Polymorphism (biology)
In biology, polymorphism[1] is the occurrence of two or more clearly different morphs or forms, also referred to as alternative phenotypes, in the population of a species.
To be classified as such, morphs must occupy the same habitat at the same time and belong to a panmictic population (one with random mating).
The term polymorphism also refers to the occurrence of structurally and functionally more than two different types of individuals, called zooids, within the same organism.
Polymorphism crosses several discipline boundaries, including ecology, genetics, evolution theory, taxonomy, cytology, and biochemistry.
In botanical taxonomy, the concept of morphs is represented with the terms "variety", "subvariety" and "form", which are formally regulated by the ICN.
This shows that polymorphisms are found to be at least as common as continuous variation in studies of natural selection, and hence just as likely to be part of the evolutionary process.
[citation needed] In simple words, the term polymorphism was originally used to describe variations in shape and form that distinguish normal individuals within a species from each other.
Presently, geneticists use the term genetic polymorphism to describe the functionally silent differences in DNA sequence between individuals that make each human genome unique.
Cases occur where a gene affects an unimportant visible characteristic, yet a change in fitness is recorded.
Pleiotropism is posing continual challenges for many clinical dysmorphologists in their attempt to explain birth defects which affect one or more organ system, with only a single underlying causative agent.
This process might involve suppression of crossing-over, translocation of chromosome fragments and possibly occasional cistron duplication.
[22] John Maynard Smith agreed with this view in his authoritative textbook,[7] but the question is still not definitively settled.
A genetic (or balanced) polymorphism usually persists over many generations, maintained by two or more opposed and powerful selection pressures.
[6] Diver (1929) found banding morphs in Cepaea nemoralis could be seen in prefossil shells going back to the Mesolithic Holocene.
The mechanism of heterozygote advantage assures the population of some alternative alleles at the locus or loci involved.
Polymorphism is strongly tied to the adaptation of a species to its environment, which may vary in colour, food supply, and predation and in many other ways including sexual harassment avoidance.
In many cases where the male is short-lived and smaller than the female, he does not compete with her during her late pre-adult and adult life.
Cook et al. (1994)[28] argued that the male-like phenotype in some females in P. dardanus population on Pemba Island, Tanzania functions to avoid detection from a mate-searching male.
In Hymenoptera (ants, bees and wasps), sex determination is by haplo-diploidy: the females are all diploid, the males are haploid.
Even with insects, the work may take many years; examples of Batesian mimicry noted in the nineteenth century are still being researched.
Polymorphism was crucial to research in ecological genetics by E. B. Ford and his co-workers from the mid-1920s to the 1970s (similar work continues today, especially on mimicry).
The work started at a time when natural selection was largely discounted as the leading mechanism for evolution,[31][32] continued through the middle period when Sewall Wright's ideas on drift were prominent, to the last quarter of the 20th century when ideas such as Kimura's neutral theory of molecular evolution was given much attention.
Evidence can be seen in Mayr's famous book Animal Species and Evolution,[34] and Ford's Ecological Genetics.
[4] Similar shifts in emphasis can be seen in most of the other participants in the evolutionary synthesis, such as Stebbins and Dobzhansky, though the latter was slow to change.
