Cyclical changes in frequencies of inversion chromosomes ST (circles) and CH (triangles) in two California populations (Pinon and Andreas) of Drosophila pseudoobscura.  Blue lines indicate June 1 in each year.  Note that the frequency of the CH allele peaks regularly around June 1 at the Pinon population.  In the Andreas population, the peak of CH frequency also occurs around June 1.  These cyclical fluctuations suggest that at certain times of year the ST chromosome is favored, while at other times of year the CH chromosome is favored by selection.

Original figure from T. Dobzhansky.  1943.  The genetics of natural populations. IX. Temporal changes in the composition of populations of Drosophila pseudoobscura.  Genetics 28: 162-186.

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