These data are described in detail in Sotka et al., 2018. From May until October 2015, we collected live thalli at each of 40 populations: 15 Japanese (six source and nine nonsource), five western North American, 10 eastern North American, and 10 European populations. We exposed a haphazardly chosen set of approximately 20 thalli per population to extreme heat (1, 2, and 4 hr in 40°C), extreme cold (45, 75, and 105 min at −20°C), and low salinities (8 days at 0, 5, and 10 ppt). For the heat tolerance assay, four apices per thallus were individually placed into unsealed 2- ml microcentrifuge tubes containing 175 μl of seawater. A control apex was kept in a 15°C growth chamber, while three apices were placed in a 40°C water bath for 1, 2, and 4 hr. For the cold tolerance assay, four apices were independently placed into sealed 250 μl PCR tubes containing 200 μl seawater. A control apex was kept in a 15°C incubator, while three apices were placed in a −20° C freezer for 45, 75, and 105 min. Apices were then placed into randomly assigned wells of 12- well tissue culture plates (2.21- cm- diameter wells) filled with 4 ml of seawater and incubated at 15°C for 8 days with a 12- hr light/dark cycle. For the salinity tolerance assay, four apices were independently placed directly into the 12- well tissue culture plates filled with 4 ml of seawater at a titration of salinities diluted to 0, 5, and 10 ppt, with 30 ppt serving as a control. Every 2 days for 8 days, we changed water and assessed thallus bleaching (i.e., loss of photosynthetic pigments; Figure S2) as a proxy of mortality, itself an important component of fitness. We generated a Bleaching Score (BS), where “1” indicated no bleaching, “2” indicated partial bleaching or color change to pink or white, and “3” indicated full bleaching.