Testing mechanisms of Bergmann’s rule: Phenotypic decline but no genetic change in body size in three posserine bird populations

TY - JOUR

T1 - Testing mechanisms of Bergmann’s rule: Phenotypic decline but no genetic change in body size in three posserine bird populations

AU - Husby, A.

AU - Hille, S.M.

AU - Visser, M.E.

N1 - Reporting year: 2011 Metis note: 5079; WAG; AnE

PY - 2011

Y1 - 2011

N2 - Bergmann’s rule predicts a decrease in body size with increasing temperature and has much empirical support. Surprisingly, we know very little about whether “Bergmann size clines” are due to a genetic response or are a consequence of phenotypic plasticity. Here, we use data on body size (mass and tarsus length) from three long-term (1979–2008) study populations of great tits (Parus major) that experienced a temperature increase to examine mechanisms behind Bergmann’s rule. We show that adult body mass decreased over the study period in all populations and that tarsus length increased in one population. Both body mass and tarsus length were heritable and under weak positive directional selection, predicting an increase, rather than a decrease, in body mass. There was no support for microevolutionary change, and thus the observed declines in body mass were likely a result of phenotypic plasticity. Interestingly, this plasticity was not in direct response to temperature changes but seemed to be due to changes in prey dynamics. Our results caution against interpreting recent phenotypic body size declines as adaptive evolutionary responses to temperature changes and highlight the importance of considering alternative environmental factors when testing size clines.

AB - Bergmann’s rule predicts a decrease in body size with increasing temperature and has much empirical support. Surprisingly, we know very little about whether “Bergmann size clines” are due to a genetic response or are a consequence of phenotypic plasticity. Here, we use data on body size (mass and tarsus length) from three long-term (1979–2008) study populations of great tits (Parus major) that experienced a temperature increase to examine mechanisms behind Bergmann’s rule. We show that adult body mass decreased over the study period in all populations and that tarsus length increased in one population. Both body mass and tarsus length were heritable and under weak positive directional selection, predicting an increase, rather than a decrease, in body mass. There was no support for microevolutionary change, and thus the observed declines in body mass were likely a result of phenotypic plasticity. Interestingly, this plasticity was not in direct response to temperature changes but seemed to be due to changes in prey dynamics. Our results caution against interpreting recent phenotypic body size declines as adaptive evolutionary responses to temperature changes and highlight the importance of considering alternative environmental factors when testing size clines.

U2 - 10.1086/660834

DO - 10.1086/660834

M3 - Article

SN - 0003-0147

VL - 178

SP - 202

EP - 213

JO - American Naturalist

JF - American Naturalist

IS - 2

ER -