Genetic polymorphism and natural selection (Answer in 150 words)

- Genetic polymorphism and natural selection (Answer in 150 words)
Introduction
Polymorphism is the persistence of two or more alleles at a locus at frequencies too high to be explained by recurrent mutation. Whether such variation is kept or removed is decided largely by selection.
Body
1. What it is: Ford's criterion is two or more discontinuous forms in a population with the rarest above about one per cent, recognised from allele frequencies and from departures from Hardy-Weinberg expectation.
2. How selection acts: balanced polymorphism is maintained by heterozygote advantage, by frequency-dependent selection favouring the rarer type, and by selection varying in space and time. Directional selection instead erodes variation, producing transient polymorphism while an allele is replaced.
3. Human demonstrations: HbS is the classic case, the AS heterozygote resisting falciparum malaria, with carrier frequencies reaching a fifth in parts of central India's tribal belt. G6PD deficiency and the Duffy null allele repeat the pattern; lactase persistence is directional selection caught in progress.
Conclusion
Selection is thus both the eliminator and the preserver of variation. The malaria-linked haemoglobin systems remain the best-evidenced instance of balancing selection in our own species.