DIGESTS doi:10.1111/evo.13154 Digest: On the temporal evolution of reproductive isolation * Spencer J. Ingley 1,2,3,4 1 University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514 2 North Carolina Museum of Natural Sciences, Raleigh, NC 27601 3 Department of Biology, Duke University, Durham, NC 27708 4 E-mail: sjingley@gmail.com Received November 29, 2016 Accepted December 7, 2016 Speciation research has long focused on understanding how re- productive barriers—sometimes referred to as the “currency” of speciation—contribute to genetic isolation between divergent lin- eages (Coyne and Orr 2004). These reproductive barriers include those that occur before mating takes place (e.g., sexual, habitat, and temporal isolation) and after (e.g., gametic incompatibility, hybrid sterility, and sexual/ecological selection against hybrids). Given that the speciation process is typically too long to observe in its entirety, researchers often approach the problem of under- standing how barriers evolve by studying barriers between taxa at different stages of divergence. This technique provides snapshots along the speciation process, and is useful for inferring the rel- ative importance of barriers across speciation. However, to date most studies have either tested for a limited number of barri- ers (e.g., sexual isolation, hybrid viability; Coyne and Orr 1989; Mendelson 2003) between many pairs of species at different lev- els of divergence, or have tested for many barriers between one or a few pairs of lineages at the earliest stages of divergence (Schemske 2010; Nosil 2012). In this issue, Lackey and Boughman (2016) use a classical system in speciation research, stickleback fishes, to provide an uncanny glimpse into which barriers matter most across specia- tion. Sticklebacks have been the subject of decades of work testing for reproductive isolation between species pairs—both young and old—that have evolved in response to divergent natural selection. Lackey and Boughman added their own impressive dataset to decades of past work to amass perhaps the most robust assess- ment of the temporal evolution of reproductive isolation to date, evaluating between six and nine barriers in pairs of stickleback ∗ This article corresponds to Lackey, A. C. R. and Boughman, J. W. (2016), Evolution of reproductive isolation in stickleback fish. Evolution. doi:10.1111/evo.13114. species from across the globe. With high levels of replication within systems, the authors were able to estimate variation in individual barriers and total isolation across space and time. Ad- ditionally, they compared measures of total isolation to other axes of divergence (ecological and genetic differentiation, divergence time, and measures of hybridization in the field), to determine how effective these measures are at predicting the level of isola- tion achieved. Lackey and Boughman’s work is important because it cor- roborates previous findings and provides novel insights into the evolution of reproductive barriers. Their work confirms the pri- macy of premating over postmating isolation early in speciation, and finds that habitat and sexual isolation evolve early and remain two of the strongest barriers across speciation. Despite evidence that extrinsic barriers are far stronger than intrinsic barriers in sticklebacks, postmating intrinsic isolation appears to be essen- tial in completing speciation. By testing for barriers between a species pair that was studied both before and after its recent collapse, Lackey and Boughman show that reverse speciation was accompanied by the loss of sexual isolation—the very barrier that played a pivotal role in catalyzing the speciation process. Finally, divergence time and genetic differentiation were found to be poor predictors of isola- tion, though ecological differentiation performed reasonably well, providing further evidence of the importance of both natural and sexual selection in the speciation process. This work raises the bar for studies that seek to understand the barriers that initiate speciation, drive it to completion, and potentially play a role in its collapse. However, this study also highlights several outstanding questions. For example, how does reproductive isolation accrue in nonecological speciation scenar- ios? Divergence in sticklebacks is largely driven by divergent 1 C  2016 The Author(s). Evolution