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4.2 explain the differences between polymorphism and clinal gradation using at least one appropriate example from human phenotypes
explain the differences between polymorphism and clinal gradation using at least one appropriate example from human phenotypes
- Polymorphism is the presence of two or more distinct forms of a genetically determined character. Humans are polymorphic for skin colour, body stature, sickle-cell anaemia, blood groups and the epicanthic eye-fold. Polymorphic differences occur as selective adaptations to different environments.
- Clinal gradation occurs as a result of interbreeding and intermediate environments. Clinal gradation is seen in skin colour, eye colour and body stature. A cline is a gradual change in gene frequency in a population. This produces a gradual change in the phenotypic characteristics of a species over a geographical range.
- An example of polymorphism and clinal gradation in humans is the gradual increase in the frequency of blonde haired blue-eyed people as you move from the Mediterranean to the countries of Scandinavia. Eye colour and skin colour are both polymorphic. The clinal gradation that occurs is because of the existence of intermediate environments where different genetic factors have an advantage.
- The presence of these clines makes it difficult to maintain the concept of race. A race is a separate breeding population within a species. As the amount of trade, migration and air travel increases the concept of race is breaking down as people from around the world interbreed and produce offspring with intermediate features. In fact it is social and cultural differences rather than geographic barriers that keep populations apart.