Mild heat stress impairs the buffering ability of mutant Hsp83 enough to cause population extinction. The horizontal axis shows time in generations after inbreeding lines had been established. The vertical axis shows the average number of offspring a female produces, i.e., female fecundity. Seven lines homozygous for wild-type Hsp83 (Hsp83+/+, blue) and for mutant Hsp83 (Hsp83P/P, red), respectively, were isogenized from each of the three geographic populations we studied here. From each isofemale line, four 3-day-old virgin females and four males were placed in a vial and allowed to mate to create the first generation. Subsequent generations were also produced from matings between four 3-day-old virgin females and four males. All offspring produced within 30 days were counted to estimate the fecundity per female. Error bars indicate one standard error of the mean among the 7 isofemale lines. Independent-sample t-tests were performed to ask whether fecundity differed between generation 1, on the one hand, and generation 2-3, on the other hand. Asterisks indicate significant differences at p < 0.05(*) and p < 0.001(***) obtained in these tests.