Evolution | Genetics | Transient polymorphism | Industrial Melanism
TRANSIENT POLYMORPHISM
Transient polymorphism is found in populations undergoing a strong selection pressure and one form or morph is being strongly favoured and the other is getting eliminated. The transient polymorphism represents a temporary condition. It lasts for a short period till the disadvantageous allele is either completely eliminated or is reduced to such a low frequency that it is only retained due to recurrent mutation.
Transient polymorphism is seen under following situations:
1. A newly arisen mutation is found to be advantageous and is favoured by selection. It spreads through the population and becomes more abundant, while the wild type becomes rare due to negative selection.
2. In a changed environment, a previously rare allele may become advantageous and is favoured by selection. It spreads throughout the population.
Examples:
1. Industrial Melanism: Industrial Melanism in peppered moth, Biston betularia, is an example of transient polymorphism.
Up to 1848, peppered moths found in and around Manchester in the nonsooty atmosphere were creamy white with black dots and darkly shaded areas. They were called nonmelanic forms. In 1848, a single black coloured moth appeared among nonmelanic forms. This melanic form is called carbonaria. By 1895, about 98% population of peppered moth in Manchester was melanic.
The black or melanic form arose by a recurring random mutation. Up to 1848, in the nonsooty atmosphere of Manchester, the tree trunks were covered with light coloured lichens. The nonmelanic forms were able to successfully camouflage with the lichens while resting and were able to survive, while the melanic forms were easily spotted out and preyed upon and were eliminated. As a result of rapid industrialization in Great Britain, large amount of soot and other gases were poured into the atmosphere. These made the environment sooty and dark and destroyed the lichens. In the dark environment light coloured nonmelanic moths were easily spotted by the predators and were being removed.
Up to 1848, peppered moths found in and around Manchester in the nonsooty atmosphere were creamy white with black dots and darkly shaded areas. They were called nonmelanic forms. In 1848, a single black coloured moth appeared among nonmelanic forms. This melanic form is called carbonaria. By 1895, about 98% population of peppered moth in Manchester was melanic.
The black or melanic form arose by a recurring random mutation. Up to 1848, in the nonsooty atmosphere of Manchester, the tree trunks were covered with light coloured lichens. The nonmelanic forms were able to successfully camouflage with the lichens while resting and were able to survive, while the melanic forms were easily spotted out and preyed upon and were eliminated. As a result of rapid industrialization in Great Britain, large amount of soot and other gases were poured into the atmosphere. These made the environment sooty and dark and destroyed the lichens. In the dark environment light coloured nonmelanic moths were easily spotted by the predators and were being removed.
The transient polymorphism in peppered moth was only for a short period and was favoured by strong selection. The transient polymorphism is seen during directional selection, where one form or character is gradually replaced by another one.
2. An intermediate variety of peppered moth is called insularia. The insularia pattern is dominant and is controlled by a different allelomorph at the same locus. Carbonaria is dominant to insularia which in tum is dominant to nonmelanic form.
The distribution of insularia also indicates transitional polymorphism. The insularia is also a melanic form but is found in the marginal regions where industrial pollution is less and carbonaria form is still rare. It means during early stages of pollution, insularia form became advantageous.
2. An intermediate variety of peppered moth is called insularia. The insularia pattern is dominant and is controlled by a different allelomorph at the same locus. Carbonaria is dominant to insularia which in tum is dominant to nonmelanic form.
The distribution of insularia also indicates transitional polymorphism. The insularia is also a melanic form but is found in the marginal regions where industrial pollution is less and carbonaria form is still rare. It means during early stages of pollution, insularia form became advantageous.
source: Organic Evolution (Evolutionary Biology), 13th edition, Medtech, by Veer Bala Rastogi , pg 249