Editing Balancing selection (section)

=== Grove snail ===
The grove snail, ''[[Cepaea nemoralis]]'', is famous for the rich polymorphism of its shell. The system is controlled by a series of [[multiple alleles]]. Unbanded is the top dominant trait, and the forms of banding are controlled by modifier genes (see [[epistasis]]).

[[Image:Schneckesnail1.jpg|thumb|200px|left|Grove snail, dark yellow shell with single band]]

In England the snail is regularly preyed upon by the [[song thrush]] ''Turdus philomelos'', which breaks them open on ''thrush anvils'' (large stones). Here fragments accumulate, permitting researchers to analyse the snails taken. The thrushes hunt by sight, and capture selectively those forms which match the habitat ''least well''. Snail colonies are found in woodland, hedgerows and grassland, and the predation determines the proportion of phenotypes (morphs) found in each colony.

[[Image:Cepaea nemoralis active pair on tree trunk.jpg|thumb|200px|right|Two active grove snails]]

A second kind of selection also operates on the snail, whereby certain heterozygotes have a physiological advantage over the homozygotes. Thirdly, [[apostatic selection]] is likely, with the birds preferentially taking the most common morph. This is the 'search pattern' effect, where a predominantly visual predator persists in targeting the morph which gave a good result, even though other morphs are available.

The polymorphism survives in almost all habitats, though the proportions of morphs varies considerably. The alleles controlling the polymorphism form a [[supergene]] with linkage so close as to be nearly absolute. This control saves the population from a high proportion of undesirable recombinants.

In this species predation by birds appears to be the main (but not the only) selective force driving the polymorphism. The snails live on heterogeneous backgrounds, and thrush are adept at detecting poor matches. The inheritance of physiological and cryptic diversity is preserved also by heterozygous advantage in the supergene.<ref>Cain A.J. and Currey J.D. Area effects in ''Cepaea''. ''[[Phil. Trans. R. Soc. B]]'' '''246''': 1-81.</ref><ref>Cain A.J. and Currey J.D. 1968. Climate and selection of banding morphs in ''Cepaea'' from the climate optimum to the present day. ''[[Phil. Trans. R. Soc. B]]'' '''253''': 483-98.</ref><ref>Cain A.J. and Sheppard P.M. 1950. Selection in the polymorphic land snail ''Cepaea nemoralis'' (L). ''Heredity'' '''4''':275-94.</ref><ref>Cain A.J. and Sheppard P.M. 1954. Natural selection in ''Cepaea''. ''Genetics'' 39: 89-116.</ref><ref>Ford E.B. 1975. ''Ecological genetics'', 4th ed. Chapman & Hall, London</ref> Recent work has included the effect of shell colour on thermoregulation,<ref>Jones J.S., Leith B.N. & Rawlings P. 1977. Polymorphism in ''Cepaea'': a problem with too many solutions. ''Annual Review of Ecology and Systematics'' '''8''', 109-143.</ref> and a wider selection of possible genetic influences is also considered.<ref>Cook L.M. 1998. A two-stage model for ''Cepaea'' polymorphism. ''[[Phil. Trans. R. Soc. B]]'' '''353''', 1577-1593.</ref>