Editing Balancing selection (section)

== More complex examples ==
Species in their natural habitat are often far more complex than the typical textbook examples.

=== 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>

=== Chromosome polymorphism in ''Drosophila'' ===
In the 1930s [[Theodosius Dobzhansky]] and his co-workers collected ''[[Drosophila pseudoobscura]]'' and ''[[Drosophila persimilis|D. persimilis]]'' from wild populations in [[California]] and neighbouring states. Using [[Theophilus Painter|Painter's]] technique,<ref>Painter T.S. 1933. "A new method for the study of chromosome rearrangements and the plotting of chromosome maps". ''Science'' '''78''': 585–586.</ref> they studied the [[polytene chromosome]]s and discovered that all the wild populations were polymorphic for [[chromosomal inversions]]. All the flies look alike whatever inversions they carry, so this is an example of a cryptic polymorphism. Evidence accumulated to show that natural selection was responsible:

[[Image:Drosophila polytene chromosomes 2.jpg|right|thumb|250 px|''Drosophila'' polytene chromosome]]

# Values for heterozygote inversions of the third chromosome were often much higher than they should be under the null assumption: if no advantage for any form the number of heterozygotes should conform to N<sub>s</sub> (number in sample) = p<sup>2</sup>+2pq+q<sup>2</sup> where 2pq is the number of heterozygotes (see [[Hardy–Weinberg principle|Hardy–Weinberg equilibrium]]).
# Using a method invented by L'Heretier and Teissier, Dobzhansky bred populations in ''population cages'', which enabled feeding, breeding and sampling whilst preventing escape. This had the benefit of eliminating [[insect migration|migration]] as a possible explanation of the results. Stocks containing inversions at a known initial frequency can be maintained in controlled conditions. It was found that the various chromosome types do not fluctuate at random, as they would if selectively neutral, but adjust to certain frequencies at which they become stabilised.
# Different proportions of chromosome morphs were found in different areas. There is, for example, a polymorph-ratio [[Cline (biology)|cline]] in ''[[Drosophila robusta|D. robusta]]'' along an {{convert|18|mi|km|adj=on}} transect near [[Gatlinburg, Tennessee|Gatlinburg]], [[Tennessee|TN]] passing from {{convert|1000|ft|m}} to 4,000 feet.<ref>Stalker H.D and Carson H.L. 1948. "An altitudinal transect of ''Drosophila robusta''". ''Evolution'' '''1''', 237–48.</ref> Also, the same areas sampled at different times of year yielded significant differences in the proportions of forms. This indicates a regular cycle of changes which adjust the population to the seasonal conditions. For these results selection is by far the most likely explanation.
# Lastly, morphs cannot be maintained at the high levels found simply by mutation, nor is drift a possible explanation when population numbers are high.

By 1951 Dobzhansky was persuaded that the chromosome morphs were being maintained in the population by the selective advantage of the heterozygotes, as with most polymorphisms.<ref>Dobzhansky T. 1970. ''Genetics of the evolutionary process''. Columbia University Press N.Y.</ref><ref>[Dobzhansky T.] 1981. ''Dobzhansky's genetics of natural populations''. eds Lewontin RC, Moore JA, Provine WB and Wallace B. Columbia University Press N.Y.</ref><ref>Ford E.B. 1975. ''Ecological genetics''. 4th ed. Chapman & Hall, London.</ref>