Selection, whether natural or artificial, changes the frequency of morphs within a population; this occurs when morphs reproduce with different degrees of success. A genetic (or balanced) polymorphism usually persists over many generations, maintained by two or more opposed and powerful selection pressures. Non-human apes have similar blood groups to humans; this strongly suggests that this kind of polymorphism is ancient, at least as far back as the last common ancestor of the apes and man, and possibly even further. in
Hawaii is partly a result of
apostatic selection. The relative proportions of the morphs may vary; the actual values are determined by the
effective fitness of the morphs at a particular time and place. The mechanism of
heterozygote advantage assures the population of some alternative
alleles at the
locus or loci involved. Only if competing selection disappears will an allele disappear. However, heterozygote advantage is not the only way a polymorphism can be maintained.
Apostatic selection, whereby a predator consumes a common morph whilst overlooking rarer morphs is possible and does occur. This would tend to preserve rarer morphs from extinction. 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. Polymorphism is one good way the opportunities get to be used; it has survival value, and the selection of modifier genes may reinforce the polymorphism. In addition, polymorphism seems to be associated with a higher rate of
speciation.
Polymorphism and niche diversity G. Evelyn Hutchinson, a founder of niche research, commented "It is very likely from an ecological point of view that all species, or at least all common species, consist of populations adapted to more than one niche". He gave as examples sexual size dimorphism and mimicry. 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. Size difference may permit both sexes to exploit different niches. In elaborate cases of
mimicry, such as the African butterfly
Papilio dardanus, female morphs mimic a range of distasteful models called Batesian mimicry, often in the same region. The fitness of each type of mimic decreases as it becomes more common, so the polymorphism is maintained by frequency-dependent selection. Thus the efficiency of the mimicry is maintained in a much increased total population. However it can exist within one gender. 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. The researchers found that male mate preference is controlled by frequency-dependent selection, which means that the rare morph suffers less from mating attempt than the common morph. The reasons why females try to avoid male sexual harassment are that male mating attempt can reduce female fitness in many ways such as fecundity and longevity.
The switch The mechanism which decides which of several morphs an individual displays is called the
switch. This switch may be genetic, or it may be environmental. Taking sex determination as the example, in humans the determination is genetic, by the
XY sex-determination system. In
Hymenoptera (
ants,
bees and
wasps), sex determination is by haplo-diploidy: the females are all
diploid, the males are
haploid. However, in some animals an environmental trigger determines the sex:
alligators are a famous case in point. Polymorphism with an environmental trigger is called
polyphenism. The polyphenic system does have a degree of environmental flexibility not present in the genetic polymorphism. However, such environmental triggers are the less common of the two methods.
Investigative methods Investigation of polymorphism requires use of both field and laboratory techniques. In the field: • detailed survey of occurrence, habits and predation • selection of an ecological area or areas, with well-defined boundaries •
capture, mark, release, recapture data • relative numbers and distribution of morphs • estimation of population sizes And in the laboratory: • genetic data from crosses • population cages •
chromosome cytology if possible • use of
chromatography, biochemistry or similar techniques if morphs are cryptic Without proper field-work, the significance of the polymorphism to the species is uncertain and without laboratory breeding the genetic basis is obscure. Even with insects, the work may take many years; examples of
Batesian mimicry noted in the nineteenth century are still being researched. == Relevance for evolutionary theory ==