Technical Comments

Comment on "Global Genetic Change Tracks Global Climate Warming in Drosophila subobscura"

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Science  16 Mar 2007:
Vol. 315, Issue 5818, pp. 1497
DOI: 10.1126/science.1136298

Abstract

Balanyà et al. (Reports, 22 September 2006, p. 1773) build on earlier claims that chromosomal inversion polymorphisms of Drosophila subobscura are rapidly evolving in response to global warming. However, that conclusion is not adequately buttressed by their data, because they overlooked the lag between calendar and climatological dates created by the progressive lengthening of the growing season in their sampling approach.

Balanyà et al. (1) recently appraised the hypothesis that chromosomal inversion polymorphisms of Drosophila subobscura are evolving in response to global warming. (24). However, their conclusions may not be adequately supported by their data owing to a potential systematic bias in their sampling approach. Balanyà et al. compared inversion frequency records collected up to 50 years ago latitudinally across three continents with the corresponding current records gathered on the same dates (5). Yet it is apparent from their data that they used calendar dates to match the samples. Using calendar dates instead of climatological or biological dates could be systematically misleading for two reasons. First, because global climate warming has lengthened the growing season, increasingly at higher latitudes (68), current biological dates are not expected to represent their corresponding calendar dates from decades ago, the disparity being greater toward the poles. Second, because chromosomal inversion polymorphisms of D. subobscura, a temperate zone species, undergo pronounced seasonal cycles, with seasonal transitions in inversion frequencies occurring in a matter of weeks (9, 10). Thus, it is possible that the long-term global genetic shift reported by Balanyà et al. is, at least in part, a sampling artifact ensuing from a biological lag between the old and new samples—especially those from higher latitudes. The new samples were collected systematically later than the old ones with respect to the historical onset of the biological spring (see Fig. 1). Even if it turns out to be less prominent, endurance of the long-term genetic shifts after correcting for the lengthened growing season would strengthen the case that chromosomal polymorphisms are being affected by ongoing global warming.

Fig. 1.

Schematic depiction of the hypothetical influence of a lengthened growing season on the seasonal cycle (pi) of a typically southern inversion in the Northern Hemisphere (9). Inversion frequencies begin to rise earlier in spring and to decline later from midsummer in the present (outer cycle) than historically (inner cycle). Sampling the same calendar date (red dot) as in the past (black dot) gives frequencies that are systematically upwardly shifted (Δ pi) with respect to the old ones, despite no change in the long-term level of the inversion.

In the near future, an increasing number of studies of biological responses to global warming will rely on updates of old measurements. Taking into account the progressive lengthening of the growing season in deciding new sampling dates should help to obtain more precise records.

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