Background
The authors write that "if genetic variation exists" -- and, of course, it does -- then environmental disturbances with large impacts on population growth rates may drive rapid evolution of tolerance," and they report that "empirical studies have now documented a growing list of species that have undergone evolutionary responses to environmental changes (Jump and Penuelas, 2005; Hoffman and Willi, 2008)."

What was done
To further explore this most important subject, Harmon et al. (2009) subjected field-caged populations of pea aphids (Acyrthosiphon pisum Harris) to an experimentally increased frequency of heat shocks by covering the aphids' mesh cages with clear plastic sheeting for four hours at midday three times a week, which increased temperatures within the cages by about 5°C and raised them above the threshold at which pea aphid fecundity is affected. In doing so, they worked with an aphid strain that was susceptible to heat shocks, as well as one that was tolerant of them, due to its containing a heat-tolerant genotype of its primary bacterial endosymbiont plus a protective secondary endosymbiont, both of which bacteria, in their words, are "invariably transmitted during parthenogenetic reproduction" and are thus "analogous to inherited traits in monoclonal aphid lines."

What was learned
The researchers determined that the heat-shock sensitive clone "had slightly higher population growth rates than did the heat-shock tolerant clone in the absence of experimental heat shocks [+11%]," but that the heat-shock sensitive clone "had greatly reduced population growth rates [relative to the heat-tolerant clone] in the presence of heat shocks [-36%]."

What it means
Harmon et al. draw three important conclusions based on their experimental findings. First, they state that the population growth rates they observed "translate into strong selection against heat-sensitive clones in the presence of heat shocks." Second, they say their observations demonstrate "the potential for rapid evolution for heat-shock tolerance." And, third, they say their findings imply that "evolution can occur so rapidly that it cannot be ignored, even in the short term."

References
Hoffman, A.A. and Willi, Y. 2008. Detecting genetic responses to environmental change. Nature Reviews Genetics 9: 421-432.

Jump, A.S. and Penuelas, J. 2005. Running to stand still: adaptation and the response of plants to rapid climate change. Ecology Letters 8: 1010-1020.