Polymorphism refers to the presence of two or more distinct forms which exist together within a single breeding population of a species. The forms are discontinuous, meaning that the population lacks individuals that are intermediate. Polymorphisms are found in many plants and animals and often exist at a frequency too high to be maintained solely by mutation. Polymorphisms are known to exist either as obvious physical variations easily detected by examination of the body of the organism, enzymes, or proteins of those organisms, and as chromosomal or deoxyribonucleic acid (DNA) variants.
The term discontinuous is essential to an understanding of polymorphism. Most traits of organisms exist in a continuous gradation between extremes. Consider pigment and height in human populations. Skin pigment varies from a deep brown to various light hues. Similarly, height in humans varies from short to extremely tall. The existence of innumerable intermediate forms in both height and pigmentation qualifies those traits as being continuously variant. Other human traits have no intermediates. Consider the human ABO blood type polymorphism. Humans may be classified as blood type A, B, AB, or O. Intermediates of those blood types do not exist. Blood type is determined by heredity and this is true of other types of polymorphism. Environmental factors such as diet and ultraviolet radiation (uv) may affect height and pigment respectively, however environment does not change the genetic determinate for ABO blood types. Thus, they are not affected by what a human may or may not be exposed to.
Pigment pattern polymorphisms exist in populations of the northern leopard frog, Rana pipiens. The leopard frog ordinarily has spots on its back and limbs. There is a remarkable variation in the number and size and shape of the spots; it is appropriate to state that spotting is a continuously variable trait of the leopard frog. However, a spotless variant, known as the burnsi morph, exists in Minnesota and contiguous states. The spotless frog is not an albino. It has pigment cells, but the cells do not aggregate into the well recognized spots of this common frog. Genetic studies reveal that the burnsi morph differs from ordinary spotted frogs by the possession of a single dominant Mendelian allele. The burnsi morph of the leopard frog seems to persist at a relatively stable frequency in many Minnesota populations, creating a polymorphic population. It is difficult to understand the biological significance of the balanced stability where it occurs, however it seems that the burnsi gene might convey an advantage when relatively rare but not when it becomes more common.
The kandiyohi morph is another example of pigment pattern polymorphism in some populations of R. pipiens in Minnesota. Kandiyohi frogs are identified by small mottled patches of pigment between the usual dark spots that characterize leopard frogs. The kandiyohi morph differs from ordinary spotted frogs by possession of a single dominant Mendelian allele. Intermediates with pigment patterns between ordinary spotted frogs and the kandiyohi frog do not exist. They are sharply distinct, and thus, kandiyohi is another example of polymorphism in leopard frogs.
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