Back Home

4.3 gather and process information from secondary sources to describe two examples of polymorphism in humans and analyse the evolutionary significance of the phenotypes displayed

gather and process information from secondary sources to identify two examples of polymorphism in humans and analyse the evolutionary significance of the phenotypes displayed.

Background information

It is very difficult to analyse the evolutionary significance of polymorphic features. Many features are trivial when considered as survival adaptations. Curly hair is an example of a polymorphic feature in humans but it would not necessarily affect the ability of the individual to reproduce and pass on genes to the next generation. Sickle-cell anaemia is a good example of the environment influencing the gene pool. Malaria is a disease that kills millions of people each year. It is a major selective agent in regions where the malaria parasite is present. The sickle-cell anaemia gene is deleterious to the individuals that possess it but it gives protection from malaria in the heterozygous form.

Example 1 Skin colour
Skin colour in humans is controlled by a number of gene pairs. The pigment melanin absorbs ultraviolet light. People with dark skin who live in areas where there is a high level of solar radiation have a selective advantage over those with paler skin. The melanin in the skin prevents the sun’s rays from penetrating to the lower layers of the skin and protects against skin cancer. In areas where the levels of solar radiation are low then light skin had a selective advantage. This is because of Vitamin D production. Vitamin D is required by the body for calcium absorption. Without Vitamin D a disease called rickets can occur. Lighter skin absorbs more ultraviolet radiation and this increases the production of Vitamin D. White skin is better at producing Vitamin D in low levels of solar radiation while dark skin prevents skin cancer in areas that have high levels of solar radiation.

Example 2 Body stature
Body stature in humans shows polymorphism. Tall thin body stature is common in hot tropical areas to allow a greater surface area to volume ratio for heat loss. A stocky short body is found in groups such as the Inuits (Eskimos). The stocky body with a smaller surface area to volume ratio allows better heat retention.

Example 3 Sickle-cell anaemia
Sickle-cell anaemia is a human genetic disease. It produces red blood cells that are folded and have a sickle shape. These cells are less efficient at carrying oxygen around the body; they block the capillaries causing intense pain and death of the surrounding cells. Few people with the disease live past ten years of age. Those who are homozygous for the disease die before they can pass the gene on but those that only carry one gene for sickle-cell anaemia (heterozygous) have a resistance to the disease malaria. Therefore the heterozygous form has an adaptive advantage in areas that have malaria present. In parts of Africa that have malaria the population is 40% heterozygous for sickle-cell anaemia.