Editing Balancing selection (section)

=== 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>