PLEIOTROPY CAUSES LONG-TERM GENETIC CONSTRAINTS ON LIFE-HISTORY EVOLUTION IN BRASSICA-RAPA

Fundamental, long-term genetic trade-offs constrain life-history evolu tion in wild crucifer populations. I studied patterns of genetic const raint in Brassica rapa by estimating genetic correlations among life-h istory components by quantitative genetic analyses among ten wild popu lations, and within four populations. Genetic correlations between age and size at first reproduction were always greater than +0.8 within a nd among all populations studied. Although quantitative genetics might provide insight about genetic constraints if genetic parameters remai n approximately constant, little evidence has been available to determ ine the constancy of genetic correlations. I found strong and consiste nt estimates of genetic correlations between life-history components, which were very similar within four natural populations. Population di fferentiation also showed these same trade-offs, resulting from long-t erm genetic constraint. For some traits, evolutionary changes among po pulations were incompatible with a model of genetic drift. Historical patterns of natural selection were inferred from population differenti ation, suggesting that correlated response to selection has caused som e traits to evolve opposite to the direct forces of natural selection. Comparison with Arabidopsis suggests that these life-history trade-of fs are caused by genes that regulate patterns of resource allocation t o different components of fitness. Ecological and energetic models may correctly predict these trade-offs because there is little additive g enetic variation for rates of resource acquisition, but resource alloc ation is genetically variable.