Polymorphism (biology)



In biology, polymorphism (from Greek: poly "many", morph "form") can be defined as discontinuous variation in a single population —in other words, the occurrence of more than one form or type of individual (whether the differences are visible or solely biochemical). The most obvious example of polymorphism is the sexual dimorphism of most higher organisms. Other examples are melanic and non-melanic forms of the same species (such as the Peppered Moth or the Jaguars shown at right).

Not all polymorphisms as defined above are genetic, but in genetics the term is reserved for variation in a population's DNA.

Rare variations are not classified as polymorphisms. The criterion may be that the frequency is too high to be the result of new mutations or simply that it is greater than 1 percent. In genetics, it may be that the most common allele (the major allele) has a frequency of 99% or less. For biallelic systems (also referred to as diallelic) the minor allele frequency (MAF) of the marker is 1% or greater.

Nomenclature
In zoology, the different forms can be called morphs or morphotypes, and the word "morpha" plus a Latin name for the morph can be added to a binomial or trinomial name. However, morphs are not recognized by the ICZN.

In botany, polymorphism is often named with the terms "variety", "subvariety", and "forms" which are formally regulated by the ICBN.

Ecology
Both natural and artificial selection change the frequency of polymorphisms; they occur when organisms of different types reproduce with different degrees of success. A polymorphism that persists over many generations is usually maintained because no form has an overall advantage or disadvantage compared to the others.

The segregation of a population into morphs, especially if the morphs are, or become, reproductively isolated, could be thought of as a precursor to sympatric speciation.

Examples
In some cases of polymorphism, the forms are visibly distinct. An example from botany is heterostyly, in which flowers occur in different forms having different arrangements of the pistil and the stamens. This polymorphism prevents inbreeding.

Visible polymorphisms in the scarlet tiger moth and in the African swallowtail Papilio dardanus (in which one of the eight races has 13 morphs) have been the subjects of considerable ecological and genetic study.

The example of the Pied Raven (Corvus corax varius morpha leucophaeus) demonstrates that although morphs do not form a discrete population, they can be suppressed to the point of extinction; incidentally, it also shows how complex zoological nomenclature can be.

The predatory mosquito Chaoborus americanus has two larval morphs, one large, yellow, and quick to develop; the other small, pale, and slow to develop. The larvae overwinter, and the polymorphism is an adaptation to unpredictable spring weather. The fast-developing larvae do well in warm springs, but die off in springtime freezes, whereas the slow-developing ones do well in springs with freezes. Many other insects with polymorphisms in development and dormancy are known.

An example of behavioral polymorphism is that anadromous individuals of brown trout (Salmo trutta morpha trutta) may occur sympatrically with a stream-resident ones (Salmo trutta morpha fario). The two morphs are part of the same species and may indeed interbreed. Nonetheless, the tendency to anadromy or to remaining stream-resident is innate in each of the morphs, indicating some degree of genetic control. Frequently, the likelihood of interbreeding is reduced because the morphs also have distinct reproductive behaviours such as variance in timing or in the selection of sites for reproduction.

The segregation of a species into sympatric morphs often can be thought of as a mechanism for the partitioning of available resources. An example of this might be certain lacustrine Arctic char populations which segregate into planktivorous and piscivorous morphs within the same lake. The planktivores feed on zooplankton in the epilimnion or on other invertebrates in the profundal zone. The piscivores feed on available forage fish (or, perhaps, on the planktivorous morph), usually over shoals in the metalimnion or by launching forays into the epilimnion. The planktivorous fish are smaller, grow more slowly and mature at a smaller size than do the piscivorous fish.

Other polymorphisms are detectable only chemically. A familiar example is human blood groups, classified are A, B, O, and AB. Another example is that several enzymes (e.g., phosphoglucomutase) taken from different individuals move with different speeds when exposed to an electric field (electrophoresis).

In genetics, polymorphisms may or may not affect an organism's phenotype. Examples include single-nucleotide polymorphisms and restriction-fragment-length polymorphisms. Polymorphisms that do not affect the phenotype are sometimes called neutral polymorphisms; the mutations that cause them are sometimes called silent mutations.

A related concept is sexual dimorphism. For instance, in humans, bones belonging to males and females (especially those of the pelvis) have different shapes.