Briefly comment on Balanced and Transient Polymorphism with suitable examples.

1. Introduction to Polymorphism:

Polymorphism refers to the presence of multiple genetic variants or alleles at a specific locus within a population. These variants may confer different phenotypic traits or molecular characteristics, contributing to genetic diversity within populations. Polymorphisms are important for evolutionary processes, adaptation to changing environments, and maintenance of population fitness.

2. Balanced Polymorphism:

Balanced polymorphism occurs when multiple alleles at a locus are maintained in a population over time, often due to natural selection favoring heterozygotes. This equilibrium is sustained by various mechanisms that preserve genetic variation and prevent the fixation of a single allele. Balanced polymorphisms can arise through different processes, including heterozygote advantage, frequency-dependent selection, and spatial or temporal variation in selection pressures.

Example of Balanced Polymorphism:

One classic example of balanced polymorphism is the sickle cell trait (HbAS) in humans, which confers resistance to malaria when present in the heterozygous state (HbAS). In regions where malaria is endemic, individuals with the sickle cell trait have a selective advantage due to reduced susceptibility to malaria infection compared to individuals with normal hemoglobin (HbAA). However, individuals homozygous for the sickle cell allele (HbSS) suffer from sickle cell anemia, a severe and often fatal genetic disorder. The heterozygote advantage maintains the balanced polymorphism, as the benefits of malaria resistance outweigh the costs of sickle cell anemia.

3. Transient Polymorphism:

Transient polymorphism refers to the temporary maintenance of multiple alleles at a locus within a population, followed by eventual fixation of one allele or loss of genetic variation. Unlike balanced polymorphism, transient polymorphism does not persist over evolutionary timescales and may result from stochastic processes, genetic drift, or fluctuating selection pressures.

Example of Transient Polymorphism:

A classic example of transient polymorphism is the peppered moth (Biston betularia) in England during the Industrial Revolution. Prior to industrialization, the light-colored morph of the peppered moth predominated in natural habitats, providing camouflage against light-colored tree bark. However, with the widespread pollution and darkening of tree bark due to industrial activities, the dark-colored morph became more advantageous, as it was better camouflaged against the darker background. This led to a transient polymorphism, with both light and dark morphs coexisting in the population. Subsequent environmental regulations and improvements in air quality resulted in a reversal of selection pressures, leading to the decline of the dark morph and the predominance of the light morph once again. In this case, transient polymorphism occurred due to fluctuating selection pressures driven by human-induced environmental changes.

Conclusion:

Balanced and transient polymorphisms are important concepts in population genetics and evolutionary biology, reflecting the dynamic nature of genetic variation within populations. Balanced polymorphisms are maintained by selective pressures that favor the persistence of multiple alleles, often resulting in heterozygote advantage or frequency-dependent selection. In contrast, transient polymorphisms arise from temporary fluctuations in selection pressures, genetic drift, or environmental changes, leading to the temporary maintenance of multiple alleles before eventual fixation or loss of genetic variation. Understanding the mechanisms underlying balanced and transient polymorphisms provides insights into the evolutionary processes shaping genetic diversity and adaptation within populations.