AN EXPERIMENTAL-STUDY OF EVOLUTION IN PROGRESS - CLINES FOR QUANTITATIVE TRAITS IN COLONIZING AND PALEARCTIC POPULATIONS OF DROSOPHILA

Drosophila sobobscura has recently colonized the American continent and is an excellent model for studying evolution in action. Previous analyses have shown that these colonizing populations have significant latitudinal dines for the frequencies of some chromosomal arrangements that parallel those dines found in the Old World. These results strongly suggest that this polymorphism is adaptive. In the present study, significant latitudinal dines for continuous morphometric variables (flies are larger in the north) have been detected in Old World populations of D. subobscura. The adaptive nature of these dines is reinforced by the fact that parallel latitudinal dines for body size have also been detected in Drosophila obscura, a closely related sympatric species, as well as previously in other Drosophila. On the other hand, no significant latitudinal dines for continuous morphometric traits, not even when using an overall size index, have been detected in colonizing populations of D. subobscura. This is a rather surprising result given the number of generations that have elapsed since the species was detected in America and given that significant dines in chromosomal inversions are already established. Thus, the adaptive response of quantitative morphometric variables is not as rapid as that found for chromosomal inversions. Nevertheless, canonical correlation analysis suggests that significant latitudinal dines for body size might soon be detected in the American continent. The results obtained here are included in a projected time series with the aim of documenting size evolution in action.