8. Aso, Y., Sitaraman, D., Ichinose, T., Kaun, K.R., Vogt, K., Belliart-Guerin, G., Placais, P.Y., Robie, A.A., Yamagata, N., Schnaitmann, C., et al. (2014). Mushroom body output neurons encode valence and guide memory- based action selection in Drosophila. eLife 3, e04580. 9. Owald, D., Felsenberg, J., Talbot, C.B., Das, G., Perisse, E., Huetteroth, W., and Waddell, S. (2015). Activity of defined mushroom body output neurons underlies learned olfactory behavior in Drosophila. Neuron 86, 417–427. 10. Sejourne, J., Placais, P.Y., Aso, Y., Siwanowicz, I., Trannoy, S., Thoma, V., Tedjakumala, S.R., Rubin, G.M., Tchenio, P., Ito, K., et al. (2011). Mushroom body efferent neurons responsible for aversive olfactory memory retrieval in Drosophila. Nat. Neurosci. 14, 903–910. 11. Owald, D., and Waddell, S. (2015). Olfactory learning skews mushroom body output pathways to steer behavioral choice in Drosophila. Curr. Opin. Neurobiol. 35, 178–184. 12. Hige, T., Aso, Y., Modi, M.N., Rubin, G.M., and Turner, G.C. (2015). Heterosynaptic plasticity underlies aversive olfactory learning in Drosophila. Neuron 88, 985–998. 13. Cohn, R., Morantte, I., and Ruta, V. (2015). Coordinated and compartmentalized neuromodulation shapes sensory processing in Drosophila. Cell 163, 1742–1755. 14. Lewis, L.P., Siju, K.P., Aso, Y., Friedrich, A.B., Bulteel, A.J., Rubin, G.M., and Grunwald Kadow, I.C. (2015). A higher brain circuit for immediate integration of conflicting sensory information in Drosophila. Curr. Biol. 25, 2203– 2214. 15. Keleman, K., Vrontou, E., Kruttner, S., Yu, J.Y., Kurtovic-Kozaric, A., and Dickson, B.J. (2012). Dopamine neurons modulate pheromone responses in Drosophila courtship learning. Nature 489, 145–149. 16. Dreosti, E., Odermatt, B., Dorostkar, M.M., and Lagnado, L. (2009). A genetically encoded reporter of synaptic activity in vivo. Nat. Methods 6, 883–889. 17. Aso, Y., Hattori, D., Yu, Y., Johnston, R.M., Iyer, N.A., Ngo, T.T., Dionne, H., Abbott, L.F., Axel, R., Tanimoto, H., et al. (2014). The neuronal architecture of the mushroom body provides a logic for associative learning. eLife 3, e04577. 18. Kim, Y.C., Lee, H.G., and Han, K.A. (2007). D1 dopamine receptor dDA1 is required in the mushroom body neurons for aversive and appetitive learning in Drosophila. J. Neurosci. 27, 7640–7647. 19. Jin, X., and Costa, R.M. (2010). Start/stop signals emerge in nigrostriatal circuits during sequence learning. Nature 466, 457–462. 20. Martin, J.R., Ernst, R., and Heisenberg, M. (1998). Mushroom bodies suppress locomotor activity in Drosophila melanogaster. Learn. Mem. 5, 179–191. Bacterial Speciation: Genetic Sweeps in Bacterial Species Frederick M. Cohan Department of Biology, Wesleyan University, Middletown, CT 06459, USA Correspondence: fcohan@wesleyan.edu http://dx.doi.org/10.1016/j.cub.2015.10.022 One theory of bacterial speciation states that bacterial and animal species share the property of cohesion, meaning that diversity within a species is constrained. A new study provides direct evidence that genome- wide sweeps can limit diversity within bacterial species. ‘‘Anything found to be true of E. coli must also be true of elephants!’’ While Jacques Monod was thinking about the unity of biochemistry, evolutionary biologists and ecologists have also sought unifying principles across all of life. Accordingly, some evolutionary ecologists have proposed universal principles for the origin and nature of species [1–3]. One particularly contentious claim is that across all of life, species are each ‘cohesive’, in the sense that the diversity within each species is constrained by some force [1,2]. Bacterial species may be subject to a special force of cohesion, owing to their low frequency of recombination [4]. This force is periodic selection, where natural selection favoring any adaptive mutation purges the genetic diversity, genome-wide, within an ecologically homogeneous species or ecotype (Figure 1A,B) [5]. However, whether periodic selection occurs widely in nature or is even possible has been debated with a passion unusual for the field of microbiology [6–8]. Contributing to the uncertainty is that genome-wide sweeps were never observed in nature, that is, until the recent work of Bendall et al. [9] — using a high-throughput, metagenomic approach to survey a bacterial community over time, these authors charted a genome-wide sweep within one natural bacterial population. In the days before metagenomics, it never seemed a good bet to search for a periodic selection event by focusing on your favorite organism — this process was thought too infrequent to be observed in one population during one grant-funding period. However, what is unlikely to be seen within a single focus population becomes eminently palpable within some population when you can observe an entire community simultaneously. The magic of metagenomics! Bendall et al. [9] surveyed the bacteria of Trout Bog Lake, Wisconsin, over eight years. From each small volume of lake water sampled, they lysed the cells and produced short DNA sequences representing the community’s diversity. Short sequences were assembled into longer sequences by joining sequences (from different organisms) that were homologous and less than 2% divergent from one another. The researchers thereby clustered the organisms into groups of close relatives, and at the same R112 Current Biology 26, R102–R124, February 8, 2016 ª2016 Elsevier Ltd All rights reserved Current Biology Dispatches