Evolution of latitudinal clines in reverse: how much do populations converge under adaptation to a novel, common environment?


Latitudinal clines are among the most appealing phenomena to study, since they represent the work of natural selection over a large geographical scale, being a clear case of repeatable patterns of differentiation in nature. They are also an emblematic example of the influence of temperature in natural populations, presenting an excellent model to study the evolutionary effects of global warming.

There are several studies of latitudinal clines in the genus Drosophila, suggesting a high differentiation on a multitude of traits, from morphology to physiology and life-history, as well as chromosomal inversions and thermal phenotypic plasticity. In spite of this, the mechanisms by which natural selection produces clinal variation remain unresolved. Also, there are inconsistencies across species and/or studies, for which time in the laboratory may contribute.

Among the cases of Drosophila clinal evolution, Drosophila subobscura is the most interesting one. The recent colonization of North and South America from Europe indicated fast, repeatable clinal evolution with latitudinal clines now present in three continents. It also gives evidence for the effects of global warming. Previous studies using populations under selection for different temperatures concluded that this factor was not the direct selective agent causing the cline. Also, one cannot discard the possibility that clinal variation is due to a balance between local selection and gene flux, with history playing also a role.

One important experiment that has yet to be performed is the study of the evolution of laboratory populations from contrasting latitudes, as they become adapted to a novel, common environment, across environments and traits, as well as the way that previous history can constrain adaptation.

With that in mind we collected flies from three points in the European cline (Groningen - Netherlands; Montpellier - France; and Sintra - Portugal) and we characterized the initial differentiation as well as the subsequent evolutionary changes of these populations.

This characterization involved: