Why are individuals in a population different from each other? Answering this question is a central goal of evolutionary biology and can be addressed by studying characteristics of individuals, features of their environment, and their genomes. The Bergland lab address this basic question by studying how temporal and spatial fluctuations in selection pressures maintain genetic variation underlying fitness related traits. The maintenance of functional genetic variation via environmental fluctuations through time and space is a form of natural selection called balancing selection and occurs when some genotypes are better in some environments than others. The Bergland lab seeks to study features of this conditionally beneficial genetic variation with the goal of testing the importance of environmental variation as a diversifying evolutionary force. We address the basic problem of balancing selection using two model systems, Drosophila and Daphnia. These species experience strong fluctuations in selection pressure over seasonal time scales providing us the opportunity to study adaptive evolution as it unfolds in the wild.
The molecular basis of local adaptation
Background. One of the major systems we work on is Drosophila melanogaster which has served for decades as an important model for understanding local adaptation. Fly populations arrayed along latitudinal and altitudinal clines exhibit patterns of genetically based phenotypic differentiation in morphological and life-history traits consistent with local adaptation to temperate environments (Paaby et al.2014, Bergland et al. 2016, Machado et al. 2016). Genetically based phenotypic differentiation in fitness related traits also occurs among seasons. For instance, flies sampled in spring - the recent descendants of individuals that survived winter - tend to be more hardy whereas those sampled during fall - the descendants of those individuals that prospered during summer - tend to invest more resources into reproduction. Quantitative genetic variation in these stress tolerance and life-history traits is thus shaped over seasonal time scales (10-15 generations) and we can identify polymorphisms that vary through time and space (Bergland et al. 2014) and underlie local adaptation.
Current project - photoperiodism & thermoperiodism. Flies that survive winter often do so by entering a physiologically protective state called ‘diapause.’ As in other species, diapause in flies is cued by the perception of short days and cold temperatures (photo- and thermoperiodism, respectively). We are currently investigating the genetic and neurological basis of photoperiodism and thermoperiodism in D. melanogaster. This work is in collaboration with Paul Schmidt at UPenn.
Current project - identification of functional polymorphisms genome-wide. Our previous work has identified hundreds of polymorphisms that vary in frequency through time and space (Bergland et al. 2014, Bergland et al. 2016). We do not know the function of many of these polymorphisms. To ameliorate this, we are conducting cis-eQTL experiments in an experimental orchard we are building at Morven.
Current project - population genomics of Drosophila. The Bergland participates in several consortia, DrosRTEC and DrosEU, whose aim is to sample and resequence fly populations on multiple continents and over decadal time scales. Such efforts will provide valuable information to the broader community about the dynamic process of adaptation in the wild. These efforts are led by members of the Bergland lab, as well as many labs throughout the world. Our consortia are open to anyone and we are actively sampling flies and continuing our organization efforts. Please be in touch if you are interested in participating.
The temporal dynamics of adaptation in the wild
Background. All organisms live in environments that vary through time and space. In response to fluctuations in aspects of the biotic and abiotic environment, many populations rapidly adapt as they track the environment. Such non-anthropogenic environmental change is often caused by genetic shifts at many loci that underly quantitative traits. However, we have little knowledge of the evolutionary dynamics of such loci and how fluctuations of these alleles affects patterns of genetic variation genome-wide. For instance, are these alleles old or do they go to fixation often and subsequently arise again de novo? Do fluctuations at these loci substantially alter patterns of polymorphism at linked neutral sites? We address these basic questions using Daphnia as a model system
Current project - dynamics of predation induced adaptation in Daphnia. In collaboration with Andrew Beckerman at Univ. of Sheffield, we are investigating the evolutionary history of loci that control quantitative genetic variation in predation defensiveness in Daphnia pulex. In response to midge predation, Daphnia grow small spikes on their back (‘neck-teeth’) but the ability to respond midge varies through space and, most likely, seasons. We are currently studying the evolutionary history of loci associated with neck-tooth induction through quantitative- and population-genomic analyses. This work seeks to examine the long term history of rapid adaptation to fluctuations in predation pressure through direct examinination of resurrected Daphnia lineages while simultaneously predicting adaptive evolutionary outcomes in the near future.
In addition to examining adaptive dynamics in response to predation regime, our work on Daphnia will bear direct relevance to our understanding of adaptation to novel anthropogenic stressors as well as adaptation to anthropogenic climate change.