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This is an example of balancing selection between the fierce selection against homozygous sickle-cell sufferers, and the selection against the standard HgbA homozygotes by malaria. The heterozygote has a permanent advantage (a higher fitness) wherever malaria exists.
For example, Obelia has feeding individuals, the gastrozooids; the individuals capable of asexual reproduction only, the gonozooids, blastostyles; and free-living or sexually reproducing individuals, the medusae. Balanced polymorphism refers to the maintenance of different phenotypes in population.
An example of a botanical genetic polymorphism is heterostyly, in which flowers occur in different forms with different arrangements of the pistils and the stamens. The system is called heteromorphic self-incompatibility , and the general 'strategy' of stamens separated from pistils is known as herkogamy .
Frequency-dependent selection may explain the high degree of polymorphism in the MHC. [12] In behavioral ecology, negative frequency-dependent selection often maintains multiple behavioral strategies within a species. A classic example is the Hawk-Dove model of interactions among individuals in a population.
Polymorphism can be maintained by selection favoring the heterozygote, and this mechanism is used to explain the occurrence of some kinds of genetic variability. A common example is the case where the heterozygote conveys both advantages and disadvantages, while both homozygotes convey a disadvantage.
Balanced polymorphism could arise when a predator like the song thrush develops a 'search image' for the commonest morph, so that the rarer morphs are less likely to be predated. Natural selection would then favour a diversity of colours and patterns as an antipredator adaptation. Most probably, the polymorphism has multiple causes. [10] [16]
If predators switch between host plants that provide toxins and plants that do not, depending on the abundance of larvae on each type, then automimicry of toxic larvae by non-toxic larvae may be maintained in a balanced polymorphism. [8] [9] A third hypothesis is that automimics are more likely to die or to be injured by a predator's attack. If ...
In natural selection, negative selection [1] or purifying selection is the selective removal of alleles that are deleterious.This can result in stabilising selection through the purging of deleterious genetic polymorphisms that arise through random mutations.