Natural selection is the foundational process of evolution, generally causing traits that improve survival and reproduction to become more common in a population over time. This standard process, known as directional selection, often leads to the reduction or elimination of less advantageous genetic variations. Balancing selection, however, represents specific mechanisms that actively work against this trend, maintaining multiple versions of a gene, or alleles, within a species. This process preserves genetic diversity (polymorphism), ensuring that no single allele becomes permanently fixed. The persistence of different traits provides a population with a deeper reservoir of genetic resources to draw upon when environmental conditions change.
Mechanisms That Maintain Genetic Variation
The maintenance of genetic variation relies on several ways that selection can favor diversity over uniformity. One major mechanism is heterozygote advantage, where an individual possessing two different alleles for a trait has higher biological fitness than individuals who are homozygous. This superior fitness of the intermediate genotype prevents either of the homozygous forms from being completely eliminated from the gene pool.
Another powerful mechanism is frequency-dependent selection, where the relative advantage of a phenotype depends directly on how common it is in the population. If the fitness of a trait increases as it becomes rarer, this negative frequency-dependence prevents its disappearance. Conversely, positive frequency-dependence, where a trait is favored when common, can also maintain specific forms, especially in systems like mimicry.
Genetic variation can also be maintained by selective pressures that vary across an organism’s environment or over time. If a single geographic range contains multiple distinct habitats, different alleles may be favored in different locations, leading to spatial heterogeneity. Similarly, if the optimal trait fluctuates from one generation to the next, this temporal variation ensures that no single allele can dominate indefinitely.
The Classic Example of Heterozygote Advantage
The most widely studied example of balancing selection in humans is the relationship between the sickle cell trait and resistance to malaria. This phenomenon centers on a mutation in the gene producing hemoglobin, the protein that carries oxygen in red blood cells. The normal allele is HbA, while the mutant allele, HbS, causes red blood cells to deform into a crescent shape under low oxygen conditions.
Individuals homozygous for the normal allele (HbAA) have no resistance to malaria and are highly susceptible to severe infection and death in endemic regions. Individuals homozygous for the mutant allele (HbSS) suffer from sickle cell disease, a severe condition causing chronic anemia, pain, and significantly reduced lifespan.
The fitness advantage emerges in the heterozygous state (HbAS), where a person carries one normal and one mutant allele. These individuals do not suffer from sickle cell disease but possess substantial resistance to the most severe symptoms of malaria.
When an HbS-containing red blood cell is infected by the parasite, the cell is more likely to sickle and be prematurely removed from circulation by the spleen. This mechanical removal disrupts the parasite’s life cycle before it can fully mature, limiting the severity of the infection.
The infection also causes oxidative stress, which triggers sickling and leads to a loss of potassium. Since the malaria parasite requires high levels of potassium to grow, this metabolic change suppresses parasite growth. The heterozygous genotype is thus the most successful in malaria-prone areas, balancing the high mortality from two different diseases and preventing the HbS allele from being eliminated.
Selection Based on Rarity or Commonness
Frequency-dependent selection is often seen in the dynamics between predators and their prey. This system typically involves negative frequency-dependence, where the selective pressure favors the less common form. For instance, a predator hunting by sight may form a “search image” based on the most frequently encountered prey pattern.
If an insect species has two color patterns, predators target the more abundant one, making the rarer pattern safer. As the rare form increases in frequency, the selective pressure flips to favor the other, now-rare morph. This constant cycle ensures that neither phenotype is eliminated, maintaining polymorphism.
This mechanism is also evident in the mating strategies of the male side-blotched lizard, which exhibits three distinct throat colors—orange, blue, and yellow—each corresponding to a different social behavior. Orange-throated males are aggressive territory holders, blue-throated males defend smaller territories, and yellow-throated males adopt a “sneaker” strategy, mimicking females to gain access to mates. The success of each strategy depends on the frequency of the others, leading to a rock-paper-scissors dynamic where no single morph can permanently dominate.
Maintaining Diversity Through Environmental Shifts
Fluctuating selection pressures across space and time maintain genetic diversity by preventing any single allele from becoming universally favored. In spatial variation, a species range is not uniform, containing diverse microclimates or soil types that create distinct local selection pressures. Different alleles are favored in different habitat patches, and gene flow between these areas prevents local adaptation from eliminating the less-favored allele from the wider population.
A classic illustration of spatial heterogeneity is found in the shell color and banding patterns of the European land snail, Cepaea nemoralis. Different habitats, such as open fields versus dense woodlands, contain different visual backgrounds and predators. For example, solid yellow shells may be camouflaged against green grass but visible against dark soil, while banded shells are favored in areas with dappled light. The diverse mix of local environments ensures that multiple shell color alleles are maintained across the species’ entire range.
Temporal variation occurs when selective conditions change over generations, sometimes cyclically. In plants, for instance, a specific allele might provide a strong advantage during a year of drought but become less advantageous during heavy rainfall. Since the climate fluctuates, selection favors one allele during dry periods and another during wet periods, preventing either from reaching fixation. This continuous shifting of the optimal trait preserves the population’s genetic capacity to survive a wide range of future conditions.