Differentiate between transient and balanced genetic polymorphism. Illustrate your answer with suitable examples from human populations.

Defining Genetic Polymorphism

Genetic polymorphism refers to the presence of two or more distinct alleles for a particular gene in a population, each occurring at a frequency greater than 1%. It is a key source of genetic variation and is the raw material for evolutionary change. The existence of polymorphism is not a static phenomenon; alleles can be lost, fixed, or maintained in a population depending on the interplay of evolutionary forces.

Transient Genetic Polymorphism

Transient polymorphisms, also known as non-adaptive polymorphisms, are alleles that have recently arisen by mutation. They are typically present at low frequencies and are not under strong selective pressure. These alleles are either eliminated from the population due to their lack of benefit or eventually become fixed (reach a frequency of 100%) through genetic drift, especially in smaller populations.

• Characteristics:
  • Recent origin through mutation
  • Low allele frequency
  • Neutral or slightly deleterious effect
  • Short lifespan – either lost or fixed
• Example: The rare allele for blood group O in certain isolated populations might initially have been a transient polymorphism. If it offered no advantage and was not subject to balancing selection, it could have been lost or become fixed depending on the population's size and genetic drift. The frequency of a newly arisen recessive disease allele is also an example.

Balanced Genetic Polymorphism

Balanced polymorphisms, also referred to as adaptive polymorphisms, are maintained at relatively stable frequencies within a population for extended periods. This maintenance occurs because different alleles confer different advantages or disadvantages under varying environmental conditions or have complex genetic interactions. The forces maintaining these polymorphisms are often opposing selective pressures.

• Characteristics:
  • Stable allele frequencies over time
  • Adaptive significance – different alleles are favored under different conditions
  • Opposing selective pressures maintain multiple alleles
• Mechanisms Maintaining Balance:
  • Heterozygote Advantage: Individuals with a heterozygous genotype have a higher fitness than either homozygous genotype.
  • Frequency-Dependent Selection: The fitness of an allele depends on its frequency in the population.
  • Oscillating Selection: Environmental conditions fluctuate, favoring different alleles at different times.
  • Negative Frequency-Dependent Selection: Rare alleles are favored because predators or parasites exploit common alleles.
• Examples:
  • Sickle-Cell Anemia and Malaria: The sickle-cell allele (HbS) confers resistance to malaria in heterozygous individuals (HbA/HbS). In regions where malaria is prevalent, the HbS allele is maintained at a relatively high frequency despite causing sickle-cell anemia in homozygous individuals (HbS/HbS).
  • Drosophila ABO blood group polymorphism: In Drosophila, the ABO blood group system exhibits balanced polymorphism due to frequency-dependent selection. Rare blood types are less susceptible to parasitoid wasps, giving them a selective advantage.
  • ABO Blood Groups in Humans: The human ABO blood group system is a classic example of balanced polymorphism. The A, B, and O alleles are maintained due to complex interactions with infectious diseases and potentially other factors. The O allele, for example, provides some protection against cholera.

Comparison Table: Transient vs. Balanced Polymorphism

Feature	Transient Polymorphism	Balanced Polymorphism
Origin	Recent mutation	Older, established alleles
Frequency	Low	Relatively stable and often moderate
Selective Pressure	Neutral or slightly deleterious	Adaptive, with opposing pressures
Lifespan	Short – lost or fixed	Long – maintained over generations
Example	Rare recessive disease alleles	ABO blood groups in humans

Case Study: Lactase Persistence

The evolution of lactase persistence (LP) in human populations is an excellent example of a shifting polymorphism. Initially, most human populations lost the ability to digest lactose after weaning (lactase non-persistence, LNP). However, in a few populations, particularly in Europe, mutations arose that allowed for continued lactase production into adulthood. The selective advantage of LP was increased milk consumption, especially during times of famine or drought. As dairy farming became more prevalent, LP became more common, demonstrating a transition from a rare, transient polymorphism to a more balanced, adaptive one. The spread of LP highlights the dynamic nature of genetic variation.

Significance in Anthropology

Understanding the difference between transient and balanced polymorphisms is crucial for reconstructing human evolutionary history and understanding adaptation to different environments. The study of these polymorphisms provides insights into population structure, migration patterns, and the interplay between genes and culture.