Dobzhansky's team also found that, for populations collected at a single site throughout the year, inversion frequencies change as well. That is, cyclic variations in chromosome arrangements occur as the seasons change, as shown in Figure 26-5. Such variation was consistently observed over a period of several years. The frequency of ST always declines during the spring, and that of CH increases in the spring.
Figure 26-5. Changes in the ST and CH arrangements in D. pseudoobscura throughout the year.