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to assess whether there is a tendency for more significant selection coefficients on the traits quantified in the selection gradient studies to involve greater plasticity, we found the original study on which each selection coefficient was based (figures 1 and 2 ) and used the midpoint between the minimum and maximum trait values in the study to calculate a plasticity score for each trait in each study (high values indicating greater plasticity). this revealed that in almost all cases (n=25) the original study had quantified the trait at a larger value for the low-plasticity than high-plasticity group. of the 20 examples of positive selection coefficients, 18 had significant higher mean plasticity scores in the high-plasticity group than the low-plasticity group, including all three significant selection coefficients. the single example of significant negative selection was for mean plasticity in arabidopsis flowering time [ 68 ] and there, the mean plasticity score in the high-plasticity group was less than half of that in the low-plasticity group. two of the seven examples of stabilizing selection had higher mean plasticity scores in the low-plasticity group than in the high-plasticity group. hence, in almost all cases, studies that found evidence for selection on plasticity also found that the trait for which selection was estimated was measured at a higher value for the low-plasticity group.

our two studies of selection on phenotypic plasticity in the context of rapid climate change [ 38, 68 ] are in some ways in the same experimental vein as the previous review by van buskirk & steiner [ 55 ], but we have focused on a different set of data. the latter had searched all published studies, whereas we only searched studies of thermal environments and had restricted our literature search to the last 5 years, which we feel has allowed a much better chance of detecting evidence of plasticity and selection. however, we also note that our sample size of studies is small relative to that of van buskirk & steiner [ 55 ], who had systematically reviewed studies over the previous 30 years and had identified a total of 79 estimates of selection on plasticity. nicotra fan selection software download

we next sought to examine whether the directions of selection coefficient estimates within and across studies was consistent. we found that the directions of linear selection coefficients were consistent across studies in more than half of the cases (figure 3 ). there were five cases of contradictory directions, with contrasting directions of selection between studies of the same phenotypic trait and the same species, in a. thaliana flowering time [ 68 ], height in wahlenbergia [ 38 ], size in prunus [ 86 ], and breeding time in o. cuniculus [ 83 ]. two of these contradictory cases are of the same studies as those that found evidence for a significant selection coefficient. there were also two studies of larus argentatus [ 84 ] and p. bairdii [ 85 ] that found evidence for stabilizing selection on plasticity. in one case (that of a. thaliana flowering time [ 68 ]), the study was based on a single, naturally-occurring population and thus was not a controlled experiment. in another case (that of p. bairdii breeding time [ 83 ]), there were two environmental conditions but only one condition of interest and so the study was not controlled.

we analysed selection on plasticity in three broad categories: (1) within populations and (2) between populations, and (3) within populations for different life stages. we also analysed selection on a broader category of plasticity, but these data are not presented here, as there was very little variation in this broad category.
our search identified 38 studies that met our criteria for selection analysis of plasticity. the table below provides a brief summary of all studies included in this review, including the species and the traits measured, and any other information that was provided (e.g. environment, phenotype, number of individuals measured). the specific traits that were measured are given in the table; these have been organised broadly into three groups: morphology, physiology, and fitness. we now review each of these categories in turn.
given the inconsistent nature of the evidence for selection on plasticity in the literature to date, we also focused on investigating the direction and strength of the relationships between plasticity and fitness. in particular, we were interested in testing whether reaction norms with steep slopes, either reflecting more plastic individuals, or a greater environmental variance of plasticity, would result in fitnesses being more strongly associated with plasticity. overall, the results of the 31 individual fitness regressions are largely consistent with this expectation. figure2 shows the three reaction norm slopes (linear regression coefficients of fitness on slope of plasticity) from the eight studies, where the coefficient for each reaction norm was calculated as the standardized regression coefficient between fitness and slope of plasticity. the mean correlation between a reaction norm slope and fitness was positive, but non-significant (r2 = 0.117, p = 0.259). when we pooled data across all eight studies, the mean correlation between the slope of a reaction norm and fitness was positive but non-significant (r2 = 0.112, p = 0.331).

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