Genetic polymorphism refers to the existence of two or more alleles (variants of a gene) at a specific locus within a population, where the frequency of the rarest allele is at least 1% (or sometimes 5%) in that population. This indicates that the variation is not due to a recent mutation but is maintained over generations. Genetic polymorphisms are crucial for understanding genetic diversity and evolution. They can be broadly categorized into two main types based on their stability and the selective forces acting upon them: balanced polymorphism and transient polymorphism.

1. Balanced Polymorphism:

• Definition: Balanced polymorphism is a stable type of genetic polymorphism where two or more alleles are maintained in a population over many generations at relatively constant frequencies. This stability is achieved because selective pressures favor the maintenance of multiple alleles, preventing any single allele from becoming fixed (reaching 100% frequency) or lost from the population.
• Mechanism: The most common mechanism for balanced polymorphism is heterozygote advantage (or overdominance). In this scenario, individuals who are heterozygous for a particular gene (carrying two different alleles) have a higher fitness (survival and reproductive success) than either of the homozygous genotypes (carrying two identical alleles). This selective advantage for heterozygotes ensures that both alleles are preserved in the gene pool.
• Example: Sickle Cell Anemia and Malaria Resistance: The classic example of balanced polymorphism in humans involves the allele for sickle cell hemoglobin (HbS) and its relationship with malaria. In regions where malaria is endemic:
  • Homozygotes for the normal allele (HbA/HbA): Are susceptible to malaria and suffer high mortality rates from the disease.
  • Homozygotes for the sickle cell allele (HbS/HbS): Develop severe sickle cell anemia, which is often fatal without modern medical intervention.
  • Heterozygotes (HbA/HbS): Do not develop severe sickle cell anemia and, crucially, exhibit significant resistance to malaria. The presence of one HbS allele provides protection against the malaria parasite, giving heterozygotes a survival advantage in malaria-prone environments.
  • Outcome: In these environments, natural selection favors the heterozygotes, leading to the maintenance of both the HbA and HbS alleles in the population at relatively stable frequencies, despite the severe disadvantage of the HbS/HbS genotype.
• Other Mechanisms: Other, less common mechanisms include frequency-dependent selection (where the fitness of a genotype depends on its frequency in the population) and varying selection pressures across different environments or times.

2. Transient Polymorphism:

• Definition: Transient polymorphism is a temporary type of genetic polymorphism where one allele is in the process of replacing another in a population. This occurs when one allele has a consistent and strong selective advantage over the other(s), leading to a directional change in allele frequencies over time.
• Mechanism: Transient polymorphism is driven by directional selection. An advantageous allele will increase in frequency, while a disadvantageous allele will decrease. If the selective pressure remains constant, the advantageous allele will eventually become fixed (reach 100% frequency), and the disadvantageous allele will be lost from the population, thus ending the polymorphism.
• Example: Industrial Melanism in Peppered Moths (Biston betularia):
  • Context: Before the Industrial Revolution in England, light-colored peppered moths were camouflaged against lichen-covered trees, while dark-colored (melanic) moths were easily spotted by predators (birds).
  • Shift: During the Industrial Revolution, pollution darkened tree trunks by killing lichens and depositing soot. The dark-colored moths then gained a selective advantage, as they were better camouflaged against the sooty trees. The frequency of the melanic allele rapidly increased in polluted areas.
  • Reversal: With improved air quality in recent decades, lichens have returned, and tree trunks have lightened. Consequently, the light-colored moths are once again favored, and the frequency of the melanic allele is decreasing.
  • Outcome: This represents a transient polymorphism because the allele frequencies are actively changing in response to a consistent selective pressure. If pollution had continued indefinitely, the melanic allele would likely have become fixed. The reversal of selection pressure has led to a new transient phase.

Key Differences:

The fundamental difference lies in the stability of allele frequencies. In balanced polymorphism, frequencies are stable due to opposing selective forces (often heterozygote advantage). In transient polymorphism, frequencies are changing directionally due to a consistent selective advantage for one allele, leading towards fixation or loss of alleles. Balanced polymorphism represents a state of equilibrium, while transient polymorphism represents a state of flux.