Replicated selection for 21-day pupa weight of Tribolium castaneum.

Response to upward selection in 10 replicate lines was linear through 15 generations, had slowed considerably in many lines by generation 23 and continued in other lines through 30 generations. The lines tended to group into those which responsed slowly during early generations and continued to respond through later generations and those which gave rapid early response and very low response later coupled with marked reduction in fitness. Response to downward selection was similar for 3 replicate lines but only 50% as great as that for the high lines. Response declined more rapidly in the low lines due to reduced fitness, declining selection differentials and a marked drop in phenotypic variance.