Editing Cytogenetics (section)

=== Natural populations of ''Drosophila'' ===
In the 1930s, [[Dobzhansky]] and his coworkers collected ''[[Drosophila pseudoobscura]]'' and ''[[Drosophila persimilis|D.&nbsp;persimilis]]'' from wild populations in [[California]] and neighboring states. Using Painter's technique<ref>{{cite journal |last1=Painter |first1=T. S. |title=A new method for the study of chromosome rearrangements and the plotting of chromosome maps |journal=Science |date=22 December 1933 |volume=78 |issue=2034 |pages=585–586 |doi=10.1126/science.78.2034.585 |pmid=17801695 |bibcode=1933Sci....78..585P }}</ref> they studied the [[polytene|polytene chromosomes]] and discovered that the wild populations were polymorphic for [[chromosomal inversions]]. All the flies look alike whatever inversions they carry: this is an example of a cryptic polymorphism.{{cn|date=February 2024}}

Evidence rapidly accumulated to show that [[natural selection]] was responsible. Using a method invented by L'Héritier 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. By the time Dobzhansky published the third edition of his book in 1951<ref>Dobzhansky T. 1951. ''Genetics and the origin of species''. 3rd&nbsp;ed, Columbia University Press, New York.</ref> he was persuaded that the chromosome morphs were being maintained in the population by the selective advantage of the heterozygotes, as with most [[polymorphism (biology)|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>