Summary:

In a world where human activity is changing the climate and habitats at a fast pace it is fundamental to understand how much and how quickly can species adapt. In the last few years we have witnessed the evolutionary response of various species to the effects of global warming. One such case is Drosophila subobscura, which presents clinal variation for body size and inversions, across three continents. Recent evidence shows that temporal changes are occurring in the clinal variation as a response to global warming, with northern populations becoming more similar to southern ones. Both local adaptation and gene flow may be involved, the latter possibly overcoming historical constraints. In this study we propose to measure the contributions of history and selection when populations initially differentiated in Nature are under a similar selective pressure, in order to test if the uniform selection erases prior genetic differences, even in the absence of gene flow. We used as scenario the adaptation to a new, common environment of three populations of Drosophila subobscura initially differentiated along the European latitudinal cline. Quick evolutionary response was observed in all foundations leading to full convergence. All foundations converged to the same adaptive peak, although at different rates and through different paths, suggesting an overall smooth fitness landscape. We concluded that although history had a strong effect during the initial generations, selection quickly overcame it, especially in fitness related traits. The fast loss of differentiation shows that, even in the absence of gene flow adaptation to a common environment can erase the variation observed in nature, a finding that raises concerns in Conservation terms.

Summary:

Chromosomal inversions are widespread in Drosophila with strong evidence supporting an adaptive cause for the evolution and maintenance of inversion polymorphisms. One emblematic example of the adaptive role associated with inversions is the repeatable clinal variation in inversions frequencies in different continents as happens in Drosophila subobscura. More recently, it has been found that these inversion polymorphisms are shifting as a response to global warming. Both local adaptation and gene flow may be involved, the latter possibly overcoming historical constraints. Nevertheless, it is still unclear what forces are important in shaping the evolution of inversion polymorphisms. An approach to this issue is to study the evolutionary dynamics of chromosomal inversions of populations initially differentiated along a cline, during adaptation to a novel, common environment in the absence of gene flow. We use this strategy analyzing laboratory adaptation in Drosophila subobscura founded from contrasting European latitudes. Will natural selection in the new environment overcome the initial historical differences, promoting convergence of inversion frequencies? During the first 25 generations of adaptation to a common environment we found that the polymorphism of chromosomal inversions was gradually reduced in all populations. We also found persistent differentiation between populations from contrasting latitudes, though these differences reduced with increasing generations in the new environment. While genetic drift seems to play an important role in inversion frequency changes, we also found consistent increase in frequency of specific inversions (initially in low frequencies) across replicate populations, which suggests that selection also played a role. Altogether this study indicates that, in the absence of gene flow, inversion polymorphism evolves under a balance of selection and genetic drift with historical constraints also playing an important role.

Summary:

Clinal variation for wing size and shape as well as for chromosomal inversion frequencies has been found in Drosophila subobscura from Europe as well as North and South America. Particularly it has been reported that standard arrangements increase in frequency towards higher latitudes, where flies are also bigger. In the New World the rapid evolution of body size clines as a follow up of clinal evolution of inversions, suggested that the wing traits cline had been driven by the inversion polymorphism. Previous studies, in favor of this hypothesis, found an intra-population association between wing traits (size and shape) and standard chromosomal inversions. Nevertheless, it was also found that the association (for shape) or details of it (for size) had opposite signs in one European population (Adraga) and one South American population (Puerto Montt). This is probably due to a bottleneck effect following the colonization of the Americas. Despite this interesting finding it is still unknown if this is a generalized difference between continents. To tackle this question we here tested for the consistency of the association between wing traits and inversions in three populations along the cline of Europe as well as South America. Surprisingly we found no clear association between wing size and the number of standard inversions in either continent. On the other hand, we confirmed that the previously reported negative association between wing shape and standard dose of Puerto Montt spread with latitude through other South America populations. This is not in agreement with the positive sign of the cline for both wing shape and inversions. Overall and contrary to previous indications, this study suggests that the inversion and wing clines in D. subobscura have evolved independently from each other.

Summary:

Natural selection is able to produce rapid adaptive responses to sustained environmental change under propitious conditions: intense selection, abundant genetic variation, and large population sizes. Consider a scenario where these conditions are not met: a population migrates to a new environment; it encounters qualitatively novel nutrition, possibly including periods in which it starves. How will its life history evolve in response to such a new environment? Furthermore, its effective population size may be reduced in the course of migration or as a result of an initial lack of adaptation to the new environment. How will effective population size affect its evolutionary response to this new environment? If this population survives and its descendants migrate back to their ancestral environment, how will this preceding period of adaptation to a new environment affect its initial life history upon return to ancestral conditions, and then its subsequent evolution? To tackle these issues requires populations of known differentiated histories, populations on which selection (both forward and reverse) is imposed with contrasting population sizes. This project involves laboratory populations of Drosophila melanogaster with controlled and replicated histories of selection. The study of their evolutionary dynamics after the imposition of new selection regimes, both at small and at large population sizes, will help resolve the interaction between genetic drift and natural selection, as a function of both previous and present selective and demographic histories