10.3 Gene pools and speciation


If a population has a gene where there are two or more alleles it is known as polymorphic.

Transient polymorphism

If one allele is gradually replacing the other (or other) then this is known as transient polymorphism.

The peppered moth Biston betularia is a good example of this process. During the 19 th century as industrialisation progressed, the colouration of lichen covered tree bark and walls became darker.

The population which was polymorphic contained a small proportion of darker (melanic) forms due to a mutant dominant allele. The proportion of the melanic allele increased as the darkening of the trees and walls favoured these individuals and meant the more common allele became a disadvantage as the moths were not as well hidden or camoflagued.

Balanced polymorphism

This where the allele frequencies stay relatively constant to each other. A good example is sickle cell anaemia. There are two alleles HbS which leads to the sickle shaped blood cells and HbA which leads to the usual round shaped blood cells.

However the HbS allele can lead to decreased life chances due to insufficient oxygen transport but also gives some immunity to the malarial parisite plasmodium.

The HbA HbA have no immunity to malaria but better oxygen transport.

The HbS HbS have resistance to malaria but can suffer from sickle cell anemia

The HbA HbS have less risk of malaria and sickle cell anaemia

This means that the natural selective advatages and disadvantages can lead to a balancing effect that retains the polymorphism in the population. In some parts of the world up to 40 per cent of the population has one copy of the HbS allele.


HL Topic 10 Genetics Copyright © by Alex Black. All Rights Reserved.

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