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Natural populations of widely-distributed animals often exhibit clinal variation in phenotypic traits or in allele frequencies of a particular gene over their geographical range. A planktotrophic intertidal snail, Littorina keenae is broadly distributed along the north-eastern Pacific coast through a large latitudinal range (24°50´N-43°18´N). We tested for latitudinal clines in two complex phenotypic traits - thermal tolerance and body size - and one single locus trait ¨C heat shock cognate 70 (HSC70) - in L. keenae along almost its entire geographical range. We found only weak evidence for a latitudinal cline in the thermal tolerance and no evidence for a cline in allele frequencies at HSC70. However, as predicted by Bergmann's rule, we detected a strong latitudinal cline that accounted for 60% of the variance in body size (R2 = 0.598; P < 0.001). In contrast, body size did not significantly affect thermal tolerance. HSC70 showed no genetic differentiation among the populations, supporting our previous mitochondrial gene-based estimate of high gene flow during this snail's free-swimming larval stage. Given that L. keenae experiences panmixia along its species range, the observed size cline may be partially or entirely caused by a phenotypically plastic response to local thermal environments rather than by genetic divergence in body size among populations in response to locally optimizing natural selection. |
