days survived without foodDays survived without food for adult female Tigriopus californicus from lines selected for increased thermal tolerance and for lines held as unselected controls. First line of file is line name. First two letters are two-letter population code (see sampling locations file), followed by line number, followed by -U for unselected or -S for selected lines.female body sizeBody length (in mm) of adult female Tigriopus californicus from lines selected for increased thermal tolerance and for lines held as unselected controls. Second column is population name (see sampling locations file). Third column is selection status: S = selected line U = unselected line. N= 6 individuals per line, 3-4 selected and unselected lines per populationmale body size# Body length (in mm) of adult male Tigriopus californicus from lines selected for increased thermal tolerance and for lines held as unselected controls. # first column is population name (see sampling locations file). # second column is selection status: S = selected line U = unselected line # N= 6 individuals per line, 3-4 selected and unselected lines per populationbrood size# Mean number of larvae from the first 3 broods of females from lines selected for increased thermal tolerance and from lines held as unselected controls. # second column is population name (see sampling locations file). # third column is selection status: S = selected line U = unselected line # N= 6 individuals per line, 3-4 selected and unselected lines per populationinterbrood interval# Mean number of days between broods for the first 3 broods of females from lines selected for increased thermal tolerance and from lines held as unselected controls. # second column is population name (see sampling locations file). # third column is selection status: S = selected line U = unselected lineminimum generation time# minimum generation time (in days) for females from lines selected for increased thermal tolerance and from lines held as unselected controls. # second column is population name (see sampling locations file). # third column is selection status: S = selected line U = unselected lineSampling locationssampling locations and abbreviations for Tigriopus californicus

Antagonistic correlations among traits may slow the rate of adaptation to a changing environment. The tide pool copepod Tigriopus californicus is locally adapted to temperature, but within populations, the response to selection for increased heat tolerance plateaus rapidly, suggesting either limited variation within populations or costs of increased tolerance. To measure possible costs of thermal tolerance, we selected for increased upper lethal limits for 10 generations in 22 lines of T. californicus from six populations. Then, for each line, we measured six fitness-related traits. Selected lines showed an overall increase in male and female body sizes, fecundity, and starvation resistance, suggesting a small benefit from (rather than costs of) increased tolerance. The effect of selection on correlated traits also varied significantly by population for five traits, indicating that the genetic basis for the selection response differed among populations. Our results suggest that adaptation was limited by the presence of variation within isolated populations rather than by costs of increased tolerance.