Genetic Balance is Maintained
The most common theoretical mechanism for maintaining balanced polymorphism is heterozygote advantage, also known as overdominance. This occurs when individuals who inherit two different versions of an allele—the heterozygous genotype—possess a higher biological fitness than individuals who are homozygous for either allele. The selection pressure specifically favors the mixed genotype, which consequently prevents the elimination of either of the less-fit homozygous forms from the population.
To illustrate this, consider a gene with two alleles, \(A_1\) and \(A_2\), leading to three genotypes: \(A_1A_1\), \(A_2A_2\), and \(A_1A_2\). If the heterozygous \(A_1A_2\) genotype has the highest survival and reproductive rate, it will be selected for by nature. Individuals with this superior mixed genotype will pass on both the \(A_1\) and \(A_2\) alleles to the next generation.
This process ensures that alleles causing reduced fitness in their homozygous states are continuously cycled back into the gene pool via the successful heterozygotes. Since the \(A_1A_2\) individual is the most successful at reproduction, both the \(A_1\) and \(A_2\) alleles are protected from being lost through natural selection. This results in a stable equilibrium frequency for both alleles.
The Classic Example of Sickle Cell Trait
The most extensively studied example of balanced polymorphism driven by heterozygote advantage involves the gene for \(beta\)-hemoglobin and its relationship with malaria. This gene has a normal allele, \(text{HbA}\), and a mutant allele, \(text{HbS}\), which causes sickle cell disease when two copies are inherited. The selective pressure that maintains the \(text{HbS}\) allele at high frequencies in certain populations is the parasitic disease malaria, specifically the form caused by Plasmodium falciparum.
The fitness trade-off is stark across the three possible genotypes in regions where malaria is common. Homozygous individuals for the normal allele (\(text{HbA}text{HbA}\) or AA) produce healthy red blood cells but are highly susceptible to severe malarial infection. Conversely, those homozygous for the mutant allele (\(text{HbS}text{HbS}\) or SS) develop sickle cell anemia, a severe blood disorder that significantly reduces life expectancy due to chronic pain and organ damage.
The heterozygous individuals (\(text{HbA}text{HbS}\) or AS), known as carriers of the sickle cell trait, exhibit the highest fitness in malarial environments. Their red blood cells are mostly normal but contain enough \(text{HbS}\) protein to briefly sickle when infected. This sickling causes the parasite to be eliminated by the immune system, granting the carrier significant resistance to severe malaria.
The heterozygous AS genotype avoids both the high mortality risk of malaria and the severe health consequences of sickle cell anemia. This superior fitness ensures the harmful \(text{HbS}\) allele is maintained in the population, sometimes reaching frequencies of 10 to 20 percent in endemic regions of Africa. This balance demonstrates how a lethal genetic disease can persist because the gene offers a survival advantage against an infectious agent.
Balancing Selection Beyond Heterozygote Advantage
While heterozygote advantage is a key mechanism, balanced polymorphism can also be maintained by other forms of balancing selection. Frequency-dependent selection (FDS) occurs when the fitness of a trait depends on how common it is in the population. In negative FDS, a trait has a survival advantage when it is rare, but that advantage diminishes as the trait becomes more widespread.
A classic illustration of negative FDS occurs in host-parasite interactions or predator-prey dynamics. For instance, a predator may develop a “search image” for the most common color pattern of its prey, making that common morph more vulnerable to being eaten. The rarer color morphs are then relatively protected, allowing their frequency to rise until they, too, become common and the cycle repeats. This constant shift in selective pressure prevents any single phenotype from dominating and maintains a stable mix of morphs.
Furthermore, environmental heterogeneity can also drive balanced polymorphism by creating a patchwork of selective pressures across space or time. If one allele is favored in hot, dry conditions and another is favored in cool, moist conditions, both alleles can persist across a species’ range that encompasses both environments. Similarly, if the selection pressure fluctuates seasonally or over many generations, no single allele can achieve fixation, keeping multiple genetic forms viable for the species’ long-term benefit.
Why Genetic Diversity Matters for Survival
The maintenance of multiple alleles through balanced polymorphism holds significant implications for the long-term survival of a species. By preserving genetic diversity, these mechanisms ensure a population has a broader reservoir of traits, even those detrimental in one context but beneficial in another. This pool of variation is sometimes referred to as the genetic load, which is the standing capacity for a species to adapt.
When a species faces a sudden, novel threat, such as a new disease or an abrupt shift in climate, this diversity becomes an asset. A genetically diverse population is more likely to possess individuals with the right combination of alleles to survive and reproduce under the new conditions. This pre-existing variation allows the population to respond rapidly to environmental changes, offering a significant advantage over genetically similar populations. Ultimately, balanced polymorphism acts as an evolutionary insurance policy, enabling a species to maintain resilience and adaptability.