Abstract. 1. Understanding evolutionary responses to global climate warming can be dauntingly complex. But, primarily, it requires careful quantification of rates of temporal change of biomonitors.
2. Long-term biomonitoring programs capitalize on traits for which there already exist retrospective measurements. Those programs are thus faced with the decision as to the appropriate time to update historical records, especially in species whose gene frequencies cycle over the successive seasons. In these cases, the time during the year at which descendant populations are sampled needs to take into account effects of the lengthening growing season that can impose direct selection on specific genotypes and, concomitantly, indirect selection on photoperiodic response.
3. Standardising new and past collections by calendar date can lead to an overestimation of the magnitude of long-term responses to global warming; standardising by equivalent seasonal temperature conditions can lead to an underestimation of the magnitude of long-term responses to global warming.
4. The results of a recent monitoring study with Drosophila indicate that caution should be exercised in how the updating of historical records is used to quantify evolutionary responses to global warming.