Evolutionary Ecology Research, 2003, 5: 431–437
Thermal environment, survival and local adaptation
in the common frog, Rana temporaria
Mats Olsson and Tobias Uller*
Department of Zoology, University of Gothenburg, Medicinaregatan 18,
Box 463, 405 30 Gothenburg, Sweden
ABSTRACT
Natural selection of organisms results in differences between populations in response to
environmental conditions – that is, local adaptation. Understanding heterogeneity at this level
requires identification of the different selection pressures that cause this differentiation.
Tadpoles of the common frog, Rana temporaria, from northern Sweden show superior growth
in warmer incubation conditions than tadpoles from the south, and vice versa in cooler conditions. A plausible explanation for this is that the thermal optimum in the northern population
has been shifted by selection to correspond better with the relatively warmer developmental
conditions experienced in the north compared with the south (northern ponds are warmer
because they are shallower and tend to heat more quickly following the melting of snow). To
test this hypothesis, we performed an experiment in which frogs from both regions were allowed
to develop in water temperatures representative of those in their natural breeding ponds and in
those at the other climatic extreme. Tadpoles in the ‘warm’ (northern) part of the range had
markedly reduced survival at low temperatures compared with those from the cooler region in
the south, with a non-significant difference at relatively high temperatures. Thus, northern frogs
appear to have evolved towards becoming high-temperature ‘specialists’, with local adaptation
being upheld by hard selection on tadpole survival.
Keywords: local adaptation, Rana temporaria, survival, temperature.
INTRODUCTION
One of the central questions in evolutionary biology is to what extent natural populations
have diverged in response to local environmental conditions – that is, local adaptation. The
study of local adaptation is fundamental to modern biology for at least two reasons. First,
it depicts an outcome of adaptive evolution. Secondly, it helps to establish whether
populations are predominantly homogeneous and close to their evolutionary equilibria, or
are small and genetically heterogeneous and relatively far removed from their adaptive
optima. Consequently, local adaptation has received much interest from evolutionary
biologists, with studies covering a wide range of taxa (e.g. Ayres and Scriber, 1994; Reznick
* Address correspondence to either author. e-mail: [email protected]; [email protected]
Consult the copyright statement on the inside front cover for non-commercial copying policies.
© 2003 Mats Olsson and Tobias Uller
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