Selection cyclic

Cyclic selectionis also frequently mentioned as a possible source of polymorphism in the population. This occurs when populations of a certain species are exposed to several opposing selection pressures.In dry summers, individuals with a certain phenotype can have greater fitness, while individuals with a different phenotype have greater fitness in wet summers. Individuals with a certain phenotype can survive better in the winter, while others survive better in the summer. As the individual periods alternate in time, selection pressures also alternate and their action increases and decreases cyclically, as do the frequencies of the individual alleles. A classical example, on which the phenomenon of cyclic selection was studied, corresponds to the coexistence of the red and black forms of two-spotted lady beetles (Adalia bipunctata). It is stated that the dark forms are at a disadvantage in the winter months, in damp and cold conditions, when their frequency decreases from the original 55-70% to 30-45%, while the red form is at a disadvantage in the warm and dry months (Timofeeff-Ressovsky 1940).

A similar effect of two opposing selection pressures on the preservation of polymorphism was also described in a system encompassing three species: the pea aphid Acyrthosiphon pisum, its parasitic wasp Aphidius ervi and the lady beetle predator (Coccinella septempunctata). In this case, the red and green forms of the aphid exist over long periods in the population, where the green form is more resistant to the predator and the red form is more resistant to parasites (Losey et al. 1997).

It is certainly not easy to decide whether polymorphism can be maintained in the long term through the action of cyclic selection alone or whether it is also necessary that frequency-dependent selection simultaneously act on the population or at least that allele recessivity effect also play a role. Mathematical models show that, in most cases, cyclic selection alone is not sufficient in the long term (Kimura 1955). However, they simultaneously show that a number of factors can affect the long-term stability of the system, including factors that are as wide-spread as sexual dimorphism or the existence of dormant stages in the particular species (Reinhold 2000). This aspect merits more detailed analysis, which would, however, substantially exceed the scope of this text.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more